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Maurizio MAZZUCCHELLI

Professore Ordinario
Dipartimento di Scienze Chimiche e Geologiche - Sede Dipartimento di Scienze Chimiche e Geologiche

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Pubblicazioni

2024 - Transition from orogenic-like to anorogenic magmatism in the Southern Alps during the Early Mesozoic: Evidence from elemental and Nd-Sr-Hf-Pb isotope geochemistry of alkali-rich dykes from the Finero Phlogopite Peridotite, Ivrea–Verbano Zone [Articolo su rivista]
Ogunyele, Abimbola C.; Bonazzi, Mattia; Giovanardi, Tommaso; Mazzucchelli, Maurizio; Salters, Vincent J. M.; Decarlis, Alessandro; Sanfilippo, Alessio; Zanetti, Alberto
abstract

The Ivrea-Verbano Zone (IVZ) in the westernmost sector of the Southern Alps is an iconic upper mantle to lower continental crust sequence of the Adriatic Plate and provides a geological window into the tectono-magmatic events that occurred at the Gondwana–Laurussia boundary from Late Paleozoic to Early Mesozoic. In this work, we document new geochemical and Nd-Sr-Hf-Pb isotopic data for Early Mesozoic alkali-rich dyke swarms which intruded the Finero Phlogopite Peridotite (northern IVZ) to provide geological constraints on the nature, origin and evolution of Early Mesozoic magmatism in the Southern Alps. The studied dykes are amphibole-phlogopite-bearing and show geochemical features varying between two end-member groups. A dyke group is characterized by HFSE-poor, Al-rich amphibole (Al2O3 up to 16 wt.%) with high LILE and LREE contents, high radiogenic 87Sr/86Sr(i) (0.704732 to 0.704934) and low radiogenic Nd isotopes (εNd(i) from –0.1 to –0.7), which support the occurrence of significant amounts of recycled continental crust components in the parental mantle melts and impart an overall “orogenic-like” affinity. This dyke group was largely derived from metasomatized lithospheric mantle sources. The second group is HFSE-rich with Al-poorer amphibole enriched in LILE and LREE, low radiogenic 87Sr/86Sr(i) (0.703761–0.704103) and higher radiogenic Nd isotopes (εNd(i) from +3.4 to +5.4) pointing to an “anorogenic” alkaline affinity and asthenospheric to deep lithospheric mantle sources. Some dykes show both orogenic and anorogenic affinities, providing evidence that the orogenic-like magmatism in the IVZ predates the alkaline anorogenic magmatism. The Finero dyke swarms therefore record a geochemical change of the Early Mesozoic magmatism of the Southern Alps from orogenic-like magmatism, typical of post-collisional settings, to anorogenic alkaline magmatism, common in intraplate to extensional settings, and places a temporal correlation of Early Mesozoic magmatism in the IVZ to those in the eastern and central sectors of the Southern Alps.

2023 - Origin of a carbonate-bearing fluorapatite from Tertiary volcanics of the Veneto Volcanic Province, Italy [Articolo su rivista]
Cipriani, A.; Giovanardi, T.; Mazzucchelli, M.; Lugli, F.; Sforna, M. C.; Gualtieri, A. F.; Di Giuseppe, D.; Gaeta, M.; Brunelli, D.
abstract

We present chemical and mineralogical data on a megacryst of a unique carbonate-bearing fluorapatite from altered Tertiary volcanics of the Veneto Volcanic Province (VVP) in the western Lessini Mountain range (Veneto, northern Italy). The cm-sized specimen was identified and characterized by scanning electron microscope (SEM), X-ray powder diffraction (XRPD), micro-Raman spectroscopy and electron probe microanalyses. Major and trace elements of the carbonate-bearing fluorapatite are consistent with the crystallization at depth from a nelsonitic melt or an evolved alkaline melt derived from a mantle source metasomatized by carbonate-rich fluids. The Sr and Nd isotopic composition fits with the lavas and xenoliths from the VVP showing a DM-HIMU affinity with addition of a crustal, possibly carbonate, component. Our data are in agreement with a recent geodynamic model for the hybridization of the VVP mantle triggered by breakdown of carbonates within the subducting Tethyan oceanic slab. Cronstedtite, chabazite-Ca, calcite associated with reaction rims of amphibole and secondary carbonate-rich fluorapatite within the megacryst originated from low temperature hydrothermal alteration of the volcanics. Cronstedtite is the first occurrence in the VVP area.

2023 - Sound velocities and single-crystal elasticity of hydrous Fo90 olivine to 12 GPa [Articolo su rivista]
Faccincani, Luca; Criniti, Giacomo; Kurnosov, Alexander; Ballaran, Tiziana Boffa; Withers, Anthony C.; Mazzucchelli, Maurizio; Nestola, Fabrizio; Coltorti, Massimo
abstract

Nominally anhydrous minerals (NAMs) may contain significant amounts of water and constitute an important reservoir for mantle hydrogen. The colloquial term ‘water’ in NAMs is related to the presence of hydroxyl-bearing (OH-) point defects in their crystal structure, where hydrogen is bonded to lattice oxygen and is charge-balanced by cation vacancies. This hydrous component may therefore have substantial effects on the thermoelastic parameters of NAMs, comparable to other major crystal-chemical substitutions (e.g., Fe, Al). Assessment of water concentrations in natural minerals from mantle xenoliths indicates that olivine commonly stores ~0 – 200 ppm of water. However, the lack of samples originating from depths exceeding ~250 km coupled with the rapid diffusion of hydrogen in olivine at magmatic temperatures makes the determination of the olivine water content in the upper mantle challenging. On the other hand, numerous experimental data show that, at pressures and temperatures corresponding to deep upper mantle conditions, the water storage capacity of olivine increases to 0.2 – 0.5 wt.%. Therefore, determining the elastic properties of olivine samples with more realistic water contents for deep upper mantle conditions may help in interpreting both seismic velocity anomalies in potentially hydrous regions of Earth's mantle as well as the observed seismic velocity and density contrasts across the 410-km discontinuity. Here, we report simultaneous single-crystal X-ray diffraction and Brillouin scattering experiments at room temperature up to 11.96(2) GPa on hydrous (0.20(3) wt.% H2O) Fo90 olivine to assess its full elastic tensor, and complement these results with a careful re-analysis of all the available single-crystal elasticity data from the literature for anhydrous Fo90 olivine. While the bulk (K) and shear (G) moduli of hydrous Fo90 olivine are virtually identical to those of the corresponding anhydrous phase, their pressure derivatives K´ and G´ are slightly larger, although consistent within mutual uncertainties. We then defined linear relations between the water concentration in Fo90 olivine, the elastic moduli and their pressure derivatives, which were then used to compute the sound velocities of Fo90 olivine with higher degrees of hydration. Even for water concentrations as high as 0.5 wt.%, the sound wave velocities of hydrous and anhydrous olivines were found to be identical within uncertainties at pressures corresponding to the base of the upper mantle. Contrary to previous claims, our data suggest that water in olivine is not seismically detectable, at least for contents consistent with deep upper mantle conditions. In addition to that, our data reveal that the hydration of olivine is unlikely to be a key factor in reconciling seismic velocity and density contrasts across the 410-km discontinuity with a pyrolitic mantle.

2022 - Age, geochemistry and mantle source of the Alto Diamantino basalts: Insights on NW Paraná Magmatic Province [Articolo su rivista]
Giovanardi, Tommaso; da Costa, Paulo Cesar Corrêa; Girardi, Vicente A. V.; Weska, Ricardo K.; Vasconcelos, Paulo M.; Thiede, David S.; Mazzucchelli, Maurizio; Cipriani, Anna
abstract

The Paranà-Etendeka is one of the largest and most thoroughly investigated large igneous provinces in the world formed in a very narrow time-range (i.e., 1.6–3.0 Ma) between 135 and 132 Ma. The South American portion of this province, i.e. the Paranà Magmatic Province, is mainly characterized by a mafic magmatism of coeval low and high-Ti magmatic terms. This bimodal activity has been explained by different degrees of melting of a homogenous mantle source or by plume activity and metasomatism of the sub-continental mantle. Despite the large body of literature, the true origin of the mantle source and processes leading to this bimodal magmatism are still debated. In this work, we present the first geochronological, geochemical and isotopic data of basalts from Alto Diamantino (southern Mato Grosso, Brazil). These basalts have tholeiitic affinity and can also be classified in two different suites based on TiO2 content (L-Ti < 3 wt%; H-Ti > 3 wt%) and Ti/Y (450 ppm), similarly to other volcanics from the Paran´a Magmatic Province. Other geochemical parameters as Zr/Y and Nb/Yb ratios range from 3.78 to 8.60 and from 3.82 to 8.31, respectively. The L-Ti and H-Ti series have significant geochemical similarities, including 87Sr/86Sr ratios, with the Ribeira and Pitanga basalts of the Paran´a Magmatic Province, respectively. The 40Ar/39Ar ages of the two series are statistically equivalent ranging between 134.4 ± 1.6 and 132.9 ± 2.0 Ma. Our data suggest that the Alto Diamantino basalts constitute the northwestern most expression of the Paranà Magmatic Province. Sr and Nd isotope ratios suggest that the original source of the Alto Diamantino basalts was a EM-I metasomatized mantle characterized by small amounts of different components (DMM and EM-II). Positive Ba and lack of Pb anomalies, together with K/Rb ratio, indicate the presence of amphibole in the mantle source of the Alto Diamantino basalts. Th/Nb and TiO2/Yb ratios suggests that the compositional difference between the L-Ti and H-Ti basalts could be explained by different degree of melting at different depths of a heterogeneously metasomatized mantle source. The H-Ti group of basalts is compatible with low degrees of partial melting at depth while the L-Ti suite records larger degrees of melting. The metasomatic event occurred after the Mesoproterozoic, possibly during the Neoproterozoic formation of the Gondwana supercontinent.

2022 - Mantle Xenoliths from Huanul Volcano (Central-West Argentina): A Poorly Depleted Mantle Source under Southern Payenia [Articolo su rivista]
Bertotto, Gustavo W.; Mazzucchelli, Maurizio; Giovanardi, Tommaso; Conceiçao, Rommulo V.; Zanetti, Alberto; Schilling, Manuel E.; Bernardi, Mauro I.; Ponce, Alexis D.; Jalowitzki, Tiago; Gervasoni, Fernanda; Cipriani, Anna
abstract

Huanul is a shield volcano with several lava flows hosting mantle xenoliths erupted during the Pleistocene (0.84 ± 0.05 Ma). It is located in the southern part of the Payenia Volcanic Province, which is among the largest Neogene-Quaternary volcanic provinces of South America. The vol-canism here has been ascribed as the northernmost expression of the back-arc volcanism of the Andean Southern Volcanic Zone. We present the first petrographic and mineral chemistry study of mantle xenoliths collected from Huanul lavas with the aim of reconstructing directly the mantle source of the Payenia Volcanic Province. Xenoliths are commonly small (<5 cm in radius) but scarcely crossed by basaltic veins. All xenoliths have a fertile lherzolitic modal composition and are equilibrated in the spinel-facies. Most of them exhibit an almost primitive-mantle geochemical affinity, characterized by slightly depleted clinopyroxene REE patterns reproducible by partial melting degrees between 0 and 4% of a PM source. Geothermobarometric P-T estimates of cli-nopyroxene-orthopyroxene couples form a linear trend between 10 and 24 kbar with constant increase of T from 814 to 1170 °C along a 50–60 mW/m2 geotherm. Evidences of interaction with the host basalts occur as spongy textures in clinopyroxene and reacted spinel, which tend to became more restitic in composition and show chromatographic or complete overprinting of the trace element compositions. The presence of plagioclase and calculated P-T values constrain this melt/rock reaction process between 6 and 14 kbar, during magma ascent, and fit the mantle ad-iabat model. Calculated melts in equilibrium with the primary clinopyroxenes do not fit the composition of the host basalt and, together with the geothermobarometric estimations, point to an asthenospheric mantle source for the magmatism in southern Payenia. The PM geochemical affinity of the xenoliths of Huanul is an extremely rare finding in the South America lithospheric mantle, which is commonly extensively refertilized by subduction-derived melts.

2021 - EoS of mantle minerals coupled with composition and thermal state of the lithosphere: Inferring the density structure of peridotitic systems [Articolo su rivista]
Faccincani, Luca; Faccini, Barbara; Casetta, Federico; Mazzucchelli, Maurizio; Nestola, Fabrizio; Coltorti, Massimo
abstract

Unravelling the physical state and properties of mantle rocks is crucial for understanding both plate tectonics, seismic activity, and volcanism. In this context, the knowledge of accurate elastic parameters of constituent mineral phases, and their variations with pressure (P) and temperature (T), is an essential requirement, that coupled with the thermal state of the lithosphere can provide a better understanding of its petrophysics and thermochemical structure. In this paper, we present an assessment of the thermoelastic parameters [in the form of P–V–T–K Equations of State (EoS)] of orthopyroxene, clinopyroxene, spinel and garnet based on X-Ray diffraction data and direct elastic measurements available in literature. The newly developed EoS are appropriate to describe the elastic behaviour of these phases under the most relevant P–T conditions and bulk compositions of the Earth’s mantle. In combination with the published EoS for mantle olivine and magnesiochromite, these EoS are suitable to calculate the physical properties of mantle peridotites and their variation with P and T. Thanks to these EoS, we can evaluate how the variations in bulk composition and thermal regimes affect the density structure of the lithospheric mantle. Accordingly, the density structure of fertile and depleted peridotitic systems was calculated along the 35, 45 and 60 mWm 2 geothermal gradients at P comprised between 1 and 8 GPa. Under very cold geothermal gradients, the density of both fertile and depleted peridotitic systems progressively increases with depth, whereas under relatively hot conditions a first downwards decrease from 1 to ca 3 GPa is observed, followed by an increase downward. In mantle sections characterized by intermediate geotherms (45 mWm 2), the behaviour of the two systems differs up to ca 4 GPa, as the density of the depleted system remains nearly constant down to this depth whereas it moderately increases in the fertile system. The results of our simplified parameterisation, in agreement with classical thermodynamic modelling, indicate that the density structure of the lithospheric mantle is predominantly controlled by the P – T gradient variations, with some compositional control mostly arising at cold-intermediate thermal conditions. Integrated by geophysical and thermodynamic modelling, the newly developed and selected EoS could provide an alternative strategy to infer the elastic properties of mineral phases and peridotite rocks, under the most relevant P–T conditions and compositions of the Earth’s mantle, without requiring sets of end-member properties and solution models.

2021 - Geology of late-Variscan Sàrrabus pluton (south-eastern Sardinia, Italy) [Articolo su rivista]
Secchi, F.; Naitza, S.; Oggiano, G.; Cuccuru, S.; Puccini, A.; Conte, A. M.; Giovanardi, T.; Mazzucchelli, M.
abstract

This paper deals with the geological mapping of the late-Variscan Sàrrabus pluton, (south-eastern Sardinia), as hallow multiple and composite igneous complex dominated by several generations of granodiorites, metaluminousandperaluminou sgranites and repeated pulses of mantle-derived mafic magmas. The map has been compiled based on geological surveys at 1:10,000 and 1:5,000 scales, assisted by in situ gamma-ray spectrometry and detailed petrographic investigations. Granite-related ore deposits have been also reported. The emplacement age of the pluton can be constrained by U/Pb dating on zircons of Cala Regina granodiorite, yielding an age of 286±9 Ma. The resulting scenario documents a bimodal magmatism controlled by an EW trending shear zone, followed by the shallower emplacement of several pulses of independent granite magmas.

2020 - Evidence of subduction-related components in sapphirine-bearing gabbroic dykes (Finero phlogopite–peridotite): Insights into the source of the Triassic–Jurassic magmatism at the Europe–Africa boundary [Articolo su rivista]
Giovanardi, Tommaso; Zanetti, Alberto; Dallai, Luigi; Morishita, Tomoaki; Hémond, Christophe; Mazzucchelli, Maurizio
abstract

A gabbroic dyke swarm containing magmatic sapphirine occurs in the Finero phlogopite–peridotite (FPP), one of the major mantle massifs in the Ivrea–Verbano Zone (IVZ; western Southern Alps). Sapphirine is part of a particular mineral assemblage, including plagioclase, titanian pargasite, titanian phlogopite, and Cl-rich apatite; the latter mineral hosts calcite inclusions. The dykes cut the mantle foliation at a high angle, are bounded by orthopyroxenite layers, and show symmetric internal banding, represented by two outer hornblendite selvages and an inner leucogabbro band. The sapphirine occurs in up to 3 cm-thick irregular patches in both hornblendite salvages, along with Al-rich amphibole and green spinel. We present major and trace elements of minerals and bulk rock, as well as mineral O, Sr, and Nd isotopic compositions of dykes and the host peridotite from two different outcrops in the FPP area. Our data show that early melt migration developed through porous flow within cm-thick channels and was characterised by orthopyroxene dissolution. Following progressive percolation and reaction, the melt became silica saturated with segregation of orthopyroxenite in the centres of the channels. The banded internal structure of the dykes was caused by three different evolutionary stages, involving opening and enlargement of the conduits. The sapphirine and green spinel segregation took place at T > 1,000 °C, in the presence of melt with transient composition, which interstitially migrated and reacted with the cumulus minerals to form the hornblendite layers. The mineral chemistry of the newly-formed amphiboles indicates that the sapphirine parental melt was Al-rich, depleted to strongly depleted in Hf, Zr, Nb, Ta, Ti, Sc, V, and middle and heavy rare earth elements, and characterised by a positive Eu anomaly and (Zr/Hf)N < 1. These data suggest a parental melt with a significant amount of normative plagioclase. However, the studied veins do not show evidence of plagioclase assimilation, and we argue that this process could have occurred in magmatic bodies that are not outcropping today to the surface or in the melt source. The δ18O values of vein amphiboles and plagioclases vary from 6.9 to 8.6‰ SMOW, which is well above the mantle range, even when considering fractionation upon cooling. Given that orthopyroxene from the wall has “normal” mantle δ18O values (5.8‰), reaction with the host metasomatised peridotite cannot be responsible for the heavy δ18O signature, and the latter must have been imparted by crustal components deeper in the mantle. Our petrographic and geochemical evidence demonstrates that the northern IVZ records an extremely prolonged release, from the Variscan orogenic cycle to the Mesozoic exhumation, of K-H2O-rich mantle-derived melts, mixed with subduction-related components. This finding provides valuable insights into the Triassic–Jurassic magmatism and the geodynamic environment at the Europe–Africa boundary.

2020 - In situ Sr isotope analysis of mantle carbonates: Constraints on the evolution and sources of metasomatic carbon-bearing fluids in a paleo-collisional setting [Articolo su rivista]
Consuma, Giulia; Braga, Roberto; Giovanardi, Tommaso; Bersani, Danilo; Konzett, Jürgen; Lugli, Federico; Mazzucchelli, Maurizio; Tropper, Peter
abstract

Carbonate-bearing wedge peridotites attest the mobilization of carbon (C) by slab fluids/melts circulating in a subduction setting. In general, COH fluids are thought to derive from the dehydration/partial melting of the crustal portions of slabs, especially during the exhumation of crust-mantle mélanges along continental subduction channels. In this study we combined textural observations with in-situ Sr isotope analyses of mantle carbonates occurring in different microstructural sites to test whether the fluids responsible for the carbonation of a mantle wedge are derived from the subducted continental crust or not.We focus on the Ulten Zone peridotites (Eastern Italian Alps) associated with high-grade felsic rocks, where carbonates occur mainly as dolomite and minor magnesite and calcite. In situ laser MC-ICP-MS analysis of peridotites representing different episodes of a complex metasomatic history, indicates that Sr isotopic variations can be linked to the differentmicrostructural positions of carbonates. The C-metasomatism of the UZ peridotites is proposed to have occurred in two stages. The first stage is the HP‑carbonation at peak (eclogite-facies) conditions,with formation of interstitial matrix dolomite in textural equilibriumwith hornblende to pargasite amphibole and Cl-apatite. This dolomite exhibits relatively unradiogenic 87Sr/86Sr present day values of 0.70487±0.00010, requiring different sourceswith respect to the associated migmatites and the overhanging mantle wedge. Carbonation continued during exhumation, with local injection of C-rich fluids forming a dolomite vein in association with tremolite and chlorite. The dolomite vein shows a wide range of 87Sr/86Sr (0.7036–0.7083), reflecting both the primary composition of carbonates and the consequent interaction with crustal fluids as expected in a crust-mantle mélange. The second stage is C-remobilization by dolomite dissolution and precipitation of brucite intergrowths with calcite during the final exhumation. This remobilization event has resulted in a similar Sr composition to the precursor dolomite. The mantle wedge is therefore capable of storing carbonates which have been shown to represent a complex metasomatic evolution fromeclogite-facies conditions to very shallowstructural levels. Therefore, fluids released fromsubducting slabs of continental lithospheremight be responsible for the crystallization of metasomatic minerals such as amphibole, phlogopite and zircon in the overlying ultramafic rocks. Conversely, the role of these metasomatic fluids on the carbonation of mantlewedge is likely overestimated. The combination of geochemical, isotopic and textural evidence suggests that dolomite inclusions and interstitial dolomite are derived in large part from a distinct source of C-bearing fluids that could be related to depleted mantle wedge sources and/or trondhjemitic igneous activity. In contrast, at the end of exhumation, residual COH-fluids released by the associated stromatic gneisses and orthogneisses resulted in late-stage dolomite veins having the highest Sr isotope values in the Ulten Zone peridotites.

2020 - Mantle heterogeneities produced by open-system melting and melt/rock reactions in Patagonian extra-Andean backarc mantle (Paso de Indios, Argentina) [Articolo su rivista]
Bertotto, Gustavo W.; Mazzucchelli, Maurizio; Zanetti, Alberto; Ponce, Alexis D.; Giovanardi, Tommaso; Brunelli, Daniele; Bernardi, Mauro I.; Hémond, Christophe; Cipriani, Anna
abstract

The Eocene basaltic extrusions in the Paso de Indios region (Chubut-Argentina) are one manifestation of the extensional tectonism of the active margin of South America during the Cenozoic. Ultramafic xenoliths embedded in these volcanics are mainly harzburgites and lherzolites with subordinate pyroxenites, estimated equilibrium temperatures ranging from 853 ± 15 to 1057 ± 32°C and pressures in the spinel stability field. Geochemical and modal evidences point to a multistage magmatic history with record of a last reactional open-system episode associated to the influx of adakitic-like melts in a orthopyroxene-rich, clinopyroxene-poor mantle column. The great variability of clinopyroxene modal and geochemical composition in a ∼20 km2 area suggests extreme variability of the physical parameters connected to melt infiltration and melt/rock reactions processes at a very small scale superimposed on a mantle with an inherited meter scale heterogeneity. Variations in the melt influx rate and residual porosity of the mantle column produced different melt/rock reactions which could be summarized in two entangled main reaction pathways: 1) opx+cpx+melt1-->ol+melt2 and 2) opx+melt1-->cpx+ol+melt2. These reactions deeply modified the trace elements content of clinopyroxenes producing variable enrichments in LREEs and LILEs related to both chromatographic and pure incremental open system processes. Petrological evidence suggests that the last reactional process occurred in the spinel stability field overprinting a strongly depleted mantle that, in a previous stage, had experienced extreme depletion in the garnet stability field, possibly under hydrous conditions. The adakitic-like nature of the influxing melt associates this episode to the subduction system along the western margin of South America, active at least since Triassic times.

2020 - Mantle-Derived Corundum-Bearing Felsic Dykes May Survive Only within the Lower (Refractory/Inert) Crust: Evidence from Zircon Geochemistry and Geochronology (Ivrea–Verbano Zone, Southern Alps, Italy) [Articolo su rivista]
Bonazzi, Mattia; Langone, Antonio; Tumiati, Simone; Dellarole, Edoardo; Mazzucchelli, Maurizio; Giovanardi, Tommaso; Zanetti, Alberto
abstract

Corundum-rich (up to 55 vol.%) felsic dykes formed with albite, +/- K-feldspar, +/- hercynite and +/- biotite-siderophyllite cut the lower crustal rocks exposed in the Ivrea–Verbano Zone (NW Italy). Zircon is an abundant accessory mineral and its investigation through laser ablation-inductively coupled plasma (multi-collector)-mass spectrometer (LA-ICP-(MC)MS) has allowed results to directly constrain the timing of emplacement, as well as petrology and geochemistry of parental melts. Zircons are characterized by very large concentration in rare earth elements (REE), Th, U, Nb and Ta, and negative Eu anomaly. U–Pb analysis points to Norian emplacement ages (223 +/- 7 Ma and 224 +/- 6 Ma), whereas large positive EHf(t) values (+13 on average) indicate a derivation from depleted to mildly enriched mantle source. The mantle signature and the corundum oversaturation were preserved thanks to limited crustal contamination of the host, high-temperature refractory granulites and mafic intrusives. According to the geochemical data and to the evidence of the development of violent explosions into the conduits, it is proposed that dykes segregated from peraluminous melts produced by exsolution processes affecting volatile-rich differentiates during alkaline magmatism. This work provides robust evidence about the transition of the geochemical affinity of Southern Alps magmatism from orogenic-like to anorogenic during Norian time, linked to a regional uprising of the asthenosphere and change of tectonic regime.

2019 - Crust-mantle coupling during continental convergence and break-up: Constraints from peridotite xenoliths from the Borborema Province, northeast Brazil [Articolo su rivista]
Liu, Shiran; Tommasi, Andréa; Vauchez, Alain; Mazzucchelli, Maurizio
abstract

We studied a suite of mantle xenoliths carried by Cenozoic volcanism in the Borborema Province, NE Brazil. These xenoliths sample a subcontinental lithospheric mantle affected by multiple continental convergence and rifting events since the Archean. Equilibrium temperatures indicate a rather hot geotherm, implying a ca. 80 km thick lithosphere. Most xenoliths have coarse-granular and coarse-porphyroclastic microstructures, recording variable degrees of annealing following deformation. The high annealing degree and equilibrated pyroxene shapes in coarse-granular peridotites equilibrated at ~900 °C indicate that the last deformation event that affected these peridotites is several hundreds of Ma old. Coarse-porphyroclastic peridotites equilibrated at 950–1100 °C probably record younger (Cretaceous?) deformation in the deep lithospheric mantle. In addition, a few xenoliths show fine-porphyroclastic microstructures and equilibrium temperatures ≥1200 °C, which imply recent deformation, probably related to the dykes that fed the Cenozoic volcanism. Chemical and microstructural evidence for reactive percolation of melts is widespread. Variation in textural and chemical equilibrium among samples implies multiple melt percolation events well spaced in time (from Neoproterozoic or older to Cenozoic). Crystal preferred orientations of olivine and pyroxenes point to deformation controlled by dislocation creep with dominant activation of the [100](010) and [001]{0kl} slip systems in olivine and pyroxenes, respectively, for all microstructures. Comparison of xenoliths' seismic properties to SKS splitting data in the nearby RCBR station together with the equilibrated microstructures in the low-temperature xenoliths point to coupled crust-mantle deformation in the Neoproterozoic (Brasiliano) continental-scale shear zones, which is still preserved in the shallow lithospheric mantle. This implies limited reworking of the lithospheric mantle in response to extension during the opening of the Equatorial Atlantic in the Cretaceous, which in the present sampling is restricted to the base of the lithosphere.

2019 - Deformation, Annealing, Melt-rock Interaction, and Seismic Properties of an Old Domain of the Equatorial Atlantic Lithospheric Mantle [Articolo su rivista]
Liu, Shiran; Tommasi, Andréa; Vauchez, Alain; Mazzucchelli, Maurizio
abstract

We analyzed a set of mantle xenoliths from Fernando de Noronha (FN) archipelago to constrain the role of deformation, cooling, annealing, and melt percolation in the evolution of the mantle lithosphere of the equatorial Atlantic. The peridotites are dominantly lherzolites with coarse granular or porphyroclastic microstructures. Equilibrium temperatures range between 850 and 1000 °C. Olivine crystal preferred orientations (CPO) have mainly orthorhombic patterns characterized by clear [100] and [010] point maxima or a fiber-[100] tendency; these patterns imply dominant activation of [100](010) slip system during dislocation creep. Olivine fiber-[010] patterns are less common; they probably result from recrystallization or melt-rock interaction. Annealing after deformation partially recovered the microstructures and reduced the olivine CPO strength. Pyroxene CPOs often have weak consistency with olivine CPO, implying post-deformation refertilization. Chemical compositions are systematically more fertile than those from abyssal peridotites from the Mid-Atlantic Ridge. These compositions together with the microstructures suggest two stages of melt-rock interaction. An early refertilization of the base lithosphere due to continuous percolation of small melt fractions and later, more important, but local chemical changes (both refertilization and dunitization) as well as recrystallization, due to reactive melt percolation likely associated with the Cenozoic volcanism. By comparing the calculated seismic properties with existing seismological data we argue that the moderate seismic anisotropy displayed by the FN mantle xenoliths likely records past horizontal asthenospheric flow parallel to the spreading direction frozen in the lithospheric mantle as the plate cooled.

2019 - Mafic dyke swarms at 1882, 535 and 200 Ma in the Carajás region, Amazonian Craton: Sr Nd isotopy, trace element geochemistry and inferences on their origin and geological settings [Articolo su rivista]
Giovanardi, Tommaso; Girardi, V. A. V.; Teixeira, W.; Mazzucchelli, M.
abstract

The Carajás-Rio Maria region, together with the Rio Maria domain of the Central Amazonian province, comprises the eastern margin of the Amazonian Craton with the Neoproterozoic Araguaia belt. This region hosts several basaltic dyke swarms whose U-Pb baddeleyite ages highlighted three intrusive events at 1882, 535 and 200 Ma. New geochemical and Sr-Nd isotopic data were obtained for the different groups of the Carajás dykes allowing new insights on i) the mantle source composition beneath the Carajás region through time and ii) the geodynamic setting of the intrusive events. The 1882 Ma swarm is coeval to the Uatumã SLIP event which is one of the oldest intraplate events of the proto-Amazonian craton. Trace elements and isotopic values suggest that the dyke parent melt for those dykes have a crustal component derived from a sedimentary source similar to GLOSS (GLObal Subducting Sediment compositions). This is consistent with the emplacement of the dykes in a supra-subduction setting or in a post-collisional setting. Trace and isotopic values of the 535 Ma dyke swarm are consistent with an enriched mantle source from EMII component. These geochemical features suggest an enrichment of the mantle from an oceanic lithosphere poor in sediments, different to that of the 1882 Ma source. The age of this swarm matches magmatic activity during a post-collisional extensive-transtensive event recorded in the marginal Araguaia belt after the amalgamation of the Amazonian Craton to the Western Gondwana during Neoproterozoic. The 200 Ma dyke swarm which is related to the CAMP (Central Atlantic Magmatic Province) and opening of the Atlantic Ocean shows trace element composition similar to Atlantic E-MORB. The coupled isotopic values are consistent with an enriched mantle source with EMII component. These particular geochemical features suggest that the plume activity responsible for the CAMP near the rifting zone has not affected the mantle beneath the Carajás region.

2018 - Filling the Gap in the Classification of Phlogopite-Bearing Ultramafic Rocks [Articolo su rivista]
Giovanardi, Tommaso; Freddo, Ivan; Mazzucchelli, Maurizio
abstract

In recent years, the many new occurrences reported in the literature of ultramafic rocks that have phlogopite as a major constituent and do not fall into the categories of kimberlites, lamproites, and lamprophyres have highlighted the need for a classification that includes this abundant mineral phase. Currently, a broadly accepted classification with phlogopite does not exist, and the only term used by scientists is “phlogopite-bearing” when this phase is above 5 vol% and up to 90 vol%. For this reason, we propose a new classification that integrates phlogopite into the current classification of ultramafic rocks without modifying the already accepted terminology or the classificative criteria (i.e., the mineral modal abundances). Phlogopite is added as an end member in the ultramafic-rocks classification diagrams, changing their shapes from triangular to tetrahedral. An Excel spreadsheet containing the new diagrams and a macro that automatically classifies the rocks is provided.

2018 - Isotopic constraints on contamination processes in the Tonian Goiás Stratiform Complex [Articolo su rivista]
Giovanardi, Tommaso; Mazzucchelli, Maurizio; Lugli, Federico; Girardi, Vicente A. V.; Correia, Ciro T.; Tassinari, Colombo C. G.; Cipriani, Anna
abstract

The Tonian Goiás Stratiform Complex (TGSC, Goiás, central Brazil), is one of the largest mafic-ultramafic layered complexes in the world, emplaced during the geotectonic events that led to the Gondwana accretion. In this study, we present trace elements and in-situ U/Pb-Lu-Hf analyses of zircons and 87Sr/86Sr ratios of plagioclases from anorthosites and gabbros of the TGSC. Although formed by three isolated bodies (Cana Brava,Niquelândia and Barro Alto), and characterized by a Lower and Upper Sequence (LS and US), our new U/Pb zircon data confirm recent geochemical, geochronological, and structural evidences that the TGSC has originated from a single intrusive body in the Neoproterozoic. New Hf and Sr isotope ratios construe a complex contamination history for the TGSC,with different geochemical signatures in the two sequences. The low Hf and high Sr isotope ratios of the Lower Sequence (εHf(t) from−4.2 down to −27.5; 87Sr/86Sr=0.706605–0.729226), suggest the presence of a crustal component and are consistent with contamination from meta-pelitic and calc-silicate rocks found as xenoliths within the Seuence. The more radiogenic Hf isotope ratios and low Sr isotope composition of the Upper Sequence (εHf(t) from 11.3 down to −8.4; 87Sr/86Sr=0.702368–0.702452), suggest a contamination from mantle-derived metabasalts in agreement with the occurrences of amphibolite xenoliths in the US stratigraphy. The differential contamination of the two sequences is explained by the intrusion of the TGSC in a stratified crust dominated bymetasedimentary rocks in its deeper part andmetavolcanics at shallower levels. Moreover, the differential thermal gradient in the two crystallizing sequences might have contributed to the preservation and recrystallization of inherited zircon grains in the US and total dissolution or magmatic overgrowth of the LS zircons via melt/rock reaction processes.

2018 - Zircon U–Pb dating of a lower crustal shear zone: a case study from the northern sector of the Ivrea-Verbano Zone (Val Cannobina, Italy). [Articolo su rivista]
Langone, A.; Zanetti, A.; Daczko, N. R.; Piazolo, S.; Tiepolo, M.; Mazzucchelli, M.
abstract

A geochronological study was performed on zircon grains from a middle-lower crustal shear zone exposed in the northern sector of the Ivrea-Verbano Zone (Southern Alps, Italy) for the first time. The shear zone developed at the boundary between mafic rocks of the External Gabbro Unit and ultramafic rocks of the Amphibole-Peridotite Unit. It is ~10–20 m wide and can be followed along a NE strike for several km and consists of an anastomosing network of mylonites and ultramylonites. Zircon grains were studied in thin section and as separates from three representative outcrops along the shear zone. Zircon grains are more abundant in the shear zone compared to wall rocks, and are generally equant, rounded to sub-rounded with dimensions up to 500m. U-Pb data are mainly discordant and the apparent 206Pb/238U dates show a large variation from Permian to Jurassic. Isotopic data, combined with microstructural, morphological and internal features of zircon, reveal an inherited age component and suggest partial zircon recrystallization under high-temperature conditions during late Triassic – early Jurassic. High-temperature deformation in the shear zone, at lower crustal levels, was coeval with amphibolite to greenschist facies mylonitic deformation at upper crustal levels, and is inferred to be related to Mesozoic rifting processes at the Adriatic margin.

2017 - Ductile–brittle deformation effects on crystal-chemistry and U–Pb ages of magmatic and metasomatic zircons from a dyke of the Finero Mafic Complex (Ivrea–Verbano Zone, Italian Alps) [Articolo su rivista]
Langone, Antonio; Padrón Navarta José, Alberto; Ji, Wei Qiang; Zanetti, Alberto; Mazzucchelli, Maurizio; Tiepolo, Massimo; Giovanardi, Tommaso; Bonazzi, Mattia
abstract

A detailed microstructural, geochemical and geochronological study was performed on zircon grains from plagioclase-rich dioritic dykes discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex (Italian Alps). This unit is exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea–Verbano Zone, Italy). The leucocratic dykes consist mainly of plagioclase (An18–24Ab79–82Or0.3–0.7) with subordinate amounts of biotite and spinel defining melanocratic layers. Zircon and corundum are common accessory phases. Both the dykes and the surrounding meta-diorites/gabbros show evidence of ductile deformation under amphibolite-facies conditions. Zircon grains/fragments (up to 2 mm in length) occur as porphyroclasts surrounded by fine-grained plagioclase within the leucocratic domains and may occur within the melanocratic layers completely or partially surrounded by biotite and spinels. Fractures are common within zircon, define networks and have associated displacements occasionally and/or they can be filled by secondary minerals. Cathodoluminescence (CL) images reveal that zircon grains from the leucocratic layers show relicts of primary magmatic (i.e. oscillatory and or sector) zoning generally related with the crystal shapes or crystallographic orientation, whereas those from the melanocratic domains do not. In both cases, zircon shows secondary CL features, i.e. mosaic-like textures, due to deformation. EBSD maps confirmed a profuse mosaic texture, resulting in an internal misorientation of ca. 10°, generally associated with fractures. Locally, zircon shows clear evidence of crystal-plastic deformation at the edges, with a gradual misorientations of up to 12°, suggesting an origin prior fragmentation. Trace elements and U–Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections. Such in situ investigations revealed a strong correlation between internal zircon structures, chemistry, U–Pb isotope ratios and mylonitic fabric. U–Pb data return highly discordant and variable ages. The 206Pb/238U ages may range from ca. 297 to 198 Ma within the same zircon grain from the leucocratic layers,whereas 206Pb/238U younger than 250Mawere systematically obtained from zircon within the melanocratic layers. The 206Pb/238U data younger than ca. 240 Ma from zircon grains within the leucocratic layers were obtained from narrow axial stripes observed in CL images and oriented parallel or at low-anglewith respect to the foliation planes. These stripes are characterized by an overall HREE, Y, U and Th enrichment possibly reflecting deformation of the grain in presence of interstitial fluid phases. Combining U–Pb data, microstructure and zircon CL features, we suggest a multistage evolution of the dykes whereby the melanocratic layers are the result of a Late Permian metasomatic event promoting modification of the pre-existing mineral assemblage of the (Late Carboniferous–Early Permian?) dykes and deformation and partial resetting of zircon porphyroclasts. This has important implications in the geology of the Ivrea–Verbano Zone because, having the dyke intruded the External Gabbro unit before Triassic (probably during Carboniferous–Permian), the latter cannot be considered a Triassic intrusion. At least part of the External Gabbro unit is Carboniferous–Permian as the other mafic bodies of the Ivrea–Verbano Zone.

2017 - In situ high spatial resolution 87Sr/86Sr ratio determination of two Middle Pleistocene (c.a. 580 ka) Stephanorhinus hundsheimensis teeth by LA–MC–ICP–MS [Articolo su rivista]
Lugli, Federico; Cipriani, Anna; Peretto, Carlo; Mazzucchelli, Maurizio; Brunelli, Daniele
abstract

Bone and tooth tissues are important biological archives to study eating habits and provenance of ancient humans and animals. By taking advantage of the high spatial resolution offered by the Laser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry (LA–MC–ICPMS) technique, we investigated the 87Sr/86Sr intra-tooth variability of two Stephanorhinus hundsheimensis tooth fragments from the Middle Pleistocene site of Isernia La Pineta. We detected significant Sr isotopes variability within the rhinos tooth enamel (enamel average ±2 sigma: sample RH–IS30 0.70951 ± 0.00014; sample RH–IS260.70976 ± 0.00015) with values higher than the “local”87Sr/86Sr ratio (dentine average ±2 sigma: sample RH–IS30 0.70918 ± 0.00013; sample RH–IS26 0.70934 ± 0.00009). This is likely linked to a different water and food intake with a Sr isotopic signature similar to volcanic soils nearby (Roccamonfina: from 0.7093 to 0.7100; Colli Albani: >0.7100) and supports the idea that the Stephanorhinus hundsheimensis species moved around seasonally. The improvement of non-destructive, accurate and precise analytical methods to decrypt the information hidden within bone and tooth hard tissues of archeological material is crucial to unravel critical questions about evolution, migration and ecology of human and animals. We have successfully took upon this challenge using three matrix-matched reference materials, with variable Sr concentration (c.a. 100–1000 ppm), to correct unresolved interferences arising from LA analyses.

2017 - Melt-Rock Interaction at Mantle Conditions: Evidences from Finero Gabbroic Dykes [Abstract in Atti di Convegno]
Giovanardi, Tommaso; Cipriani, Anna; Lugli, Federico; Morishita, Tomoaki; Zanetti, Alberto; Mazzucchelli, Maurizio
abstract

The Finero Phlogopite-Peridotite (FPP) is a worldwide famous mantle massif recrystallized through several events of melt migrations. These events have enriched the FPP in hydrous phases and crustal components and have been commonly interpreted as related to a subduction/post orogenic geodynamic setting. The last of these metasomatic events has produced composite sapphirine-bearing gabbroic dykes, interpreted as the result of the interaction of channelized migrating melts with the host rock in a two-steps intrusion process. In the first step, the melt reacted with the FPP rocks and evolved by fractional crystallization of amphibole cumulates. In the second step, the evolved melt reacted with the first cumulates producing magmatic sapphirine and segregating plagioclase-rich bands containing abundant apatites at the nucleus of the dike. New data suggest, however, a more complex evolution. New O and in situ Sr isotopes on minerals suggest that the gabbroic dykes have evolved from melt(s) that progressively were contaminated by the interaction with the FPP rocks during its fractionation. The δ18O increases from 5.81‰ in orthopyroxenes at the dykes border to ~6.90‰ in cumulitic amphiboles and 8.60‰ in plagioclases. The 87Sr/86Sr values for plagioclase and coexisting apatite show isotopic disequilibrium between the two phases (plagioclases at 0.70474 ± 0.00033, n=23, and apatites at 0.70369 ± 0.00025, n=6). These isotopic variations could be explained with an AFC-like process between mantle-derived melt(s) and a crustal-enriched host (the FPP). In situ Sr isotope analyses were performed at the CIGS laboratory of the Università di Modena e Reggio Emilia using a Thermo Fisher Scientific Neptune™ coupled to a 213 nm Nd:YAG laser ablation system (New Wave Research™). During the analytical sessions a new in house plagioclase reference material for Rb-Sr systematic, named BC84, has been successfully tested and used.

2017 - New U-Pb SHRIMP-II zircon intrusion ages of the Cana Brava and Barro Alto layered complexes, central Brazil: constraints on the genesis and evolution of the Tonian Goias Stratiform Complex [Articolo su rivista]
Giovanardi, Tommaso; Girardi, Vicente A. V.; Correia, Ciro T.; Tassinari, Colombo C. G.; Sato, Kei; Cipriani, Anna; Mazzucchelli, Maurizio
abstract

The Cana Brava, Niquelândia and Barro Alto complexes (Goiás, central Brazil) are three of the largest mafic-ultramafic layered complexes in the world and their origin has been a matter of debate for several decades. One hypothesis suggests that Niquelândia and Barro Alto were both formed by two distinct igneous events at 1.3 Ga and at 790Ma and were later overlapped during tectonic exhumation at 650 Ma; according to this reconstruction Cana Brava belongs to the youngest intrusion at 790 Ma. A second hypothesis suggests that the three complexes formed during the same event. Here we provide new U-Pb SHRIMP-II zircon ages for the Cana Brava and Barro Alto complexes, constraining their intrusion age to the Neoproterozoic (between 770 and 800 Ma), coeval with Niquelândia. A review of new and literature ages indicate that these complexes formed during a single igneous event andwere notmodified by regional metamorphism.We propose that the complexes represent fragments of the larger Tonian Goiás Stratiform Complex, which was likely part of a back-arc environment connected to the formation of the GoiásMagmatic Arc at about 790Ma, later disrupted and accreted to the São Francisco craton.

2017 - The growth and contamination mechanism of the Cana Brava layered mafic-ultramafic complex: new field and geochemical evidences [Articolo su rivista]
Giovanardi, Tommaso; Girardi, Vicente A. V.; Correia, Ciro T.; Sinigoi, Silvano; Tassinari, Colombo C. G.; Mazzucchelli, Maurizio
abstract

The Cana Brava complex is the northernmost of three layered complexes outcropping in the Goiás state (central Brasil). New field and geochemical evidences suggest that Cana Brava underwent hyper- to subsolidus deformation during its growth, acquiring a high-temperature foliation that is generally interpreted as the result of a granulite- facies metamorphic event. The increase along the stratigraphy of the incompatible elements abundances (LREE, Rb, Ba) and of the Sr isotopic composition, coupled with a decrease in εNd(790), indicate that the complex was contaminated by the embedded xenoliths from the Palmeirópolis Sequence. The geochemical data suggest that the contamination occurred along the entire magma column during the crystallization of the Upper Mafic Zone, with in situ variations determined by the abundance and composition of the xenoliths. These features of the Cana Brava complex point to an extremely similarity with the Lower Sequence of the most known Niquelândia intrusion (the central of the three complexes). This, together with the evidences that the two complexes have the same age (c.a. 790 Ma) and their thickness and units decrease northwards suggests that Cana Brava and Niquelândia are part of a single giant Brasilia body grown through several melt impulses.

2016 - Brittle-ductile deformation effects on zircon crystal-chemistry and U-Pb ages: an example from the Finero Mafic Complex (Ivrea-Verbano Zone, western Alps) [Abstract in Rivista]
Antonio, Langone; Alberto, Padrón Navarta José; Alberto, Zanetti; Mazzucchelli, Maurizio; Massimo, Tiepolo; Giovanardi, Tommaso; Mattia, Bonazzi
abstract

A detailed structural, geochemical and geochronological survey was performed on zircon grains from a leucocratic dioritic dyke discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex. This latter is nowadays exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea-Verbano Zone, Italy). The leucocratic dyke consists mainly of plagioclase (An18-24Ab79-82Or0.3-0.7) with subordinate amounts of biotite, spinel, zircon and corundum. Both the leucocratic dyke and the surrounding meta-diorites show evidence of ductile deformation occurred under amphibolite-facies conditions. Zircon grains (up to 2 mm in length) occur mainly as euhedral grains surrounded by fine grained plagioclasedominated matrix and pressure shadows, typically filled by oxides. Fractures and cracks within zircon are common and can be associated with grain displacement or they can be filled by secondary minerals (oxides and chlorite). Cathodoluminescence (CL) images show that zircon grains have internal features typical of magmatic growth, but with local disturbances. However EBSD maps on two selected zircon grains revealed a profuse mosaic texture resulting in an internal misorientation of ca. 10o. The majority of the domains of the mosaic texture are related to parting and fractures, but some domains show no clear relation with brittle features. Rotation angles related to the mosaic texture are not crystallographically controlled. In addition, one of the analysed zircons shows clear evidence of plastic deformation at one of its corners due to indentation. Plastic deformation results in gradual misorientations of up to 12o, which are crystallographically controlled. Trace elements and U-Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections and designed to cover the entire exposed surface of selected grains. Such investigations revealed a strong correlation between internal zircon structures, chemistry, U-Pb isotope ratios and mylonitic fabric. U-Pb data return highly discordant and variable ages: in particular, the 206Pb/238U ages range from Carboniferous to Triassic within the same zircon grain. The youngest 206Pb/238U data derive from narrow axial stripes oriented parallel or at low angle with respect to the foliation planes. These stripes are characterized by an overall HREE, Y, U and Th enrichment possibly reflecting deformation of the grain in presence of interstitial fluid phases, likely related to a concomitant magmatic activity. Deformation related structures (cracks and fractures) within zircon grains acted as fast-diffusion pathways allowing fluids to modify the geochemistry and isotopic systems of zircon. Our results suggest that fluid-assisted brittle-ductile deformation can severely modify the trace elements and isotopic composition of zircon with unexpected patterns constrained by stress regime. In similar cases, our observations suggest that, for a more appropriate interpretation of the petrologic evolution and age variability, a direct characterization of the internal structures of zircons still placed in their microtextural site is highly recommended.

2016 - Comparing the Cana Brava and Niquelândia complexes: large mafic-ultramafic intrusions in the lower crust and contamination processes [Abstract in Atti di Convegno]
Giovanardi, Tommaso; Vicente, V. A. V.; Correia, C. T.; Sinigoi, S.; Tassinari, C. C. G.; Mazzucchelli, Maurizio; Sforna, MARIE CATHERINE
abstract

Mafic-ultramafic complexes offer a unique opportunity to study how intrusions of mantlederived melts growth into the deep crust and interact with the country rocks. The Cana Brava and Niquelândia complexes are two mafic-ultramafic bodies which outcrop within the Brasilia Belt (Goias, central Brazil) and that intruded the metavolcanicmetasedimentary sequences of Palmeiropolis and Indaianopolis during a Neoproterozoic continental rifting. The two complexes are parts, together with the Barro Alto complex, of a ~350 km NNE-trend belt of layered bodies which were exhumed during the Gondwana formation. New field, geochemical and isotopic data give new constraints on the model of growth of these complexes and the interactions between parent melts and the lower crust. Field evidences suggest that the complexes grow via multiple-melt intrusions under hyper- to subsolidus shear conditions. During the complex growth, the upper metavolcanic-metasedimentary sequence was delaminated and xenoliths were incorporated and deformed within the crystal mush. The increase of the 87Sr/86Sr(790) along the complex stratigraphy, coupled with a decrease of the εNd(790), provides evidences of strong crustal contamination by the embedded xenoliths. The enrichment in most incompatible elements (e.g. K, Ba and LREE) and hydrous phases (biotite and amphibole) in rocks containing more xenoliths supports also the crustal contamination. The almost linear trend of isotopic contamination suggests that this process involved all the magma colum, similarly to AFC. However, the increase abundance of incompatible elements and H2O contents toward xenoliths-rich bands provide for a local effect of contamination.

2016 - DATING THE ONSET OF A LOWER CRUSTAL SHEAR ZONE: A (LUCKY) CASE FROM THE NORTHERN SECTOR OF THE IVREA-VERBANO ZONE (VAL CANNOBINA, ITALY) [Abstract in Rivista]
Langone, A.; Zanetti, A.; Tiepolo, M.; Mazzucchelli, Maurizio
abstract

A detailed textural and U-Pb geochronological investigation has been performed on zircons from a ductile shear zone established in lower crustal mafie intrusives. The investigated shear zone is hosted in the Finero mafic-ultramafic complex of the northeastern sector of the Ivrea-Verbano Zone, Southern Alps. Il developed al the base of the lower lo middle continental crust section, where mafie lo ultramafic rocks intruded into a sequence of metapelites and metabasites (i.e., Kinzigite Formation) constituting the polymetamorphic basement of the Adria plate. The shear zone forms a braided belt that can be followed for several km, from the Cannobino River lo the MI. Gridone, and developed entirely within gabbroic/dioritic rocks, partly intruded during Triassic lime (- 232 Ma; Zanetti et al., 2013). The shear zone is characterized by a well-developed foliation, a lineation, and a compositional banding, where amphibole, clinopyroxene and game! occur as large rounded lo elliptical porphyroclasts, resulting embedded in a fine grained matrix mainly consisting of plagioclase, amphibole, pyroxenes and ilmenite. The shear zone was active during uplift and cooling of the wall rock, and recorded retrograde metamorphic conditions ranging from 650 lo 500"C al 0.6-0.4 GPa (Kenkmann, 2000). The mylonitic deformation is generally placed no earlier !han 230 Ma, lasting until about 170 Ma al the latest (e.g., Handy & Zingg, 1991). Numerous zircon grains were obtained from the minerai separation of one (- 2 kg weight) sample collected in the Cannobino River (northem Ivrea-Verbano Zone, Southern Alps). Zircon grains were examined also directly on petrographic lhin sections of samples collected throughout the (ultra)mylonitic belt. According to petrographic and microstructural analyses zircon occurs as rounded grains within the mylonitic matrix, easily recognisable due lo their dimensions up lo 110 micron. Cathodoluminescence (CL) study al the SEM revealed a well developed systematic zoning of zircon grains with dark cores surrounded by brighter domains, locally asymmetric, and with a thickness up to 30 micron. The dark inner core can show zoning features suggesting magmatic growlh. Preliminary LA-ICP-MS U-Pb results from zircon separates yielded mainly late Triassic concordant ages of about 235 Ma for the dark CL cores and about 204 Ma for the brighter CL (mylonitic) overgrowths. The sizes, distributions, shapes and CL features of zircon grains provide strong evidence of a metamorphic response of zircon during ductile deformation. The observed features can be interpreted as evidence of dissolution/recrystallization in response lo fluid influx during high-temperature shearing. Further microstructural investigations, minerai chemistry and trace element characterization of different zircon domains are in progress and will provide more evidence for the petrochronological evolution of such a lower crustal shear zone.

2016 - Geochemistry of sapphirine-apatite-calcite-bearing gabbroic dykes from the Finero Phlogopite Peridotite (Ivrea-Verbano Zone): evidence for multistage interaction with the ambient peridotite [Abstract in Rivista]
Giovanardi, Tommaso; Alberto, Zanetti; Mazzucchelli, Maurizio; Tomoaki, Morishita; Antonio, Langone
abstract

The Finero Phlogopite-Peridotite (FPP) is a mantle unit outcropping in the northernmost tip of the Ivrea-Verbano Zone (IVZ, Southern Alps). It shows a virtually complete recrystallization due to pervasive to channelled melt migration. The pervasive metasomatism formed a main lithologic association constituted by phlogopite harzburgites associated to phlogopite pyroxenites (mainly olivine-websterites, websterites and orthopyroxenites). These lithologies are also rich in amphibole and do not show significant chemical gradients among them (Zanetti et al., 1999). The channelled migration stages formed dunite bodies, which sometimes contain stratiform chromitites and, more rarely, pyroxenite layers similar to those associated to phlogopite harzburgite. The FPP also shows a discrete number of other, subordinate rock-types, which are characterised by the presence of apatite usually associated to carbonates (i.e. calcite or dolomite) and exhibit marked modal and chemical gradients with respect to the host phlogopite harzburgite. Examples of these lithologies are apatite-dolomitebearing wehrlites and harzburgites (e.g. Zanetti et al. 1999; Morishita et al., 2008), apatite-calcite zircon-syenites and hornblendites. Ar-Ar amphibole analysis and U-Pb zircon and apatite data return Triassic ages for these rocks, which have been considered to document the time of melt/fluid injection. Notwithstanding the apparent mineralogical and chemical differences with the main lithologic sequences, apatite-carbonates-bearing rocks have been frequently interpreted as cogenetic to phlogopite harzburgites. To debate the petrogenesis of these rocks, a detailed field, petrological and geochemical investigation has been carried out on a swarm of apatite-calcite-bearing gabbroic veins that randomly cut the main lithologic association. Preliminary investigation evidenced as these veins show complex metasomatic haloes and a symmetric internal layering, characterised by crystallisation of magmatic sapphirine (Giovanardi et al., 2013). The mineral assemblage of the veins is dominated by titanian pargasite towards the host peridotite and by plagioclase at the vein centre. The veins also present phlogopite and spinel. Field and petrographic evidence, major and trace element data and the O isotopic composition of such gabbroic veins indicate that they formed at shallow mantle conditions by multistage fractional crystallisation of a migrating melt unrelated to those forming phlogopite harzburgites. Besides, local strong enrichments in LILE, LREE and 18O in vein minerals confirm that such melt was deeply modified by interaction with the host phlogopite peridotite. The genetic relationships with other intrusive events recorded by the FPP and the associated crustal sequence will be addressed with the aim of placing new constraints on the petrologic and geodynamic evolution of the IVZ.

2016 - In situ 87Sr/86Sr LA-MC-ICPMS on biogenic apatites: a matrix-matched standard correction approach [Abstract in Atti di Convegno]
Lugli, Federico; Cipriani, Anna; Mazzucchelli, Maurizio; Sforna, MARIE CATHERINE; Brunelli, Daniele
abstract

Strontium isotope ratios are a strong tool to study ancient hominin and animal migrations, hence the increasing need to have a simple, fast and microdestructive analytical technique to obtain accurate and precise 87Sr/86Sr ratios of precious tooth enamel and bone tissue. The traditional analysis by the TIMS or MC-ICPMS tecniques requires sample dissolution; therefore, several LA-MC-ICPMS methods have been developed to prevent sample destruction, particularly for prehistoric human teeth. Instrumental calibration on human enamel is difficult because of the typical low-Sr concentration and analytical interferences. In fact, the methodology for data reduction of in situ Sr isotopes of biogenic apatite is largely debated in the literature [e.g. 1, 2]. While monoatomic interferences (Kr, Rb, REE2+) are routinely corrected, the correction of polyatomic interferences (CaCa, CaAr and 40Ca31P16O) are challenging. In particular, the CaPO molecule strongly interferes on mass 87, hindering the achievement of precise and accurate 87Sr/86Sr ratios. Following on the work of Horstwood et al. (2008), we developed a method based on the concurrent analyses of multiple matrix-matched standard materials. We show how the linear regression of 87Sr/86Sr accuracy vs. 1/88Sr of at least three standards allows correction of this interference. During each analytical session, we analyse our four in-house matrix-matched standards (a human tooth, a bovine tooth, a swine tooth and a shark tooth) covering a wide range of Sr concentrations (from c.a. 100 ppm of the human tooth to the 1000 ppm of the shark tooth). A daily CaPO model is then built to predict the expected accuracy of the analysis. This correction gives an external reproducibility to the 4th decimal digit (e.g. 2σ-human enamel = 0.00047; c.a. 100 ppm) and an accuracy between the 4th and the 5th decimal digit when applied to analyses with a laser spot sizes of 100μm and a linear dynamic ablation pattern. Monitoring of the CaPO molecule formation during analysis is also achieved by performing several high resolution mass scans.

2016 - MELT-PERIDOTITE MULTISTAGE INTERACTION AT MANTLE CONDITIONS: PETROLOGICAL ANO GEOCHEMICAL EVIDENCES FROM SAPPHIRINE-APATITE-CALCITE-BEARING GABBROIC DYKES FROM THE FINERO PHLOGOPITE PERIDOTITE (IVREA-VERBANO ZONE) [Abstract in Rivista]
Giovanardi, Tommaso; Zanetti, A.; Mazzucchelli, Maurizio; Langone, A.; Morishita, T.
abstract

The Finero Phlogopite-Peridotite (FPP) is a mantle uni! outcropping in the northernmost pari of the Ivrea-Verbano Zone (IVZ, Southern Alps). Multistage pervasive lo channelled meli migrations had completely recrystallized the entire FPP. The main metasomatic event pervasively formed an association of amphibole-rich phlogopite harzburgite with subordinated phlogopite-pyroxenites which do not show geochemical gradients (Zanelli et al., 1999). Channelled migrations lately formed dunite bodies, sometimes containing stratiform chromitites and, more rarely, pyroxenite layers similar lo those associated lo phlogopite harzburgite. Several other lithologies, showing geochemical gradients with rocks of the main FPP association and characterized by the presence of apatite sometimes associated lo carbonates (i.e. dolomite and calcite), are subordinated in volumes and abundances. Commonly these lithologies occur as dykes or veins along deformation zones. Geochronological data from apatite-calcite zircon syenites and apatite-dolomite wehrlites provide Triassic ages assumed lo document the lime of the meltlfluid migrations. Notwithstanding the apparent mineralogica! and chemical differences with the main lithologic sequences, apatite-carbonates-bearing rocks have been frequently interpreted as cogenetic lo phlogopite harzburgites and related lo the main metasomatic event. Recently, apatite-calcite-bearing gabbroic dykes randomly crosscutting the FPP lithologic associations were recognized as possibly the las! (or one of the las!) melt migration event within the mantle unii (Giovanardi et al., 2013). The dykes show symmetrical internal layering formed by melanocratic bands towards the host peridotite dominated by titanian pargasite and a centrai leucocratic zone dominated by plagioclase. Magmatic sapphirine occurs in plagues al the contaci of the leucocratic zone within the melanocartic bands. New field, petrographic and geochemical studies were conducted lo constrain the gabbroic veins intrusion and their genetic relationships with other FPP metasomatic events. Petrographic evidences, major and trace element data and the O isotopic composition of such gabbroic veins indicate that they formed al shallow mantle conditions by multistage fractional crystallisation of a migrating meli unrelated lo those forming the harzburgite-pyroxenite association and the dunite bodies. However, local strong enrichments in LILE, LREE and 1i180 in vein minerals confirm that such melt was deeply modified by interaction with the host phlogopite peridotite. However, the amphiboles in textural equilibrium with sapphirine show a marked M-H REE and Y depletion associated lo a marked positive Eu anomaly, which suppor! meli evolution through plagioclase assimilation. The genetic relationships with other intrusive events recorded by the FPP and the associated crustal sequence will be addressed with the aim of placing piace new constraints on the petrologic and geodynamic evolution of the IVZ.

2016 - Mafic Dykes: Petrogenesis and Tectonic Inferences [Capitolo/Saggio]
Girardi, Vicente A. V.; Teixeira, Wilson; Mazzucchelli, Maurizio; de Oliveira, Elson P.; Corrêa da Costa, Paulo C.
abstract

The main mafic dyke swarms of the São Francisco craton range in age from Late Archean to the Early Neoproterozoic, and record extensional and transpression tectonic events. The Uauá and Lavras regions include norite (2.73 and 2.66 Ga) and tholeiite swarms (2.62 and 1.97 Ga) respectively. Other swarms are made up of tholeiites: Paraopeba (2.10 Ga), Pará de Minas (1.71 Ga), Curaçá and Chapada Diamantina (1.5 Ga), Diamantina (0.93 Ga) and Salvador-Olivença (0.92 Ga). Geochemical and Sr–Nd isotopic data indicate variable scales of mantle heterogeneity, caused by recurrent metasomatic effects since Archean times. The main processes are attributed to the action of slab fluids from recycling of oceanic crust and overlying sediments on the sub-lithospheric mantle in continental settings, and by upwelling of OIB-like material. These processes are consistent with the protracted geologic history of the country rocks, given by episodic juvenile accretion and successive continental amalgamations from Archean to ca. 2.0 Ga. The adopted mixing geochemical model permits to evaluate the variation range of N-MORB melts, OIB and slab fluids and melts, which are considered to represent the main components of the parental mantle composition of the studied dykes.

2016 - Origin and age of zircon-bearing chromitite layers from the Finero phlogopite peridotite (Ivrea–Verbano Zone, Western Alps) and geodynamic consequences [Articolo su rivista]
Alberto, Zanetti; Giovanardi, Tommaso; Antonio, Langone; Massimo, Tiepolo; Fu Yuan, Wu; Luigi, Dallai; Mazzucchelli, Maurizio
abstract

An investigation has been performed on three chromitite layers segregated in dunite bodies of the Phlogopite Peridotite mantle unit in the Finero complex (FPP, Ivrea–Verbano Zone, Southern Alps) aimed at providing new constraints to their origin and evolution. Field relationships, the sub-chondritic Hf isotopic composition of the zircons (εHf(188) as low as − 5.4), the heavy O isotopic composition of zircons and pyroxenes (δ18O up to 6.9‰), the strict similarity of the trace element composition between the clinopyroxenes and amphiboles from the chromitites and those from the phlogopite harzburgites and pyroxenites forming the typical FPP association, as well as the REE composition of zircons, which approaches equilibrium with the associate clinopyroxene, suggest that the studied chromitites were segregated from melts, highly contaminated from continental crust, during the pervasive cycle of metasomatism recorded by the FPP. An LA-ICP-HRMS survey of chromitite zircon grains has provided Early Jurassic U–Pb ages mostly between 199 ± 3 Ma and 178 ± 2 Ma, with a pronounced peak at 187 Ma. Relevant exceptions are inherited domains of two grains giving Triassic ages of 242 ± 7 Ma and 229 ± 7 Ma, and a third homogeneous zircon giving 208 ± 3 Ma. Our geochronological data and those reported in the literature show that the FPP chromitites have zircon populations with different internal CL textures, but the same sub-chondritic Hf isotopic composition, which define an overall U–Pb age span from ~ 290 Ma to 180. The segregation of the chromitite layers and the main pervasive metasomatism likely occurred in the Early Permian (in a post-collisional, transtensional setting) or before (possibly, in a subduction-related setting). The rejuvenation of the zircon ages was accompanied by a progressive disappearance of the internal zoning, interpreted as the result of a prolonged residence at mantle depths with progressive re-equilibration of the U–Pb system due to thermal perturbations. The age peak at ~ 187 Ma is argued to constrain the timing of FPP exhumation at shallower, crustal levels. This process was characterised by an important reheating event, possibly due to lithospheric hyperextension. The evolution of the FPP appears completely different than that of mantle bodies of the central IVZ (i.e., the Val Sesia-type bodies), which were emplaced within the continental crust, as part of accretionary prisms, at or before the end of the Variscan orogeny.

2016 - Origin of the DUPAL anomaly in mantle xenoliths of Patagonia (Argentina) and geodynamic consequences [Articolo su rivista]
Mazzucchelli, Maurizio; Cipriani, Anna; Hémond, Christophe; Zanetti, Alberto; Bertotto, Gustavo Walter; Alberto, C. i. n. g. o. l. a. n. i. Carlos
abstract

The sub-continental lithospheric mantle of South America has been known for some time to carry the DUPAL isotope anomaly as seen in volcanics from the Paraná volcanic province. However, this has not allowed discriminating whether the DUPAL anomaly is a primary feature of the mantle source or acquired during the upwelling and emplacement of the primary magmas. We discovered mantle xenoliths from the Tres Lagos location in Patagonia that carry evidence of percolation by metasomatic melts that imparted the DUPAL isotope anomaly signature. We discuss a model that requires four isotope components (LCC, EM2, HIMU and DM) to account for the Sr, Nd and Pb isotope variability of our samples. We propose that upwelling of hot astenosphere during the Miocene could have triggered the melting of the LCC and EM2 components carrying the DUPAL anomaly, previously entrained in the subcontinental mantle by subduction. These ascending melts would have then metasomatised the local SCLM characterised by DMM and HIMU geochemical affinity generating the hybrid DUPAL-bearing mantle sampled by the Tres Lagos xenoliths.

2016 - PRELIMINARY U-Pb LA-ICPMS ZIRCON ANALYSES FROM THE GOIAS COMPLEXES: SHRIMP COMPARISON AND INTRUSION AGE [Abstract in Rivista]
Lugli, Federico; Giovanardi, Tommaso; Girardi, V. A. V.; Correla, C. T.; Tassinari, C. C. G.; Sinigoi, S.; Cipriani, Anna; Mazzucchelli, Maurizio
abstract

Cana Brava, Niquelàndia and Barro Alto are three mafic-ultramafic layered intrusions (from N to S) which form a - 350 km, NNE-trend belt within the Brasilia Belt (Goiés state, Brazil). Presently, their intrusion ages and geologica! evolution are stili debated. The Niquelàndia and Barro Alto complexes are forrned by two main sequences: the upper sequence and the lower one. Some authors suggest that the two sequences represent two separate intrusions: the upper sequence would be a Mesoproterozoic intrusion at - 1.3 Ga, whereas the lower sequence a Neoproterozic one at - 790 Ma. According to this interpretation, the two sequences were re-crystallized by Neoproterozoic metamorphism and exhumed and juxtaposed during the Brazilian event of formation of the Gondwana continent. Another model suggests that the two sequences are part of the same intrusion, which occurred during the Neoproterozoic and was exhumed during the Brazilian event. New U-Pb SHRIMP-II zircon analyses were perforrned at the Universidade de Sào Paulo from samples from Cana Brava and Barro Alto, the two least-known complexes in order to clarify the sequence of events that led to their forrnation. Analyses were then replicated at the CIGS of the Università di Modena e Reggio Emilia using a X Series" quadrupole ICP-MS coupled with a New Wave UP-213 Nd:YAG laser ablation system. Zircons were sampled through a 40 micron spot (static mode), using a He flux of 0.6 l/min, with an energy density of - 6 J/cm2. Daily instrument calibration was perforrned with the NIST 610 standard, monitoring also the oxide production rate {232Th160/232Th << 0.01 %). Laser-induced elemental fractionation was corrected by repeated analyses of the standard zircon TEMORA2 (Black et al., 2004). A secondary reference materia! (zircon CZ3) was used to check the precision and accuracy of the corrections. Our LA-ICP-MS data are preliminary, but very promising being the accuracy of the measured ratio within the SHRIMP variability. We are currently working to improve the precision of our methodology, which however is now comparable with literature LA-ICP-MS data (propagated 2SE - 2-6%; Horstwood et al., 2008). Overall, the isotopic data of Cana Brava and Barro Alto complexes previde for a coeval Neoproterozoic intrusion age at - 790 Ma. These ages are consistent with those reported in literature for Niquelàndia. Mesoproterozoic ages, consistent with the forrnation age of the metavolcanic-metasedimentary sequence in magmatic contaci with the complexes, were found in inherited zircon cores. Our data clearly show that the Goiàs complexes are formed by single bodies intruded during the Neoproterozoic at -790 Ma and that the hypothesis of two separate intrusions juxtaposed by tectonic must be discarded.

2016 - Precambrian Dykes in the São Francisco Craton Revisited: Geochemical-isotopic Signatures and Tectonic Significance [Articolo su rivista]
Teixeira, W.; Girardi, V. A. V.; Mazzucchelli, M.; Oliveira, E. P.; CorrEa da costa, P. C.
abstract

2016 - THE LARGE LAYERED GOIAS COMPLEXES: NEW U-Pb ANO PRELIMINARY Lu-Hf IN SITU ZIRCON ANALYSES FROM BARRO ALTO AND CANA BRAVA [Abstract in Rivista]
Giovanardi, Tommaso; Lugli, Federico; Girardi, V. A. V.; Correla, C. T.; Tassinari, C. C. G.; Sinigoi, S.; Cipriani, Anna; Mazzucchelli, Maurizio
abstract

The large layered Goias complexes are three mafic-ultramafic layered intrusions which outcrop in a -350 km, NNE-trend bel! within the Brasilia Belt (centrai Brazil). Fom N lo S, they are: Cana Brava, Niquelllndia and Barro Alto. The intrusion age and geologica! hystory of these intrusions is stili presently debated. New U-Pb SHRIMP-II zircon analyses were perforrned in samples from Cana Brava and Barro Alto, the two poorest-known complexes among them, providing fora coeval Neoproterozoic intrusion age al -790 Ma of both complexes, consistent with literature ages for the Niquelandia complex. lnherited zircons with Mesoproterozoic ages are consistent with the formation age of the metavolcanic-metasedimentary sequences in magmatic contaci with these three complexes and suggest some degree of contamination of the complexes. This contamination in Niquelandia and Cana Brava is well known in the literature as revealed by bulk-rock Rb-Sr and Sm-Nd systematics. The two complexes show enrichment of the contamination in their gabbroic sequence with local enrichment where the crustal xenoliths are more abundant. Conversely, the upper pari of Niquelandia, mainly formed by anorthosite, is almost or totally uncontaminated. The Lu-Hf analyses on zircons from the Goiés complexes were perforrned during the instrument calibration of the Lu-Hf methodology al the laboratories of Centro lnterdipartimentale Grandi Strumenti al the Università di Modena e Reggio Emilia. The measurements were carried out using a Neptune MC-ICPMS coupled with a New Wave UP-213 laser ablation. The instrument was firstly calibrated on a standard solution and successively in situ analyses on complexes zircons were carried out together with CZ3 and TEMORA2 zircon standards. Mass bias and isobaric interference were corrected offiine using the lsotopeMaker free software of Zhang et al. (2015). Preliminary Lu-Hf in situ zircon data show negative EHf(t) values which are consistent with crustal contamination of the Barro Alto and Cana Brava parental melts as show by Rb-Sr and Sm-Nd in Cana Brava. The new Lu-Hf data, togheter with a review of literature data, show, for Barro Alto, a contamination similar to Niquelandia for the Rb-Sr and Sm-Nd systematics: zircons from the gabbroic sequence are contaminated, while zircons from the upper anorthosites are poorly contaminated. lnherited zircons commonly show positive EHf(t) values, which suggest mantle-derived melts for the magmatism of the metavolcanicmetasedimentary sequence.

2016 - The Classification of Igneous Rocks: Diorite vs. Syenite – A suggestion in order to avoid a contradiction [Articolo su rivista]
Mazzucchelli, Maurizio
abstract

In the recommendations of the Subcommission on the Systematics of Igneous Rocks of the International Union of Geological Sciences, in the volume edited by R.W. Le Maitre (2002), an evident contradiction in the definition of the dioritic and syenitic plutonic rocks is reported. According to Le Maitre (2002) the parameters used for the modal classification of the plutonic rocks are: Q, A, P, F and M, where Q = quartz, tridymite, cristobalite; A = alkali feldspar, including orthoclase, microcline, perthite, anorthoclase, sanidine and albitic plagioclase (An0 to An5); P = plagioclase (An5 to An100) and scapolite; F = feldspathoids including nepheline, leucite, kalsilite, analcime, sodalite, nosean, haûyne, cancrinite and pseudoleucite; M = mafic and related minerals, e.g. mica, amphibole, pyroxene olivine, opaque minerals, accessory minerals (e.g. zircon, apatite, titanite), epidote, allanite, garnet, melilite, monticellite, primary carbonate. When the rock is defined felsic, e.g. M < 90%, the modal classification is based on the QAPF diagram (Streckeisen, 1973, Figs. 1 and 2). To use the QAPF classification, the modal amounts of Q, A, P and F must be known and recalculated so that their sum is 100%. When a rock plot in the following fields: 4, 9, 10, 13 and 14 (Fig. 1), the root names proposed by the classification are: diorite and gabbro (and anorthosite in field 10, with M < 10%). A Diorite is defined when plagioclase = An0 to An50, whereas a Gabbro when plagioclase = An50 to An100. Now a contradiction arises whenever the composition of the plagioclase = An0 to An5. From the definition of A and P reported above, the albitic plagioclase should be part of the parameter A and, correspondingly, the parameter P would be = 0 (unless scapolite is present). Rocks containing albitic plagioclase (An0 to An5) would therefore fall inevitably in the fields closer to the vertex A of the QAPF diagram (e.g. the rocks that are supposed to fall in the field 4 would plot within the field 2, those in the fields 9 and 10 in field 6 and those in the fields 13 and 14 in field 11). This contradiction does not help in resolving the confusion in the classification of rocks manifested in several publications in various scientific journals. Cases of dioritic rocks defined as syenites and vice versa are not uncommon. Examples of the first case are reported in: a) Stähle et al. (1990), where pegmatites without K-feldspar and containing plagioclase = An07-An11 are defined mica syenites and nepheline syenites; b) Stähle et al. (2001), where a dike consisting of hornblende and plagioclase = An08-An13 is defined hornblende syenite; c) Schaltegger et al. (2015), where zircon, biotite, plagioclase-bearing pegmatites are defined miaskitic nepheline syenite; the plagioclase composition (An06-An07) of these samples is reported in Weiss et al. (2007). An example of the second case is reported in Yu et al. (2011) who describes albite diorite rocks for the Zhongjiu iron deposit, following the quite old quantitative mineralogical classification of igneous rocks of Johannsen (1920), while these rocks in reality are syenites. The suggestion proposed here is to change the definition of the diorite root as follows: A Diorite is defined when plagioclase = An5 to An50. This is also in accordance with the first definition of Streckeisen (1973), who only had place the limit between diorite and gabbro the composition An = 50, without setting the lower limit for the diorite root, as already implicit in the definition of the parameter A. Alternatively, the classification of plutonic rocks based on the chemical composition, as proposed by Bellieni et al. (1995), would be recommended.

2015 - Comparing the Cana Brava and Niquelândia complexes: different contamination and fractionation processes in coeval intrusions [Abstract in Rivista]
Giovanardi, Tommaso; Girardi, V. A. V.; Correia, C. T.; Sinigoi, S.; Tassinari, C. C. G.; Mazzucchelli, Maurizio
abstract

Contamination processes during the intrusion in the crust of mantle-derived melt which produced huge layered complexes are recognized in almost all the layered complexes (e.g. Val Sesia magmatic system). However, the contamination does not always occur (e.g. Finero mafic complex) or occurred in different intrusions with different modalities. The Niquelândia and Cana Brava complexes (Goiás, central Brazil) are part of a 300 km long, North-trending belt of mafic-ultramafic massifs outcropping in the Brasilia Belt. Among the three complexes forming the belt (together with the Barro Alto one), the Cana Brava complex is the less known while the Niquelândia complex is the better known. The intrusion of the complexes occurred during a continental rift in the lower crust and the parent melt compositions were estimated to be MORB-like. Notwithstanding these, the stratigraphy of the two complexes is different: the Niquelândia complex shows anorthositic rocks forming the so-called Upper Sequence while the Cana Brava complex is similar to the Niquelândia Lower Sequence. New U-Pb SHRIMP-II analyses on zircons from 4 samples from the Cana Brava complex provided for concordia ages between 798.7±2.2 Ma and 779.3±1.3 Ma. These ages constrain the Cana Brava intrusion at 800-780 Ma, similarly to the intrusion ages estimated in literature for the Barro Alto and Niquelândia complexes. Literature data suggests that the Niquelândia complex suffered crustal contamination as a late event during its growth only locally and in the Lower Sequence. The contamination enriched the melt in incompatible elements (e.g. LREE and Ba)and affected the Rb-Sr and Sm-Nd isotopes. New bulk-rock major and trace elements analyses from the Cana Brava complex show strong enrichments for themost incompatible elements at the top of the complex which suggest, together with the occurrence of xenoliths, that the parent melt was affected by crustal contamination. Rb-Sr and Sm-Nd isotopic analyses confirm this hypothesis, showing an increasing contamination trend along the stratigraphy (87Sr/86Sr(790) between 0.708243-0.736590 and ɛNd(790) between 1.71 and -8.47 from the bottom to the top). This suggests a continuous contamination process during the complex growth. The comparison of the two complexes evidenced a different development of the contamination processes. AlphaMELTS models for the Niquelândia and Cana Brava complexes provide evidences of different fractionation of the parent melts, thus suggesting that the different development of crustal contamination is led to the fractionation processes and a different melt compositions.

2015 - Gabbroic dykes in the Finero Phlogopite-Peridotite Massif: evidence for meltperidotite interactions at mantle conditions and igneous sapphirine formation by autometasomatism [Abstract in Rivista]
Giovanardi, Tommaso; Zanetti, A.; Morishita, T.; Langone, A.; Dallai, L.; Mazzucchelli, Maurizio
abstract

The Finero Phlogopite Peridotite (FPP: Ivrea-Verbano Zone, western Alps, Italy) is a mantle sequence completely recrystallized by several events of melt migration. Literature studies provide evidence that a main metasomatic event induced the pervasive crystallisation of Amphibole-Phlogopite-bearing mineral assemblages (harzburgites and pyroxenites; Amph, Phl), as well as the formation of dunite bodies. These lithologies have similar geochemical compositions characterised by strong crustal components, as testified by enrichment in K, Mg, H2O, LREE and LILE and depletion in HREE and HSFE, radiogenic Sr and Pb, and unradiogenic Nd. Besides, the FPP shows bands, veins or pockets with variable mineral assemblages, but usually rich in Apatite (Ap) and Carbonates (Crb). Late gabbroic dyke swarms, different from all the other lithologies and containing Sapphirine (Spr), were recently described (Giovanardi et al., 2013). These dykes were formed by multi-stage magma intrusions via hydraulic-fracturing, characterised by early crystallisation of Amph ± Ap ± Phl (i.e the Early Amph Zone) followed by segregation at the vein core of Plagioclase ± Amph ± Crb (the Leucocratic Zone). Spr occurred in a reaction zone placed between them. Another reaction zone occurs at the contact between the host peridotite and the vein, being characterised by the complete replacement of peridotite Olivine by secondary Orthopyroxene (the Opx Zone). The occurrence of Spr only in the Late Amph Zone was possibly interpreted as i) direct magmatic crystallization from an evolved Al-rich melt or ii) crystallization by auto-metasomatic process with interaction between the Early Amph Zone with the most evolved melt segregating the Leucocratic Zone. A new study on the Spr-bearing gabbroic dykes of the FPP allowed us: i) to constrain the occurrence of local (up to 8 cm from the veins) melt interaction with the host harzburgite, which provides high δ18O, Al, Mg, K, H2O, locally associated to pronounced enrichments in U, Th, LILE and LREE, which unravels different degrees of contamination by host rock; ii) to provide new evidence supporting the Spr formation by auto-metasomatic process, after reaction of the first cumulates (i.e. the Early Amph Zone) with the most evolved melt which crystallized the Leucocratic Zone; iii) to give new constraints on the nature of the gabbroic dykes parent melts. The formation of the Opx Zone within the host peridotite and the cumulus crystallization of hydrous phases (i.e. Amph and Phl) suggest to a silica-saturated, hydrous evolved melt as parent melt of the gabbroic dykes. The L/MREE-enriched convex-upward patterns of the Amph, point to an alkaline geochemical affinity of the parent melts.

2015 - In situ trace element analysis of human hard tissues by Laser Ablation ICP–MS [Abstract in Atti di Convegno]
Lugli, Federico; Cipriani, Anna; Brunelli, Daniele; Giorgio, Gruppioni; Mazzucchelli, Maurizio
abstract

After the advent of the laser ablation system (LA), the analysis of micro-samples of hard materials has spread to many scientific fields (i.e. geology, engineering, archaeometry, anthropology). The Laser Ablation Coupled with a Mass Spectrometer (as in the LA–ICP–MS) allows to investigate any hard material at the scale of a few µms, for example, defining the elemental compositional profile of bone, teeth and nephroliths, measuring any element with an atomic mass between 2 and 255 amu (i.e. heavy metals as Pb, Cr, Cd, As, etc.). Moreover, one of the main advantages offered by LA, compared to more traditional techniques, is the drastic reduction of analytical time, because the sample is analysed as it is without chemical dissolution or separation of elements. In this work, we have tested and developed a routine to measure all trace elements in human bones and teeth with the LA–ICPMS housed at the University of Modena and Reggio Emilia. We analyse all trace element but we have focused here our attention on Zn and Pb, given their importance and significance in dentistry, human health and forensic pathology studies. We have then analyzed two human teeth, measuring their trace elements compositional profile and drawing conclusions on the health history of these two individuals. Materials and Methods. We analysed two human teeth (molar 1HT–R and premolar 2HT–R) sampled from two post-medieval mummies (Roccapelago, Modena, 16th – 18th cent.). The concentrations of Pb and Zn were measured in situ through a 213 nm Nd:YAG Laser Ablation system coupled to a quadrupole ICP–MS system (Thermo Fisher Scientific® X SeriesII). Before analysis, each sample was cut in half with a diamond low-speed wheel saw, to expose the inner parts of the tooth. Eighteen LA–spots (diameter 100 µm) were analysed along two profiles (n=10 sample 1HT–R; n=8 sample 2HT–R), exploring enamel and dentine. The elemental concentrations of our samples have been corrected through a calibration with NIST 1486 Bone Meal as a multi-element standard. To correct any matrix-related effect, the calcium concentration, measured with an ESEM Quanta 200, has been used as the internal standard. To ensure the accuracy of the method, a two-tailed t-test was performed on two certified elements of the standard. Statistical analysis shows no significant difference between expected and measured values (p<0.05; mean of 3 analyses). Results. The first tooth (1HT–R) shows a Zn concentration ranging from 230 in the outer enamel to 110 ppm in the inner primary dentine (x = 150 ± 38 ppm, 1σ) and, respectively, a Pb concentration ranging from 3 to 1 ppm (x = 2 ± 1 ppm, 1σ). From the outer enamel to the inner dentine, the second tooth (2HT–R) shows a Zn concentration ranging from 301 to 123 ppm (x = 177 ± 66 ppm, 1σ) and a Pb concentration ranging from 6 to 2 ppm (x = 3 ± 1 ppm, 1σ). Conclusions. Our results are in agreement with Pb and Zn profiles described in literature for human teeth (Humphrey et al. 2008). The concentrations of both elements tend to increase from the enamel-dentine junction (EDJ) to the enamel external surface and tend to decrease from the EDJ to the inner dentine. The Pb concentration of both samples is on average lower than the threshold of poisoning reported for human bones (5 ppm; Hess et al. 2013). The Zn concentrations are low, in particular in the outer enamel. As reported by Lynch (2011), this could be related to tooth wear or to the old age of the two individuals (~ 40 yr). The LA–ICP–MS is a non-destructive in–situ analytical technique, fundamental in forensic pathology and human health studies to measure the elemental composition of human hard tissue and in particular, in outlining the history of the tooth by trace element compositional profile from the inner to the outer portions of the tooth.

2015 - LAM U-Pb zircon Early Jurassic exhumation age of the Finero Phlogopite Peridotite (Ivrea-Verbano Zone, Western Alps) and its geodynamic consequences [Abstract in Rivista]
Zanetti, A.; Giovanardi, Tommaso; Mazzucchelli, Maurizio; Langone, A.; Tiepolo, T.; Wu, F. Y.
abstract

A new LA-ICP-HRMS investigation of transparent zircons, unzoned and smoky at cathodoluminescence (CL), separated from three chromitite layers segregated in mantle dunite bodies belonging to the Phlogopite Peridotite unit (hereafter PP) of the Finero Complex (Ivrea-Verbano Zone, Southern Alps) provides single-spot 206Pb/238U Lower Jurassic ages between 200 to 180 Ma, with a pronounced peak at ~190 Ma. Relevant exception is represented by two pinky zircons showing relics of zoning at CL, with darker cores that give Triassic ages from 240 to 230 Ma. The presence of continental crust component(s) evidenced by the negative eHf of the zircons, the strict similarity of the trace element contents shown by clinopyroxenes and amphiboles from chromitites and the phlogopite harzburgites and pyroxenites hosting the dunite bodies, as well as the complete to partial disappearance of olivine replaced by orthopyroxene, indicate that the parent melts of the chromitites had a cognate origin with the hydrous LILE-enriched silica-saturated melts responsible of the pervasive metasomatism recorded by the Finero mantle sequence. The combination of our data with those reported in literature for the PP chromitite zircons determines a large age interval ranging from 290 to 180 Ma. However, zircon populations with different U-Pb ages show eHf very similar to that found in this study. The latter evidence, together with the rejuvenation of the ages with the disappearing of the internal structures suggest that the large age variability is the result of a prolonged residence at mantle/lower crustal depths of the PP, characterised by progressive re-equilibration stages of the U-Pb zircon system. Thus, it is here proposed that the segregation of the zircon-bearing chromitite layers was related to the pervasive,metasomatic event, which occurred at ~290 Ma or before. Successively, the U-Pb zircon system remained virtually unperturbed until Middle Triassic, when the area was affected by at least two main magmatic cycles with tholeiitic to Na-alkaline geochemical affinity associated to tectonic instability. The consequent thermal perturbations induced reequilibration stages of the chromitite zircons, which ended with the Early Jurassic exhumation documented by the U-Pb ages of chromitite zircons of this study. Our data suggest that the Early Jurassic extensional tectonics was characterised by an important reheating event at 190 Ma, possibly due to lithospheric hyperextension. Such a scenario considers that the PP unit resided at mantle depths during Early Permian, being possibly emplaced at crustal levels only thanks to trans-lithospheric faults during the Early Jurassic. This evolution is completely different with respect to the present day interpretation of the geodynamic history of the mantle bodies in the Val Sesia area, which are believed to have been emplaced within the continental crust, as part of accretionary prisms, since the end of the Variscan orogeny or before. This evidence confirms that the northernmost part of the Ivrea-Verbano Zone underwent peculiar Paleozoic to Mesozoic geodynamic processes, thus unravelling important additional complexities to the interpretation of the geodynamic evolution of the area now related to the Southern Alps.

2015 - LAM U-Pb zircon Early Jurassic exhumation age of the Finero Phlogopite Peridotite (Ivrea-Verbano Zone, Western Alps) and its geodynamic consequences [Abstract in Rivista]
Zanetti, Alberto; Giovanardi, Tommaso; Mazzucchelli, Maurizio; Langone, Antonio; Tiepolo, Massimo
abstract

A new LA-ICP-HRMS investigation of transparent zircons, unzoned and smoky at cathodoluminescence (CL), separated from chromitite layers segregated in mantle dunite bodies belonging to the Phlogopite Peridotite unit (hereafter PP) of the Finero Complex (Ivrea –Verbano Zone, Southern Alps) provides single-spot 206Pb/238U Lower Jurassic ages between 200 to 180 Ma, with a pronounced peak at around 190 Ma. Relevant exception is represented by two pinky zircons showing relics of zoning at CL, with darker cores that give Triassic ages from 240 to 230 Ma. The presence of continental crust component(s) evidenced by the negative EpsilonHf of the zircons, the strict similarity of the trace element contents shown by clinopyroxenes and amphiboles from chromitites and the phlogopite harzburgites and pyroxenites hosting the dunite bodies, as well as the complete to partial disappearance of olivine replaced by orthopyroxene, indicate that the parent melts of the chromitites had a cognate origin with the hydrous LILE-enriched silica-saturated melts responsible of the pervasive metasomatism recorded by the Finero mantle sequence. The combination of our data with those reported in literature for the PP chromitite zircons determines a large age interval ranging from 290 to 180 Ma. However, zircon populations with different U-Pb ages show EpsilonHf very similar to that found in this study. The latter evidence, together with the rejuvenation of the ages with the disappearing of the internal structures suggest that the large age variability is the result of a prolonged residence at mantle/lower crustal depths of the PP, characterised by progressive reequilibration stages of the U-Pb zircon system. Thus, it is here proposed that the segregation of the zircon-bearing chromitite layers was related to the pervasive metasomatic event, which occurred at ~290 Ma or before. Successively, the U-Pb zircon system remained virtually unperturbed until Middle Triassic,when the area was affected by at least two main magmatic cycles with tholeiitic to Na-alkaline geochemical affinity associated to tectonic instability. The consequent thermal perturbations induced re-equilibration stages of the chromitite zircons, which ended with the Early Jurassic exhumation documented by the U-Pb ages of chromitite zircons of this study. Our data are in agreement with the interpretation that the Early Jurassic extensional tectonics was characterised by an important reheating event at 190 Ma, possibly due to lithospheric hyperextension. Such a scenario considers that the PP unit resided at mantle depths during Early Permian, being possibly emplaced at crustal levels only thanks to trans-lithospheric faults during the Early Jurassic. This evolution is completely different with respect to the present day interpretation of the geodynamic history of the mantle bodies in the Val Sesia area, which are believed to have been emplaced within the continental crust, as part of accretionary prisms, since the end of the Variscan orogeny or before. This evidence confirms that the northernmost part of the Ivrea-Verbano Zone underwent peculiar Paleozoic to Mesozoic geodynamic processes, thus unravelling important additional complexities to the interpretation of the geodynamic evolution of the area now related to the Southern Alps.

2015 - Multiple refertilisation of a strongly refractory mantle column in the extra-Andean back-arc (Paso de Indios, Argentina) [Abstract in Rivista]
Zanetti, Alberto; Bertotto, Gustavo W.; Mazzucchelli, Maurizio; Ponce, Alexis D.; Giovanardi, Tommaso; Brunelli, Daniele; Aragón, Eugenio; Bernardi, Mauro I.; Hémond, Christophe
abstract

In the central part of the Chubut province, close to the village of Paso de Indios, there are several outcrops of Cenozoic basalts carrying spinel-facies mantle xenoliths. In this area, located in the extra-Andean back-arc, basaltic necks and dikes outcrop between 43° 36′ – 43° 50′ S and 68° 53′ – 69° 02′ W, along with remnants of lava flows, being divided in two groups of Paleocene and Eocene age. This volcanism was generated by extensional tectonic related to an episode of transform plate margin that affected the southern sector of South America western margin from the Paleocene to the Oligocene, as the Aluk plate detached and a slab window opened beneath the study area. In this contribution, the petrochemical processes experienced by mantle xenoliths hosted in Eocene basalts belonging to the Matilde lava flow, the Leon volcano and the Chenque dike, are presented and discussed. The studied samples are mainly spinel-facies harzburgites and clinopyroxene(Cpx)-poor lherzolites, with some dunites. The Chenque xenoliths mainly display porphyroclastic to equigranular texture, whereas those from Matilde and Leon volcanoes have coarse-granular to porphyroclastic textures. Estimated equilibrium temperatures based on pyroxenes solvus range from 800 to 940°C. The overall refractory character of the mineral assemblages is matched by the major element mineral compositions, which are mostly Al-poor and Mg-and-Cr-rich. Spinel composition is consistent with melt extraction from 8 to 14% for Chenque and Leon samples, and from 14 to 18% for the Matilde ones. The estimated degree of partial melting rises up to 24% considering the literature spinel data. However, the occurrence of melt related open-system processes is suggested by local trends of positive correlation between Na and Cr# in Cpx, being fully confirmed by the trace element compositions. In particular, the Matilde harzburgites ubiquitously show Cpx with transient U-shaped REE patterns. The LREE fractionation is very strong, with LaN up to 100 and REE patterns minimum in the M-HREE region between 0.1-1 xCI. The HREE level content (LuN down to 1 xCI) is consistent with 20-23% fractional melting of spinel DM. V-to-U-shaped REE patterns are also shown by Chenque lherzolites and harzburgites. Their M/HREE are significantly more fractionated than that expected in residue after spinel-facies basalt removal, thus suggesting an onset of the partial melting process at garnet facies conditions. Other Chenque lherzolites experienced a more pronounced refertilisation process led by LREE-enriched to LREE-depleted melts. The latter gave rise to peculiar, transient LREE-depleted sinusoidal patterns in Cpxs through reaction with the depleted ambient peridotite. A refertilised geochemical composition is also shown by the Leon samples, with harzburgite Cpxs resulting enriched in highly-incompatible elements such as U, Th, Sr and LREE. The data presented in this study, in combination with those from the literature, allow us to conclude that the shallow mantle column beneath Paso de Indios was strongly refractory in origin, being successively affected by multiple events of melt migration. These letter, however, were able to produce only an incomplete refertilisation of the depleted protoliths, which still record geochemical gradients developed during the interaction with both LREEenriched and LREE-depleted migrating melts. These petrochemical features make the Paso de Indios mantle column a unique study case in the Patagonian region, where the composition of the shallow mantle is usually completely overprinted by multiple stages of melt/fluid migration.

2015 - New insights into the evolution of the Finero Mafic Complex, north-eastern Ivrea-Verbano Zone [Abstract in Rivista]
Langone, A.; Zanetti, A.; Renna, M. R.; Tiepolo, M.; Mazzucchelli, Maurizio; Giovanardi, Tommaso
abstract

The Finero Mafic Complex outcrops in the northern sector of the Ivrea-Verbano Zone (IVZ, Southern Alps). It occurs at the flank of an antiform which core is constituted by mantle peridotites, and it consists of mafic/ultramafic rocks subdivided in three units: a) the Layered Internal Zone (LIZ), in tectonic contact with the mantle unit; b) the Amphibole Peridotite (Amph-Pd); c) the External Gabbro (EG), which is in tectonic contact with the Variscan crystalline basement (Rivalenti et al., 1984; Siena &amp; Coltorti, 1989). Recent studies point to a Middle Triassic emplacement age for the EG unit (Zanetti et al., 2013), suggesting that it is unrelated to the Permian Mafic Complex of the central IVZ. Owing to the lack of a detailed petrochemical characterisation of the FMC, we performed new major (EMP) and trace element (LA-ICPMS) analyses on representative samples from the LIZ and Amph-Pd. The LIZ mainly consists of hornblende-gabbros; anorthosites and garnet hornblendites with minor pyroxenites. The Amph-Pd is mostly made up of Amph-bearing harzburgites and dunites with minor piroxenites. Locally, Amph-rich veins with a variable thickness from a few cm to about 1 m crosscut the magmatic layering. Olivine (Fo87-82) only occurs in the perfrom Amph-Pd, whereas amphibole and clinopyroxene are common throughout the entire sequence. The Mg# of Cpx and Amph tends to increase from the LIZ towards the upper part of the Amph-Pd whereas the Al2O3 content in Cpx and Amph is up to 11 and 18 wt.%, respectively and show an opposite trend. In garnet-free pyroxenites and hornblendites from LIZ, Amph and Cpx have slightly LREE-depleted patterns with flat HREE (at 2xCI in Cpx) and marked positive Eu, Sr, Pb and U anomalies. Similar features are shown by the Cpx and Amph from the associated gabbros, but they are strongly depleted in HREE indicating chemical equilibration with garnet. Cpx and Amph from the Amph-bearing peridotites (Amph-Pd) have instead LREE-enriched spoon-shape patterns with HREE contents comparable with those of the LIZ lithologies, being also characterised by Eu, Sr and U enrichments. The LILE enrichments and fractionation can be reconciled by an interaction dominated by ion exchange chromatographic-type process with strongly LILEenriched melts: the composition of the latter is recorded by the amphibole-dominated lithologies. The new data suggest that the LIZ with a clear crustal signature. Instead, the trace element variations in the Amph-Pd cannot be explained via a closed-system evolution, pointing to the presence of significant changes in the composition of the uprising mantle melts.

2015 - Petrology and geochemistry of the back-arc lithospheric mantle beneath the Payenia volcanic province (La Pampa, Argentina) [Abstract in Rivista]
Zanetti, Alberto; Bertotto, Gustavo W.; Mazzucchelli, Maurizio; Hémond, Christophe; Conceiçao, Rommulo V.; Schilling, Manuel
abstract

Insights into the petrochemical composition and evolution of the lithospheric mantle beneath the Payenia volcanic province (PVP) (Mendoza and La Pampa provinces, centralwestern Argentina) are provided thanks to the characterization of mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca and Huanul volcanoes. The PVP is related to the mantle wedge of the Argentine back-arc area in correspondence to the Transitional Southern Volcanic Zone. Agua Poca is a pyroclastic cone formed 0.6 Ma and located in the eastern PVP at 37°01′S and 68°07′W, at about 530 km east of the Chilean trench. The Cerro Huanul is instead located in the southern part of PVP, at 37º17'S and 68º32'W, about 480 km east of the Chilean trench. It is a shield volcano crowned by a ring of lava 3.9 km in diameter and 20 m height. Inside this ring, there are various heights formed by pyroclastic cones and a lava flow. The latter contains ultramafic xenoliths mostly smaller than 5 cm. According to K-Ar dating, Huanul volcanic rocks are 0.84 ± 0.05 Ma (Bertotto et al. 2006). The studied mantle xenoliths from both the localities are mainly anhydrous spinel lherzolites, with very subordinate amounts of harzburgites, wehrlites and pyroxenites. Most of the peridotites show porphyroclastic texture, whereas pyroxenites are equigranular. These lithologies show highly variable equilibrium temperatures ranging from 780°C to 1080°C at 1.0 to 2.0 GPa, with the relevant exception of two peridotite samples from Huanul, reaching 1200°C of equilibrium T. This constitutes evidence that the studied ultramafic xenoliths are representative of a large portion of the lithospheric mantle column. Major element mineral composition of Agua Poca and Huanul spinel lherzolites is characterized by fertile character, as evidenced by low Fo in olivine, large Al content in pyroxenes and spinel, large Na in clinopyroxene. CI-normalised REE patterns of lherzolite clinopyroxenes are LREE depleted with flat M-HREE region at 10 xCI (see also Bertotto et al. 2013). They also show extreme depletion in highly incompatible elements such as Nb, Ta, U and Th. REE whole rock and clinopyroxene composition indicate that lherzolites can be considered to be refractory residua after 1% to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. These estimates are consisting with those obtained by means of spinel composition. Isotope composition of clinopyroxene separates from Agua Poca samples is characterized by low radiogenic Sr and large radiogenic Nd. As a whole, all the petrochemical data converge in indicating that most of the mantle column of this sector of PVP consists of rocks belonging to Depleted Mantle reservoir. Spinel harzburgites show bulk and trace element composition characterized by LREEenrichment over HREE. These enrichments are the evidence of interaction of the mantle peridotite with LREE-enriched melts, which induced the development of transient chemical gradients, but that locally was also able to induce a complete metasomatic overprint of the trace element composition. The Sr and Nd isotopic composition of clinopyroxenes from Agua Poca harzburgites lies very close to the DM field, thus not evidencing any clear geochemical fingerprint imparted by contributions from slab-derived crustal components. A banded websterites from Agua Poca shows peculiar major and trace element composition. In particular, REE patterns are characterized by slight LREE depletion (La down to 0.8 xCI) and flat HREE, with the content of these latter down to 3 xCI. Isotope composition of the websterite clinopyroxene exhibits very low radiogenic Sr, similar to DM. Also the Nd isotope composition in the pyroxenite slab at the contact with the peridotite is very close to that of the Agua Poca lherzolites, but it becomes markedly more enriched in radiogenic Nd in the central part of pyroxenite. As a whole, the geochemical data suggests

2015 - Short-scale variability of the SCLM beneath the extra-Andean back-arc (Paso de Indios, Argentina): Evidence from spinel-facies mantle xenoliths [Articolo su rivista]
Ponce, Alexis D.; Bertotto, Gustavo W.; Zanetti, Alberto; Brunelli, Daniele; Giovanardi, Tommaso; Aragón, Eugenio; Bernardi, Mauro I.; Hémond, Christophe; Mazzucchelli, Maurizio
abstract

Matilde, León and Chenque hills are some of the several occurrences of Cenozoic basalts carrying ultramafic xenoliths in Paso de Indios region, Argentina. The mantle xenoliths from the Chenque and León hills mainly present porphyroclastic texture, whereas the Matilde hill ones have coarse-grained to porphyroclastic textures. The equilibrium temperatures are in the range of 780 to 940°C, indicating a provenance from shallow sectors of the lithospheric mantle column that was subjected to a relatively low heat flux at Cenozoic Era. According to the modal compositions, the mantle columns beneath Matilde and León hills mostly record partial meting events larger than 22%, while less depleted peridotites occur in the Chenque suite (starting from 10% partial melting). Such an observation is confirmed by the partial melting estimates based on Cr#Sp, which vary from 8 to 14% for the selected Chenque samples and from 14 to 18% for the Matilde ones. The common melting trend is overlapped by short scale cross-cutting local trends. Local trends can be generated by open-system processes, such as open-system partial melting and/or post partial-melting metasomatic migration of exotic Na-Cr-rich melts. Petrographic survey evidences the occurrence of two main mineralogical reaction schemes due to channelled and/or pervasive melt extraction/migration. These are: i) pyroxenes dissolution and segregation of new olivine in olivine-rich peridotites, and ii) replacement of primary olivine by orthopyroxene±clinopyroxene in orthopyroxene-rich peridotites. Enhanced pyroxene dissolution is attributed to channelling of silica-undersaturated melts, whereas replacement of primary olivine by orthopyroxene±clinopyroxene points to reaction with silica-saturated melts. Late disequilibrium reactions identified in the xenoliths comprise: the breakdown of orthopyroxene in contact with the host basalt and (rarely) reaction coronae on orthopyroxene, clinopyroxene and spinel linked to glassy veins. Such features are apparently related to the injection of melt, likely during entrainment into the host basalts and ascent to the surface.

2015 - U-Pb zircon SHRIMP data from the Cana Brava layered complex: new constraints for the mafic-ultramafic intrusions of Northern Goiás, Brazil [Articolo su rivista]
Giovanardi, Tommaso; Girardi, V. A. V.; Correia, C. T.; Sinigoi, S.; Tassinari, C. C. G.; Mazzucchelli, Maurizio
abstract

The Cana Brava Complex is the northernmost and less-known layered intrusion of a discontinuous belt of mafic-ultramafic massifs within the Brasilia Belt, which also comprises the Niquelândia and Barro Alto complexes. Available geochronological determination by means of different systematics (K/Ar, Ar/Ar, Rb/Sr, Sm/Nd and U/Pb) provide a range of possible ages (time span from 3.9 Ga to 450 Ma), hence a precise and statistically reliable age for the Cana Brava Complex is still lacking. Also, preliminary isotopic and geochemical data of the Cana Brava Complex suggest a significant crustal contamination, which could have affected bulk-rock Sr and Nd systematics resulting in meaningless age determinations. In this paper, we present new U-Pb SHRIMP zircon analyses from four samples of different units of the Cana Brava Complex which suggest that the intrusion occurred during the Neoproterozoic, between 800 and 780 Ma, i.e. at the same age of Niquelândia. Discordant older 206Pb/238U ages are provided by inherited zircons, and match the age of the metamorphism of the encasing Palmeirópolis Sequence.

2014 - Erratum. SHRIMP U^Pb ZirconTriassic Intrusion Age of the Finero Mafic Complex (Ivrea-Verbano Zone, Western Alps) and its Geodynamic Implications [Articolo su rivista]
A., Zanetti; Mazzucchelli, Maurizio; S., Sinigoi; Giovanardi, Tommaso; G., Peressini; M., Fanning
abstract

Fig. 2 in colour

2014 - Evidence for strong depletion, followed by multiple refertilisation, in the mantle column of the extra-Andean backarc (Paso de Indios, Argentina) [Abstract in Rivista]
Mazzucchelli, Maurizio; A. D., Ponce; G. W., Bertotto; A., Zanetti; Brunelli, Daniele; Giovanardi, Tommaso; E., Aragón; M., Bernardi
abstract

In the central part of the Chubut province, close to the town of Paso de Indios, there are several outcrops of Cenozoic basalts carrying spinel-facies ultramafic xenoliths. In this area, located in the extra-Andean back-arc region, basaltic necks and dikes outcrop between 43º 36′ – 43º 50′ S and 68º 53′ – 69º 02′ W, along with remnants of lava flows divided in two groups of Paleocene and Eocene age. This volcanism was generated by extensional tectonic related to a transform plate margin episode that affected the southern South America active margin from the Paleocene to the Oligocene, as the Aluk plate detached and a slab window opened beneath the study area. Here, the petrochemical processes experienced by spinel-facies mantle xenoliths, hosted in Eocene basalts of the Matilde lava flow remnants, the León volcano and the Chenque dike, are presented. The studied samples are mainly spinel-facies harzburgites and clinopyroxene(Cpx)-poor lherzolites, with some dunites. The Chenque xenoliths mainly present porphyroclastic to equigranular texture, whereas those from Matilde and León volcanoes have coarse-grained to porphyroclastic textures. Estimated equilibrium temperatures based on pyroxenes solvus range from 800 to 940°C. The refractory character of the mineral assemblages is matched by the major element mineral compositions, which are mostly Al poor and Mg and Cr rich. Spinel composition is consistent with melt extraction from 8 to 14% (Chenque and León) and 14 to 18% (Matilde). The estimated degree of melting rises up to 24% considering the literature spinel data. However, the occurrence of melt-related open-system processes is suggested by local trends of positive correlation between Na and Cr# in Cpx, being fully confirmed by the trace element compositions. Cpxs from a harzburgitic sample from León volcano show composition rich in U, Th, Sr and LREE. The Matilde harzburgites ubiquitously show Cpx with transient U-shaped REE patterns. The LREE fractionation is very strong, with LaN up to 100 and minimum at the M-HREE region between 0.1-1xCI. The HREE level content (LuN down to 1) is consistent with 20-23% fractional melting of spinel DM. V-to-U-shaped REE patterns are also shown by Chenque lherzolites and harzburgites. Their MHREE are more fractionated than that expected in residue after spinel facies basal removal, thus suggesting an onset of the partial melting at garnet facies conditions. Other Chenque lherzolites seem to result from a more extensive refertilisation processes led by LREE-enriched to LREE-depleted melts. The latter gave rise to transient LREE-depleted sinusoidal patterns through reaction with the depleted ambient peridotite. It is, thus, concluded that the shallow mantle column beneath Paso de Indios records an incomplete refertilisation of strongly depleted protoliths. This represents a unique example for the Patagonian region, where the mantle is usually completely overprinted by multiple stages of melt/fluid migration.

2014 - Igneous evolutions across the Ivrea crustal section: the Permian Sesia Magmatic System and the Triassic Finero intrusion and mantle [Articolo su rivista]
Mazzucchelli, Maurizio; J. E., Quick; S., Sinigoi; A., Zanetti; Giovanardi, Tommaso
abstract

The famous deep crustal section of the Ivrea-Verbano Zone (IVZ, western Alps, Fig. A) has received enormous attention over the last three decades as one of the best examples of continental “magmatic underplating”. Recent investigations, comprising structural, petrochemical and geochronological data, point to the occurrence of a “Sesia-type IVZ” (i.e. central IVZ) and a “Finero-type IVZ” (i.e. northern IVZ), which underwent different magmatic and tectonic evolution. In the Sesia area (box a in Fig. A), the Permian gabbroic pluton known as the mafic complex (reaching thicknesses > 8 km) intruded the deepest rocks of the crustal section, comprising amphibolite to granulite-facies paragneiss and interlayered mantle peridotite bodies, while they were resident in the deep crust. The broader magmatic context of this voluminous intrusion remained unclear until Quick et al. (2009) demonstrated that the emplacement of the mafic complex was coeval to the activity of a mainly silicic volcanic field, including extensive caldera deposits, and to the growth of silicic plutons in the upper crust of the adjacent “Serie dei Laghi”. The Sesia magmatic system constitutes an exposure of the plumbing system of a caldera from the surface to a depth of about 25 km (Quick et al., 2009). In this framework, the mafic complex records processes occurring in the deep crust beneath the caldera. The onset of volcanic activity correlates strictly with the climax of the growth of the upper mafic complex, when the mafic intrusion invaded fertile crustal levels and the crust was pervasively heated. In the upper crust, igneous activity was dominated by hybrid silicic melts produced by anatexis in the deep crust, but including significant amounts of mantle component. During the life of the volcanic activity, the mafic complex grew from a relatively small but continuously fed magma chamber according to the “gabbro glacier” process. The excursion will transect the entire igneous system, starting from the deepest exposures of the mafic complex up to reach the outcrops of megabreccia within the caldera fill. The Finero area (box b in Fig. A) is characterized by the occurrence of a pervasively-metasomatised mantle unit made by phlogopite-bearing ultramafic rocks (i.e. spinel-facies harzburgites, dunites and pyroxenites). These rock types were produced by several episodes of pervasive-to-channeled porous flow migration of K-LILE-Mg-enriched hydrous melts containing large amount of crustal components. The mantle unit is surrounded by a layered mafic-ultramafic intrusion, i.e. the Finero mafic complex, comprising garnet hornblendites, cumulus amphibole peridotites, amphibole gabbros and diorites, with tholeiitic to transitional geochemical affinity. Recent U-Pb zircon data point to a Middle-Triassic intrusion age for the Finero mafic complex, which may thus represent the deep-crustal counterparts of the Middle-Upper Triassic volcanism widespread throughout the Southern Alps. In any case, the Finero mafic complex can no longer be considered as part of the Permian mafic complex exposed in the Sesia area. Instead, U-Pb ages of zircons from massive chromitites of the mantle unit are Lower Jurassic. The marked age span of the Finero mafic complex and the associated mantle unit suggests that they experienced different evolutions until Lower Jurassic, and were subsequently tectonically juxtaposed during the opening of the Jurassic Neo-Tethys or later. The geodynamic setting related to the intrusion of the Finero mafic complex, the sources, the age and geodynamic environment of the metasomatism of the mantle unit and the age of the emplacement of the latter in contact with the crustal rocks are some of the issues that will be discussed in this guide. This excursion aims to illustrate the Sesia-type IVZ as a complete sequence of a section from the mantle to a supervolcano and show the different geochronological, petrographic and ge

2014 - Is the Ivrea-Verbano Zone subdivided in two sectors? New insights from the Kinzigite Formation exposed in the Val Cannobina (northern Italy) [Abstract in Rivista]
A., Langone; M., Tiepolo; A., Zanetti; Mazzucchelli, Maurizio; C., Bergamaschi
abstract

The Ivrea-Verbano Zone (IVZ, Southern Alps), in northern Italy, is part of the pre-Alpine basement characterized by the exposure of a well-preserved lower crustal section. The middle-lower crustal rocks of the IVZ have commonly been subdivided in three major units, from bottom to top: i) different ultramafic mantle bodies; ii) the Mafic Complex (MC), which represents an intrusive complex (gabbros, norites and diorites); iii) the Kinzigite Formation (KF), consisting of amphibolite- to granulite-facies metapelites with subordinate metabasites, marbles and calc-silicate gneisses. U-Pb zircon data on the MC indicate an emplacement at 295-280 Ma in the central sector of the IVZ (Peressini et al., 2007, and references therein) and at about 232 Ma in the north-eastern sector (External Gabbro, Finero Complex; Zanetti et al., 2013). The Permian intrusion was coeval with granite intrusions in the middle-upper crust and rhyolitic volcanism, while the Triassic intrusion was concurrent with the activity of (low-/high-temperature) shear zones. In order to shed light on the observed diachronism of the MC and to provide useful insights on the tectonic evolution, new P-T-t data from the KF exposed in the Val Cannobina (Cursolo-Orasso area) were carried out. Here, the KF shows the narrowest thickness and is partially affected by mylonitic deformation. It mainly consists of micaschists and migmatitic paragneisses, dominant at the higher structural levels (SE), and amphibolites, prevailing at the contact with the MC (lower structural levels; NW; Boriani & Burlini, 1995). Septa, both mafic and metapelitic in composition, occur within the MC and show granulite-facies metamorphism. Metapelites are mainly composed of Pl, Qtz, Bt, Grt and Sil. With increasing the metamorphic grade (towards NW), the abundance of Bt decreases, that of Grt increases andfibrolitic Sil is progressively replaced by prismatic Sil. Preliminary P-T estimates on metapelites indicate P of about 0.4 GPa and T ranging from 620 to 700°C. The septa show T higher than 800°C. Preliminary U-Pb dates on monazite fromthe migmatitic paragneisses zone indicate that an HT metamorphic event occurred at about 277 Ma, coherently with other areas of the IVZ. A single age at 225±7 Ma obtained from a narrow rim suggests a later perturbation of the U-Pb system. Boriani A. & Burlini L. 1995. Carta Geologica della Valle Cannobina. Scala 1:25000. Centro di Studio per la Geodinamica Alpina e Quaternaria del CNR-Milano. Grafica Diodoro, Milano. Peressini G., Quick J.E., Sinigoi S., Hofmann A.W. & Fanning M. 2007. Duration of a large mafic intrusion and heat transfer in the lower crust: a SHRIMP U/Pb zircon study in the Ivrea-Verbano Zone. J. Petrol. 48, 1185-1218. Zanetti A., Mazzucchelli M., Sinigoi S., Giovanardi T., Peressini G. & Fanning M. 2013. SHRIMP U-Pb zircon Triassic intrusion age of the Finero Mafic Complex (Ivrea-Verbano Zone, Western Alps) and its geodynamic implications. J. Petrol., 54, 2235-2265.

2014 - Mafic dykes from the São Francico Craton, Brazil: ages and inferences regarding the mantle sources [Articolo su rivista]
Girardi, V. A. V.; Teixeira, W.; Mazzucchelli, Maurizio; Oliveira, E. P.; Corrêa da Costa, P. C.
abstract

Mafic dyke swarms crosscut Archean to Paleoproterozoic country-rocks of the São Francisco craton. The Uauá and Lavras regions include norite (2.73 Ga and 2.66 Ga) and tholeiite dykes (2.62 Ga and 1.97 Ga) respectively. The other swarms are made up of tholeiites: Paraopebas (2.10 Ga), Pará de Minas (1.71 Ga), Curaçá and Chapada Diamantina (1.5 Ga), Diamantina (0.93 Ga) and Salvador-Olivença (0.92 Ga). Geochemical and Sr-Nd isotopic data indicate variable scale of mantle heterogeneity, caused by recurrent metasomatic effects since Archean times, mainly due to the action of slab fluids from recycling of oceanic crusts and overlying sediments on the sub-lithospheric mantle, and by upwelling of OIB-like material. These processes are consistent with the protracted geologic history of the country rocks, given by episodic juvenile accretion and eventual assembly from Archean to ca. 2.0 Ga leading to emplacement of dyke swarms under transpression and/or extensional tectonics. The adopted mixing model, supported by geochemical and isotopic data, permits to evaluate the variation range of slab fluids, OIB and MORB components on the parental mantle composition of the studied dykes.

2014 - Melting of Plume Residue beneath the Afar: Implications for Axial Basalts Geochemistry [Abstract in Rivista]
Barbieri, Emiliano; Brunelli, Daniele; Cipriani, Anna; Mazzucchelli, Maurizio
abstract

The Afar is the place to investigate both the evolution of the lithosphere from continental break up to incipient seafloor spreading and the interaction between rifting processes and a mantle plume. The plume has been evoked as one of the main factors involved in the development of the Afar, but its persistence beneath the depression is still a matter of debate. Recent studies have shown the lack of a well developed plume structure beneath the Afar (Hammond et al., 2013), thus suggesting its partial exhaustion. Geophysical investigations hint at an uppermost mantle dominated by broad asthenospheric upwelling (Rychert et al., 2012; Hammond et al., 2013) affected by decompression melting, feeding the magma chambers stored within both the crust and mantle, and the Afar plume magmatism. However, modern basalts erupted along the northern Afar show a strong enrichment in incompatible and trace elements that partially disagree with a shallow depleted mantle reservoir. Recent geochemical analyses indicate that part of the mantle melting process, still occurs at greater depths (> 80 km) (Ferguson et al., 2013) and several authors suggest the presence of focused diapiric upwelling (Hammond et al., 2013), which probably enhances the melting at greater depths. EMPA and LA-ICP-MS were used to investigate the composition of modern lavas sampled from the Erta Ale Chain and the Asal region in 2011 and 2013. The plume markers are recognizable but with less intensity compared to the Oligocene High Ti lavas, according to the lower activity of the Afar hot spot. Our results suggest a hybrid source characterized by two main reservoirs: an enriched mantle melting in the grt field and a shallow depleted mantle. We elaborated a numerical model that predicts the composition of the axial basalts through the mixing of melts obtained by melting of these theoretical reservoirs. We propose for the Afar region the presence of isolated volumes of enriched material genetically related to the remains of the plume. These bodies generate enriched melts that pollute the liquids obtained by the surrounding asthenosphere before they reach the surface. Isotopic investigations are in progress and will help to better define the involvement of each reservoir. Ferguson D.J., Maclennan J., Bastow I.D., Pyle D.M., Jones S.M., Keir D., Blundy J.D., Plank T. & Yirgu G. 2013. Melting during Late-Stage Rifting in Afar Is Hot and Deep. Nature 499 (7456) (July 4), 70–3. Rychert C.A., Hammond J.O.S., Harmon N., Kendall J.M., Keir D., Ebinger C.J., Bastow I. D., Ayele A., Belachew M., & Stuart G. 2012. Volcanism in the Afar Rift Sustained by Decompression Melting with Minimal Plume Influence. Nature Geoscience 5 (6), 406–409. Hammond J.O.S., Kendall J.M., Stuart G.W., Ebinger C.J., Bastow I.D., Keir D., Ayele A., Belachew M., Goitom B., Ogubazghi G. & Wright T.J. 2013. Mantle Upwelling and Initiation of Rift Segmentation beneath the Afar Depression. Geology 41(6), 635–638.

2014 - Occurrence of Phlogopite in the Finero Mafic Layered Complex [Articolo su rivista]
Giovanardi, Tommaso; Mazzucchelli, Maurizio; A., Zanetti; A., Langone; M., Tiepolo; Cipriani, Anna
abstract

Phlogopite-bearing lithologies are the main constituent of the Phlogopite-Peridotite unit of the Finero sequence and the result of pervasive migration of metasomatizing melts/fluids. Conversely, the presence of phlogopite within the associated Finero Mafic Complex, a mafic-ultramafic pluton intruded into the metamorphic basement of the Adria plate, is mentioned in literature as rare. Recent detailed fieldwork has evidenced the presence of two distinct phlogopite-rich ultramafic lithologies within the Amphibole-Peridotite unit of the Finero Mafic Complex, where phlogopite is always associated with amphibole. Field and petrographic features of these occurrences, as well as major- and trace-element mineral chemistry, are here presented to i) place constraints on the nature of the parent melt from which they have been generated and ii) to address their relationship with the other lithologies of the Finero Complex. We find that these rocks were formed by late melt migrations along shear zones under high-T conditions. The geochemical affinity of these lithologies is different to the tholeiitic-transitional affinity reported in literature for the Finero Mafic Complex. The enrichment in LREE, Th, U and Sr of the associated amphibole possibly suggests that these phlogopite-bearing lithologies are genetically related to the metasomatic events that have affected the Finero mantle massif.

2014 - Sources, migration mechanisms and geodynamic environment of K-LILE-Mg-rich melts: Evidence from the Finero Complex (Southern Alps) [Abstract in Rivista]
A., Zanetti; Giovanardi, Tommaso; Mazzucchelli, Maurizio; M., Tiepolo; L., Dallai; F. Y., Wu; T., Morishita; A., Langone; R., Vannucci
abstract

The Finero Complex is located in the northern sector of the Ivrea-Verbano Zone (IVZ, Southern Alps). It outcrops with an antiformal structure showing a mantle unit at the core and a layered mafic-ultramafic intrusion on the flanks, i.e. the Finero Mafic Complex. The youngest unit of the latter (i.e. the External Gabbro) emplaced at the bottom of the continental basement at 232±3 Ma (Zanetti et al., 2013). The Finero Complex records the unique worldwide example of subcontinental lithospheric mantle column in offcraton setting extensively metasomatised with segregation of phlogopite-bearing mineral assemblages. Presently, there is a large consensus in considering that the mantle unit experienced a multi-stage melt migration in a supra-subduction environment (Zanetti et al., 1999). However, the sequence of metasomatic stages and the mechanisms of melt migration are poorly constrained. Besides, the nature of the crustal component(s) (oceanic vs. continental crust) occurring in the ascending melts is still controversial, rendering doubtful the geodynamic reconstruction. This contribution is aimed at providing new data about field relationships, petrographic features, major and trace elements mineral chemistry of the main lithologies of the mantle unit (e.g. phlogopite-amphibole-bearing harzburgites, dunites with chromitite and pyroxenite bands, phlogopite-bearing websterite, orthopyroxenites), as well as about the O isotope mineral composition and in-situ U-Pb and Lu-Hf isotope data for zircons from chromitite layers. Our investigation points out that the mantle unit experienced a virtually complete metasomatic recrystallization as a results of a discrete number of episodes of pervasive-to-channelled porous flow migration of hydrous melts, alternated with stages of melt migration in open fractures. The latter mechanism formed pyroxenites usually containing Opx, Cpx, Amph and Phl. Both peridotites and pyroxenites display a similar geochemical signature, characterized by low contents in Al, Ti, Nb, Ta, HREE and Y, associated to large content in Mg, K, Th, U, Sr, Pb, Ba and LREE. The finding of δ18OOpx vs. SMOW‰ up to +8 and of negative εHf in chromitite zircons suggest that large volumes of crustal component were present in the migrating melts. New U-Pb zircon data for the chromitite layers provide Lower Jurassic ages. The sources of the migrating liquids, the age and the geodynamic environment of the mantle metasomatism, as well as the age of the crustal accretion of the mantle unit will be addressed. In particular, it will be stressed out the possibility that the K-LILE-Mg-enriched melt migration took place in a late-orogenic environment, similarly to the high-MgO ultrapotassic, lamproitic magmatism widespread in different Mediterranean areas from Oligocene to Pleistocene in association with shoshonitic and calk-alkaline rocks. Zanetti A., Mazzucchelli M., Rivalenti G., Vannucci R. 1999. The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism. Contrib. Mineral. Petrol., 134, 107-122. Zanetti A., Mazzucchelli M., Sinigoi S., Giovanardi T., Peressini G., Fanning M. 2013. SHRIMP U-Pb Zircon Triassic Intrusion Age of the Finero Mafic Complex (Ivrea-Verbano Zone, Western Alps) and its Geodynamic Implications. J. Petrol., 54, 2225-2265.

2013 - Igneous sapphirine as a product of melt-peridotite interactions in the Finero Phlogopite-Peridotite Massif, Western Italian Alps [Articolo su rivista]
Giovanardi, Tommaso; T., Morishita; A., Zanetti; Mazzucchelli, Maurizio; R., Vannucci
abstract

Sapphirine is generally interpreted to be of metamorphic origin in high-MgO-Al2O3 rocks. Igneous sapphirine, i.e. sapphirine crystallised from melt, is very rare. We examined sapphirine-bearing magmatic veins in the Finero Phlogopite- Peridotite Massif, Western Italian Alps, to investigate a possible igneous origin for sapphirine by crystallisation from a melt, acquiring particularly high Al2O3 content via melt-rock reaction and fractional crystallisation. Sapphirine locally occurs in melanocratic zones placed between leucogabbroic veins and the host peridotite. The leucogabbroic veins cut at high angle the mantle foliation and the lithological layering of the peridotite massif, which is defined by alternating phlogopite-rich harzburgites and pyroxenites. This observation, along with their peculiar major-element mineral chemistry, indicates that leucogabbroic veins were unrelated to the pervasive metasomatic recrystallisation of the host mantle sequence, recording a later, distinct event of melt injection. Melanocratic seams are observed on both sides of the leucogabbroic veins. They show a marked zoning of the mode: an orthopyroxene-rich zone overgrows upon the host peridotite (OPX zone), whereas an amphibole-rich zone occurs towards the leucogabbroic vein (AMPH zone). Sapphirine commonly mantles spinel or occurs as discrete grains, located either (1) within large light-brown pargasite crystals in the AMPH zone or (2) interstitially, between light-brown pargasite in both AMPH zone and OPX zone. Light-brown pargasite can also enclose spinel that does not have sapphirine envelope. To explain the petrochemical features of the sapphirine-bearing veins and the host peridotite a four-stage process involving melt–rock reactions and fractional crystallisation is here proposed. During the first stage (Stage A), the interaction between the uprising SiO2-saturated melt and the host peridotite caused the replacement of peridotite olivine, amphibole and phlogopite by newly formed orthopyroxene close to the contact and Al2O3, TiO2, FeO enrichments in the host peridotite beyond the recrystallisation front. This mineralogical reaction resulted in high Al2O3/SiO2 and MgO/FeO ratios in the migrating melt. The modified melt crystallised Al2O3-rich dark-brown pargasite (16.5 wt% Al2O3) and apatite in the open conduit (Stage B). Sapphirine/spinel saturation was actually achieved later, in the Stage C, in presence of a more differentiated melt, which reacted with the early dark-brown pargasite locally producing pseudo-symplectite textures made by lightbrown pargasite and spinel/sapphirine. A peculiar Al2O3-enriched composition for the parent melt segregating sapphirine is indicated by the composition of the associated light-brown pargasite (17.5 wt% Al2O3), phlogopite and spinel. Locally, this melt percolated also the OPX zone, segregating sapphirine-bearing mineral assemblages. The sapphirine-free leucogabbroic vein was finally segregated during the Stage D, after splitting of the AMPH zone likely due to hydraulic fracturing.

2013 - New insights into the evolution of the Finero Mafic Complex [Abstract in Rivista]
A., Langone; M. R., Renna; M., Tiepolo; A., Zanetti; Mazzucchelli, Maurizio; Giovanardi, Tommaso
abstract

The Finero Complex outcrops as an antiform in the northern sector of the Ivrea-Verbano Zone (Southern Alps). The antiform core is constituted by a mantle unit surrounded by a cumulitic sequence, i.e. the Finero Mafic Complex (FMC) [1,2]. The complex is divided in three units: a) the Layered Internal Zone (LIZ), in tectonic contact with the mantle unit; b) the Amphibole Peridotite (Amph-Pd); c) the External Gabbro. Owing to the lack of a detailed petrochemical characterisation of the FMC, we performed new major and trace element (LA-ICPMS) analyses on representative samples from the LIZ and Amph-Pd. The LIZ mainly consists of Grt-hornblendites and Hbl-gabbros, with minor anorthosites and pyroxenites. The Amph-Pd is mostly made up of Amph-bearing harburgites and dunites (Ol: Fo87-82), with recrystallisation fronts along which the peridotites become modally-dominated by Amph. The Al2O3 content is up to 11 and 18 wt% in Cpx and Amph, respectively: it increases from the peridotites (Amph-Pd) through gabbros to the hornblendites and pyroxenites (LIZ). In the garnet-free pyroxenites and hornblendites from LIZ, Amph and Cpx have slightly LREE-depleted patterns with flat HREE (at 2 CI in Cpx) and marked positive Eu, Sr, Pb and U anomalies. Similar features are shown by the Cpx and Amph from the associated gabbros, they differ in having HREE-depleted patterns, thereby indicating chemical equilibration with garnet. Cpx and Amph from the Amph-Pd have variable LREE-enriched spoon-shaped patterns, with nearly flat HREE-pattern and positive Eu, Sr and U anomalies. The LREE gradient can be explained by interaction with percolating LREE-enriched melts, dominated by ion exchange processes. Amph-enriched peridotites, which contain the highest LREE contents are a proxy for the composition of the percolating melts. The new data suggest that the LIZ and Amph-Pd units may have crystallised from melts of cognate origin with a clear crustal component. However, the recrystallisation of the Amph-Pd cannot be explained by a closed-system evolution, pointing to significant changes in the composition of the uprising mantle melts.

2013 - Petrology and geochemistry of the back-arc lithospheric mantle beneath eastern Payunia (La Pampa, Argentina): Evidence from Agua Poca peridotite xenoliths [Articolo su rivista]
G. W., Bertotto; Mazzucchelli, Maurizio; A., Zanetti; R., Vannucci
abstract

This paper presents the results of new petrochemical studies carried out on mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca volcano in central-western Argentina. Mantle xenoliths studied are shown to be mainly anhydrous spinel lherzolites with minor amounts of harzburgite and banded pyroxenite, showing highly variable equilibrium temperatures ranging from 820°C to 1030°C at 1.0 to 2.0 GPa. This constitutes evidence that the mantle xenoliths are representative of a large portion of the lithospheric mantle column and that the geothermal gradient is not very elevated as reported in some other Patagonian provinces. Geochemical characteristics of clinopyroxene in the mantle xenoliths allow classification into two groups; Groups 1 and 2. Group 1 contains most of the lherzolites and has light-REE depletion, with slightly positive anomalies of Eu in some samples and extreme Nb and Ta depletion. Group 2 consists of two harzburgitic samples, has flat REE patterns with lower Sm to Lu concentrations, with enriched Sr and negative HFSE anomalies. Based on mineral and residua compositions estimated assuming equilibrium with clinopyroxenes, Group 1 can be considered to be refractory residua after up to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. Group 2 can be considered to be after ca. 13% of partial melting. It is inferred that partial melting events in the lithospheric mantle beneath the Agua Poca occurred in different ages since the Proterozoic, but compared with Group 1, the metasomatic overprint is dominant in Group 2 mantle xenoliths. The calculated melt compositions from Group 2 are interpreted to be transient liquid compositions developed during melt-peridotite interaction, and are different from the host alkaline basalts. The HFSE-depleted composition estimated for the rising melt suggests the presence of a slab-derived component, although the possibility cannot be disregarded (on the basis of present data) that such a geochemical feature is due to segregation of HFSE-bearing minerals during the interaction with the peridotite. Thus, we attribute the metasomatic agent to a basaltic melt and to a minor amount of slab-derived fluids.

2013 - SHRIMP U^Pb ZirconTriassic Intrusion Age of the Finero Mafic Complex (Ivrea-Verbano Zone, Western Alps) and its Geodynamic Implications [Articolo su rivista]
A., Zanetti; Mazzucchelli, Maurizio; S., Sinigoi; Giovanardi, Tommaso; G., Peressini; M., Fanning
abstract

Interstitial, subhedral, slightly pink zircons have been separated from a diorite of the External Gabbro unit of the Finero Mafic Complex, outcropping in the northernmost tip of the Ivrea-Verbano Zone (IVZ, Southern Alps). They are characterised by oscillatory zoning at cathodoluminescence (CL), high Th/U ratio (0.9-0.4), associated to high U, Th, Pb, REE, Ta, Nb, Li and P contents, consistent with a magmatic nature. The mean of the single concordant ages is 232±3 Ma, which is here proposed to constrain the timing of intrusion of the Finero Mafic Complex. Most of these magmatic zircons exhibit an outer rim, up to 50-μm thick, with a bright, “white pest”-like CL emission, with transgressive and/or sub-concordant contacts to the internal, oscillatory-zoned domains, in which the Th/U ratio is decreased, as well as the U, Th, Pb, REE, Ta and Nb contents. They result in younger, slightly discordant 206Pb/238U ages, between 219±3 Ma to 205±3 Ma, which are interpreted to highlight a fluid-assisted recrystallisation event. Gabbros also showed an interstitial zircons population consisting of rounded, small, colourless, bright-to-milky crystals with blurred patterns in CL. These zircons show low U, Th, Pb, Nb and Ta contents, and low Th/U ratio (down to 0.08). The overall metamorphic character, together with the field evidence of the presence of blocks of paragneiss belonging to the crystalline basement at the roof of the External Gabbro unit, indicate that these zircons are inherited from country crustal rocks. They have U and Pb isotopic compositions strongly altered and discordant, showing upper limit of the recrystallization consistent with the Middle Triassic emplacement age obtained from the magmatic zircons. It is concluded that the Finero Mafic Complex cannot be longer considered as a part of the huge Permian Mafic Complex of the central IVZ. It follows that the intrusive record preserved in the IVZ allows to look at the interplay between the continental crust and mantle melts through time (i.e. Permian through Triassic). In this scenario, the modest crustal assimilation shown by the Finero Mafic Complex can be interpreted in first instance as the consequence of the limited volume of intruded melt (less than 2 km in thickness). However, the possible role played by the decreased fertility of the metamorphic country rocks due to a inferred Permian magmatic event must be better constrained. The intrusion of the Finero Mafic Complex may represent the deep-crustal counterparts of the Ladinian volcanism widespread throughout the Southern Alps. The geodynamic setting of this magmatism is still strongly debated. The results presented in this study, along with several other structural, petrochemical and age heterogeneities documented in literature, suggest the occurrence of a “Finero-type IVZ” (i.e. northern IVZ) and a “Balmuccia-type IVZ” (i.e. central IVZ), which underwent different magmatic and tectonic evolutions.

2013 - Sr-Nd constraints and trace-elements geochemistry of selected Paleo and Mesoproterozoic mafic dikes and related intrusions from the South American Platform: Insights into their mantle sources and geodynamic implications [Articolo su rivista]
V. A. V., Girardi; W., Teixeira; Mazzucchelli, Maurizio; P. C., Corrêa da Costa
abstract

The study of selected mafic intrusions from cratonic areas of the South American Platform shows considerable differences among their mantle sources and geodynamic features, particularly regarding the dikes from the SW Amazonian and São Francisco Cratons. The tholeiites from the SW Amazonian Craton, which belong to the Serra da Providência Intrusive Suite (1.55 Ga), the Nova Lacerda swarm (1.44 Ga), the Colorado Complex (1.35 Ga), and the Nova Brasilândia Group (1.10 Ga), originated from a mantle source composed mainly of a N-MORB end-member, with a variable addition of slab fluids (up to 30%, according to the adopted model) from oceanic lithosphere due to episodic subductions during the Mesoproterozoic. Mafic intrusions from Nova Lacerda swarm and Colorado Complex are related to arc settings formed during the 1.47e1.35 Ga closure of the oceanic domain separating the Amazonian Craton and the Paraguá Terrane, whereas the tholeiites from the Serra da Providência Intrusive Suite and the Nova Brasilândia Group are considered intracratonic. The dike swarms of the São Francisco Craton are associated with intra-plate events. The inferred composition of the mantle source of the Lavras swarm (1.9 Ga) has a predominant E-MORB signature, and a modest contribution of up to 10% of an OIB component. The mantle composition underwent considerable changes during the Proterozoic, as indicated by the sources of the younger dikes, represented by the Diamantina (0.93 Ga) and the Salvador eOlivença swarms (0.92 Ga), to which considerable amount of slab derived fluids, probably from recycled crustal material, and OIB component were added. Changes in mantle composition and dikes intrusions could be related to the initial disruption of the Rodinia Supercontinent. The Florida (1.79 Ga) and Tandil (2.0 Ga) dikes are associated with extensional events of the Rio de La Plata Craton. In spite of the similarities between the tectonic framework of these swarms and that of Lavras (1.9 Ga), the composition of the Paleoproterozoic mantle of both cratons is very different. The parent mantle of the Florida dikes is more enriched and more heterogeneous than the others, probably due to the recycling of old crusts and OIB metasomatism. Isotopic data from the Tandil dikes also suggest an enriched original source, which could be a characteristic feature of the mantle of the Rio de La Plata Craton in Paleoproterozoic times. The CrixáseGoiás (2.49 Ga) swarm originated from a very heterogeneous mantle source, probably contaminated by OIB metasomatic fluids and crustal addition, suggesting that crustal recycling has occurred since Archean times.

2013 - U-Pb and Hf isotopes in zircons from mantle chromitites of the Finero Peridotite (Ivrea Verbano Zone) [Abstract in Rivista]
Giovanardi, Tommaso; A., Zanetti; Mazzucchelli, Maurizio; M., Tiepolo; F. W., Wu; A., Langone; R., Vannucci
abstract

The Finero Phlogopite Peridotite unit (hereafter Ph-Pd, Ivrea-Verbano Zone, IVZ: Southern Alps) is the unique worldwide example of orogenic mantle massif completely constituted by phlogopite-bearing ultramafics (mainly harzburgites, websterites and dunites) after pervasive to channelled melt migration events. A precious opportunity to provide further constraints on the petrologic and geodynamic evolution of such a mantle sequence is given by the occurrence of zircons in chromitites usually composed by Chromite+Orthopyroxene±Clinopyroxene±Amphibole. Zircons show low CL emission and are generally homogeneous. Few grains display cores slightly darker than the rims. Zircons have up to 1420 and 800 ppm of U and Th, respectively, with Th/U ratio up to 1.6. U-Pb LA-ICP-HRMS analyses yield most concordant Lower Jurassic dates with a weighted average 206Pb/238U age at 187±2 Ma. A few darker cores yield Middle Triassic concordant 206Pb/238U ages from 242±7 Ma to 229±7 Ma. Zircons also show 176Hf/177Hf ratios in the range of 0.282486-0.282610, which give subchondrititic epsilon Hf(188) (-6.0 to -1.6). The relatively high U and Th contents and the large Th/U ratios are the evidence that the chromitite zircons crystallised from a melt, which, according to the low epsilon Hf(188) values, had a marked crustal signature. This melt is analogous to that at the origin of the phlogopite harzburgites and websterites. The absence of internal zoning in zircon is interpreted as the result of homogenisation after a prolonged residence at high temperature mantle conditions. In this frame, the Lower Jurassic ages are proposed to date the cooling of the mantle sequence during exhumation for the opening of the Alpine Tethys. Conversely, the few Middle Triassic dates could represent the age of mantle metasomatism. As a whole, our data strongly support that the Finero area experienced a different geodynamic evolution with respect to the rest of IVZ.

2013 - U–Pb (ID-TIMS) baddeleyite ages and paleomagnetism of 1.79 and 1.59 Ga tholeiitic dyke swarms, and position of the Rio de la Plata Craton within the Columbia supercontinent [Articolo su rivista]
W., Teixeira; M. S., D'Agrella Filho; M. A., Hamilton; R. E., Ernst; V. A. V., Girardi; Mazzucchelli, Maurizio; J. S., Bettencourt
abstract

The Tandilia Terrane (southernmost fringe of the Rio de la Plata Craton) is an igneous and metamorphic complex produced by an accretionary orogeny (2.25–2.02 Ga). Calc-alkaline acidic dykes with E–W strike and a major shear zone with similar orientation are related with the late orogeny stage, as supported by field relations. In a previous study the acid dykes gave 40Ar–39Ar ages of 2007±24 Ma to 2020±24 Ma. A N and NW trending tholeiitic dyke swarm (Tandil swarm) is also present in the Tandilia Terrane. One sample from the NW-trending subset previously gave a U–Pb (ID-TIMS) baddeleyite age of 1588±11 Ma. New precise U–Pb (ID-TIMS) baddeleyite dating of both N- and NW-trending Tandil dykes yielded crystallization ages of 1589±3 Ma, 1588±3 Ma and 1588±3 Ma. Significantly older tholeiitic dykes known as the Florida swarm occur in the Northern Rio de la Plata Craton, for which a U–Pb (ID-TIMS) baddeleyite age of 1790± 5 Ma was previously reported. Consequently intermittent rifting (1.79, 1.59 Ga) took place after tectonic stabilization of the late Paleoproterozoic lithosphere (proto-Rio de la Plata Craton). The available geochemical data for the 1.59 Ga Tandil dykes define low- and high-TiO2 trends, although, only the low-TiO2 subgroup is firmly dated. Both the Tandil and Florida dykes have geochemical and Nd–Sr characteristics consistent with derivation from heterogeneous mantle sources that underwent metasomatic effects. The Tandil dykes may be linked with the 1.57±0.02 Ga Capivarita anorthosite which occurs to the east of the northern part of the craton. Correlatives on other crustal blocks may include those in Baltica such as bimodal rock association (including the Breven-Hällefors and Åland-Åboland diabase dykes) and in the reconstructed Gawler Craton/NW Laurentia dolerites, bimodal magmatism and IOCG deposits. Contemporary within-plate bimodal associations are also present in the SW Amazonian Craton. Paleomagnetic data for the 1790 Ma Florida dykes allows three possible reconstruction scenarios for the position of Rio de la Plata Craton: i) in the southern hemisphere linked to the Pampia terrane, and the Amazonia+Rio Apa landmass as close as they are in present day; ii) in the northern hemisphere, as a nearest neighbor to the reconstructed NW Laurentia and Gawler blocks (proto-Australia); or iii) in the northern hemisphere near the boundary of Amazonia (proto-Amazonian Craton) and Baltica. Positions 2 and 3 are consistent with all three criteria: paleomagnetic poles and proximity to matching ca. 1790 Ma and 1590 Ma mafic magmatism. Our data are consistent with the idea that the Columbia supercontinent started major break up at 1.59 Ga.

2012 - Diopside-titanian pargasite intergrowth: crystallography and formation mechanism. [Abstract in Rivista]
Tarantino, S. C.; Zanetti, A.; Mazzucchelli, Maurizio; Zema, M.; Heidelbach, F.; Miyajiima, N.; Ghigna, P.; Olivi, L.; Gasparini, E.
abstract

2012 - Evidence for the development of a convergent setting in the Southern Alps domain during the early Mesozoic: insights from the Finero Complex (Ivrea-Verbano Zone) [Abstract in Rivista]
Giovanardi, T.; Zanetti, A.; Mazzucchelli, Maurizio; Tiepolo, M.; Vannucci, R.; Morishita, T.
abstract

The Finero Mafic-Ultramafic Complex is located in the northernmost sector of the Ivrea-Verbano Zone (hereafter IVZ, Southern Alps). It consists of a pervasively metasomatized dunitic-harzburgitic phlogopite-amphibole-rich mantle unit surrounded by a layered and strongly hydrous mafic-ultramafic pluton (the Finero Mafic Complex) that underplated the lower crust of the Adria plate. A number of different geodynamic scenarios, among which i) aborted rifting processes, ii) mantle plume activity and iii) development of a subduction zone, have been proposed to account for the mantle metasomatism and the melts intrusion in the Finero area. All these scenarios, however, are commonly considered from pre-Hercynian to Permian in age, in analogy with the petrogenetic processes which occurred in the central and southern sectors of IVZ. In this contribution, new geochronological and petrochemical data are presented, along with a review of the literature age determinations, which suggest that the metasomatic events of the Finero mantle unit, as well as the emplacement of the layered intrusion, occurred over a time span covering Middle Triassic to Lower Jurassic. Trace element and isotopic evidence point to the occurrence of large amount of crustal component in the melts migrating through the mantle unit, which, consistently with regional structural features, has been proposed to be related development of roll-back subduction during the early Mesozoic. In this scenario, the intrusion of the Finero Mafic Complex predates the mantle metasomatism and occurred during Upper Anisian-Ladinian, as a consequence of the uprising of melts produced by large degrees of fluid-assisted partial melting in a supra-subduction regime. The mantle unit and the mafic-ultramafic complex were tectonically juxtaposed later on, possibly during the opening of the Middle Jurassic Neo-Tethys.

2012 - The growth of large mafic intrusions: comparing Niquelândia and Ivrea igneous complexes [Articolo su rivista]
Correia, C. T.; Sinigoi, S.; Girardi, V. A. V.; Mazzucchelli, Maurizio; Tassinari, C. C. G.; Giovanardi, T.
abstract

The Niquelândia Complex, Brazil, is one of theworld's largestmafic–ultramafic plutonic complexes. Like the Mafic Complex of the Ivrea-Verbano Zone, it is affected by a pervasive high-T foliation and shows hypersolidus deformation structures, contains significant inclusions of country-rock paragneiss, and is subdivided into a Lower and an Upper Complex. In this paper, we present new SHRIMP U–Pb zircon ages that provide compelling evidence that the Upper and the Lower Niquelândia Complexes formed during the same igneous event at ca. 790 Ma. Coexistence of syn-magmatic and high-T subsolidus deformation structures indicates that both complexes grew incrementally as large crystal mush bodies which were continuously stretched while fed by pulses of fresh magma. Syn-magmatic recrystallization during this deformation resulted in textures and structures which, although appearing metamorphic, are not ascribable to post-magmatic metamorphic event(s), but are instead characteristic of the growth process in huge and deep mafic intrusions such as both the Niquelândia and Ivrea Complexes. Melting of incorporated country-rock paragneiss continued producing hybrid rocks during the last, vanishing stages of magmatic crystallization. This resulted in the formation of minor, late-stage hybrid rocks, whose presence obscures the record of the main processes of interaction between mantle magmas and crustal components, which may be active at the peak of the igneous events and lead to the generation of eruptible hybrid magmas.

2011 - Insights into the geodynamic evolution of the Finero mafic-ultramafic sequence (Southern Alps). [Abstract in Rivista]
Giovanardi, T.; Zanetti, A.; Mazzucchelli, Maurizio; Morishita, T.; Tiepolo, M.; Vannucci, R.
abstract

The Finero sequence is located in the northernmost sector of the Ivrea-Verbano Zone (Southern Alps) in contact with the Insubric line. It consists of a strongly-metasomatised mantle body, surrounded by a mafic-ultramafic intrusive sequence [1,2,3], documenting the alternation of mantle and crustal rocks placed at the bottom of the Adria plate before the opening of Ligurian-Piedmontese branch of the Jurassic Tethys. The metasomatisedperidotite is enriched in phlogopite and LILE, being considered by several authors as related to the migration of melts containing significant slab-derived components.Unlike the intrusive sequences of the central and southern sectors of the Ivrea-Verbano Zone, characterised by Permo-Carboniferous emplacement ages, the Finero massif shows abundant radiometric evidence of intrusion of basic melts at the bottom of the continental crust during Trias, which formed the cumulitic sequences of the so-called Basic Complex of Finero. Besides, in Triassic times, the mantle sequence of Finero suffered a virtually complete metasomatic recrystallisation triggered by several episodes of pervasive to channelled porous flowmigration of (mostly hydrous) melts. Later on, but yet in Triassic time, the mantle sequence experienced the intrusion of basic veins-dykes (locally characterised by the presence of sapphirine), which discordantly cut the mantle foliation. Thus, the mafic–ultramafic Finero sequence represents a unique opportunity to characterise the composition of Triassic melts migrating through the Adria realm escaping significant interaction with the continental crust. Notwithstanding that several papers have been devoted to the petrologic investigation of themafic-ultramafic Finero sequence since the beginning of the seventies, its petrochemical and geodynamic evolution is presently very poorly constrained. Crucial issues still debated are: 1) the sources of the liquids that percolated the mantle sequence, the timing and geodynamic setting of the mantle metasomatism; 2) the age of accretion of the mantle sequence to the bottom of the continental crust; 3) the geochemical composition of the parent melts of the Basic Complex, their differentiation processes, the timing of the different melt injections and their potential relationships with the melt-related events recorded by the associated mantle sequence. In the frame of this contribution, new data about the major and trace mineral chemistry of the three main units of the Basic Complex(i.e Internal Gabbro, Amphibole Peridotite, External Gabbro) and of the various peridotitic (e.g. phlogopite harzburgites, dolomite-apatite-bearing wehrlites, dunites with chromitites bands and/or pyroxenite-hornblendite veins), pyroxenitic (e.g. phlogopite-bearing websterite, orthopyroxenites, clinopyroxenites) and femic (e.g. sapphirine-bearing amphibole gabbros) lithologies of the mantle sequence will be provided, in order to constrainthe geodynamic setting of the melt-related processes.

2011 - Jurassic U-Pb ELA-ICP-MS zircon ages for segregation of huge chromitite layers in the Finero mantle body: new insights into the geodynamic evolution of the Southern Alps. [Abstract in Rivista]
Giovanardi, T.; Zanetti, A.; Tiepolo, M.; Mazzucchelli, Maurizio; Vannucci, R.
abstract

The mafic-ultramafic Finero complex represents the northernmost sectorof the Ivrea-Verbano Zone (IVZ), Southern Alps. It consists of a mantlebody, surrounded by a mafic-ultramafic intrusive sequence [1],documenting an association of mantle and crustal rocks inferred to beplaced at the bottom of the continental crust of the Adria plate before theopening of Ligurian-Piedmontese branch of the Jurassic neo-Tethys.Nevertheless, the Finero complex shows several tectonic andpetrochemical differences with respect to those of the central IVZ. Forinstance, published geochronological U-Pb zircon data constrain theemplacement of the Mafic Complex in the central sector of the IVZ at280-to-295 Ma ([2] and references therein). Conversely, the intrusion ageof almost part of the Mafic Complex in the Finero area (SHRIMP U-Pbzircon dating) indicate that the External Gabbro, the largest unit formingthe Finero Mafic Complex, intruded the bottom of the Kinzigite Formationduring Ladinian [3]. A Triassic ages have been also determined for themultiple metasomatic and intrusive events affecting the associated mantlebody ([4] and references therein). This is mainly formed by harzburgitemodally enriched in phlogopite and amphibole and with large LILEcontents. Based on the geochemical and isotopic composition, severalauthors proposed that the growth of phlogopite-bearing assemblages wasrelated to the migration of melts containing significant crustal-derivedcomponents. With the aim to place constraints on the timing of meltmigration through the Finero mantle body, zircons were separated fromthree, dm-thick chromitite bands enclosed in huge dunite bodiesoutcropping along the Cannobino River and on the Mt. Sasso Rosso. U-PbELA-ICP-MS data result in concordant ages comprised in the range of188-186 Ma, which is believed to date the segregation of the chromititebands during channeled porous-flow melt migration. These ages aresignificantly younger than that determined through conventionalmultigrain dating by [4], whose data define an intercept Triassic age of208±2 Ma. Although the studied chromitite bands do not show primaryphlogopite, the major and trace element mineral chemistry point to acognate origin with the migrating melts forming the phlogopiteharzburgite. The finding that the porous-flow ascent of LILE-enrichedmelts through the Finero mantle unit postdates the emplacement of theFinero Mafic Complex, and the absence in the latter of the record of thismagmatic stage, suggest that the Finero mantle unit might have beenemplaced tectonically in contact with the lower crust at some time afterlower Jurassic. This hypothesis is supported by the observation that thecontact between mantle unit and Mafic Complex is always tectonic. As awhole these data confirm that the northernmost part of the IVZ,represented by the Finero complex, cannot be longer considered as a partof the central IVZ, thus suggesting that the entire geological setting of thearea should be revised.

2011 - Magmatic sapphirine from gabbroic veins cutting the Finero mantlesequence (Southern Alps): Petrology, geochemistry and geodynamiccontext. [Abstract in Rivista]
Giovanardi, T.; Zanetti, A.; Mazzucchelli, Maurizio; Morishita, T.; Tiepolo, M.; Vannucci, R.
abstract

A late swarm of sapphirine-bearing amphibole gabbroic veins discordantly crosscut the main layering of the Finero phlogopite-peridotite massif, Western Italian Alps. Sapphirine locally occurs in a melanocratic zone placed between a leucocratic gabbroic band, forming the central part of the intrusion, and the host peridotite. The melanocratic zones are observed on both sides of the leucocratic gabbroic vein and consist of (i) an outer orthopyroxene-rich zone along the host peridotite and (ii) an inner amphibole-rich zone placed along the leucocratic gabbroic band side. Sapphirine either overgrows spinel or occurs as isolated inclusion within large amphiboles in the amphibole-rich melanocratic zone. Spinels without sapphirine envelopes also microtexturally co-exist with independent sapphirine grains. EMPA and LA-ICP-MS analyses of minerals from the melanocraticand leucocratic bands evidence significant differences in terms of both major and trace elements in the composition of the parent melts. In particular, the amphiboles in the melanocratic zones show higher TiO2, Na2O and K2O, M-HREE and HFSE than those in the leucocratic ones. The Al2O3 content of amphibole and the Fo in olivine of the host peridotite are significantly higher and lower, respectively, than those in other Finero peridotites far from the amphibole gabbroic veins. Moreover, amphiboles from the host peridotite are characterised by LREE-enriched convex-upward patterns significantly different with respect to those documented in literature for the Finero phlogopite-peridotites [1]. Mineral assemblages and mineral chemistry in both the melanocratic zone and the host peridotites are interpreted as the result of different stages of melt migration associated withmelt-rock interaction. In particular, the major and trace element compositions of the amphiboles from the wall peridotite suggest that during an early stage, possibly before the opening of the conduits, the peridotite suffered porous flow migration of a melt more enriched in REE with respect to those forming the gabbroic bands. The crystallisation of the melanocratic bands is related to a second stage, in which the precipitation of large amphiboleswas accompanied by a strong reaction between host peridotite and melt flowing into the conduit that determined the complete substitution of peridotite olivine with orthopyroxene at the peridotite-vein contact. A third stage was characterised by with the precipitation of the leucocratic band, associated to a further enlargement of the vein. Petrographic survey highlights that parent melt of leucocratic zone reacted with the minerals of the melanocratic one, inducing sapphirine growth around spinels. The genetic relationships occurring between the parent melts ofthe melanocratic and leucocratic zones must be yet established. Working hypotheses consider the parent melt of the leucocratic zone either related to a late injection or a residual differentiate after precipitation of melanocratic band in the frame of flow differentiation process. Quantitative considerations suggest that the selective addition of Al-rich phases, like amphiboles and micas, to a basalt can determine the large Al/Si ratios required for sapphirine precipitation. Modelling results indicate that the eutectic T of Finero phlogopite-peridotite is <1000C and thatthe first partial melts are saturated in corundum. Thus, it is proposed that injection of basaltic melts triggered the partial melting of limited volumes of phlogopite peridotite producing Al-rich melts: the mixing of such Al-rich components with the migrating basalt is believed to have played a fundamental role in favouring the precipitation of sapphirine.

2011 - Segregation of igneous Sapphirine in gabbroic veins cutting the Finero mantle sequence (Southern Alps): petrology, geochemistry and geodynamic context. [Abstract in Rivista]
Giovanardi, T.; Zanetti, A.; Mazzucchelli, Maurizio; Morishita, T.; Vannucci, R.
abstract

Sapphirine is generally interpreted to be of metamorphic origin in highMg-Al rocks. Igneous sapphirine, i.e. sapphirine crystallized from melt, isvery rare. We examined sapphirine-bearing rocks in the FineroPhlogopite-Peridotite Massif, Western Italian Alps, to investigate a possibleigneous origin for sapphirine in a melt modified via melt-peridotiteinteraction. Sapphirine locally occurs in a melanocratic zone between aleucogabbroic vein and the host peridotite. The leucogabbroic vein cutsthe foliation and lithologic layering of the peridotite massif, which isdefined by alternating phlogopite-rich harzburgites and pyroxenites,indicating that its emplacement occurred after the main metasomaticevents in the massif. Melanocratic seams are observed on both sides ofthe leucogabbroic vein. These mainly consist of orthopyroxene andamphibole, and show a marked zoning in modal compositions: anorthopyroxene-rich zone overgrown on the host peridotite side (OPXzone), whereas an amphibole-rich zone occurs on the leucogabbroic veinside (AMPH zone). Sapphirine precipitated in the AMPH zone asindependent interstitial grains (up to 3 mm long), as independent grainswithin large amphibole grains or as overgrowth on spinel. The amphiboleswith sapphirine inclusions can also enclose spinel crystals that do not havesapphirine envelopes. Amphibole in the sapphirine-free melanocratic zoneis more abundant in incompatible elements, such as TiO2, and K2O, thanthat in the gabbroic veins and the OPX zone, excluding the development ofdiffusion-controlled subsolidus reaction. A pronounced enrichment inAl2O3 of the parent hydrous melts is indicated by the composition of theamphiboles and phlogopites of this study that show higher Al2O3 thanthose crystallised by basaltic melts. Mineral assemblages and chemistry inboth the melanocratic zone and the host peridotite can be explained bymelt-peridotite interactions, which resulted in replacement of peridotiteolivine by secondary orthopyroxene in the OPX zone, and by Al2O3, TiO2,FeO enrichments in the host peridotite beyond the recrystallization front.Interactions between peridotite and a hydrous, high Al2O3,orthopyroxene-oversaturated, mafic melt related to the formation of theleucogabbroic vein caused the formation of orthopyroxene at the expenseof peridotite olivine. This resulted in high MgO/FeO and high Al2O3/SiO2ratios in a modified melt, allowing for precipitation of igneous sapphirine.

2011 - Tectono-magmatic response to major convergence changes in the North Patagonian suprasubduction system; the Paleogene subduction-transcurrent plate margin transition [Articolo su rivista]
E., Aragon; F., D‘eramo; A., Castro; L., Pinotti; Brunelli, Daniele; O., Rabbia; Rivalenti, Giorgio; R., Varela; W., Spakman; M., Demartis; C. E., Cavarozzi; Y. E., Aguilera; Mazzucchelli, Maurizio; A., Ribot
abstract

The southern and central Andes reflect significant along-strike differences of tectonic activity, including shortening, alternating flat-to-normal subduction styles and magmatism. In northern Patagonia, the subduction/supra-subduction system, fore arc, arc and back arc basins developed in an extensional setting during the Paleogene. This was accompanied by landward migration of calcalkalic magmatism which changed to synextensional bimodal volcanism of rhyolitic ignimbrites and interbedded tholeiitic and alkalic basalts. These Paleogene events occurred during a time when the Farallon-Aluk active ridge reached the South American plate, and the Farallon plate subduction was interrupted. They represent a new tectonic regime, characterized by a transcurrent plate margin. The presence in the back arc of a rigid lithosphericblock of 100,000 km2 represented by the North Patagonian Massif focused the rotation of the coastal blocks. This resulted in the development of two Paleogene extensional regions to the north and south, respectively, of the Massif and replaced the former back arc. Plate rearrangement caused by the inauguration of the Nazca plate and its regime of orthogonal subduction at the beginning of the Miocene, re-established typical calc-alkaline arc magmatism at the former upper Cretaceous arc locus. Present seismic activity in the subducted plate andtomographic modeling of p-wave velocity anomalies in the upper mantle also suggest the presence of a subduction gap that lasted for most of the Paleogene in northern Patagonia.

2011 - U-Pb zircon data for tuffaceous layers from the sedimentary cover of the Strona-Ceneri Zone and Po Plain: constraints on the Triassic geodynamic evolution of the Southern Alps. [Abstract in Rivista]
Giovanardi, T.; Zanetti, A.; Tiepolo, M.; Mazzucchelli, Maurizio; Decarlis, A.; Fantoni, R.; Vannucci, R.
abstract

In the western sector of the Southern Alps, tuffaceous levels areoccasionally found in the Triassic sedimentary sequences. The direct agecharacterisation of these layers is very limited: in fact, a U-Pb zircon ageof 245±1 Ma is available only for the tuffites from Mt. San Giorgio area(Lugano, CH: [1]), which results slightly older than that expectedaccording to the stratigraphic position. Besides, no geochemicalinformation is presently available for these layers and the geodynamicsetting of the related volcanism is still matter of debate.Thus, with the aim to place further constraints of the geodynamicevolution of the Southern Alps in Mesozoic times and increase theaccuracy of the age record of the sedimentary sequence, we haveperformed a mineralogical, geochemical and geochronological study ontuffaceous deposits outcropping within the Anisian-Ladinian succession ofthe sedimentary cover of the Strona-Ceneri Zone in the Borgosesia area[2,3] and recovered at the same stratigraphic position in wells of theVillafortuna-Trecate oil field (western Po Plain, Piedmont region, NW Italy:[4]). CO2 concentration was determined by Dietrich-Früling calcimeter.XRD data indicate that such tuffaceous deposits are constituted by avariable mixture of magmatic and sedimentary components. Major andtrace element compostions, assessed by means of XRF analysis, suggest acalc-alkaline affinity for the magmatic component. To provide accurategeochronological constraints, zircons have been separated withconventional methods from four tuffaceaous outcrops, namely "Lembo diSostegno", "Lembo di Crevacuore" and "Lembo di Monte Fenera"outcrops, as well as from a buried tuffaceous sample of theVillafortuna-Trecate oil field. Zircons were mounted in epoxy resin andcharacterised under cathodoluminescence (CL). Based on colour,morphology and internal structure, they have been divided in twopopulations. One group is constituted by light-pink coloured zircons withprismatic habits and tight oscillatory zoning suggesting growth undermagmatic conditions. The zircons from the second group are colourless,rounded in shape and with only relics of magmatic zoning, consistent withmetamorphic recrystallization. U-Pb ELA-ICP-MS data point to ages of237±8 Ma and 229±9 Ma for the magmatic growth of the zircons from thetuffites of "Lembo di Crevacuore" and "Lembo di Sostegno", respectively,which are in agreement with their stratigraphic position. The volcanicactivity producing these layers might be linked to the intrusive Triassicmagmatism documented in the Finero Complex (NE Ivrea-Verbano Zone[5,6]). Conversely, the zircons from the "Lembo di Monte Fenera" andVillafortuna-Trecate oil field produce a very large range ofcrystallisation-recrystallisation ages, which span from Proterozoic toPaleozoic to the Permian-Triassic boundary, thus indicating a dominantcontribution of the crystalline basement to the zircon population.

2010 - Age and geochemistry of mantle peridotites and diorite dykes from the Baldissero body: Insights into the Paleozoic–Mesozoic evolution of the Southern Alps [Articolo su rivista]
Mazzucchelli, Maurizio; Zanetti, A.; Rivalenti, G.; Vannucci, R.; Correia, C. T.; Tassinari, C. C. G.
abstract

Trace element and isotopic data obtained for mantle spinel lherzolites and diorite dykes from the Baldisseromassif (Ivrea–Verbano Zone, Western Italy) provide new, valuable constraints on the petrologic and geodynamic evolution of the Southern Alps in Paleozoic to Mesozoic times. Whole rock and mineral chemistry indicates that Baldissero lherzolites can be regarded as refractory mantle residues following limited melt extraction. In particular, the Light Rare Earth Elements (LREE)-depleted and fractionated compositions of whole rock and clinopyroxene closely match modelling results for refractory residues after low degrees (~4–5%) of near-fractional melting of depleted mantle, possibly under garnet-facies conditions.Following this, the peridotite sequence experienced subsolidus re-equilibration at lithospheric spinel-faciesconditions and intrusion of several generations of dykes. However, lherzolites far from dykes show verymodest metasomatic changes, as evidenced by the crystallisation of accessory titanian pargasite and theoccurrence of very slight enrichments in highly incompatible trace elements (e.g. Nb). The Re–Os data for lherzolites far from the dykes yield a 376 Ma (Upper Devonian) model age that is considered to record apartial melting event related to the Variscan orogenic cycle s.l.Dioritic dykes cutting the mantle sequence have whole rock, clinopyroxene and plagioclase characterised byhigh radiogenic Nd and low radiogenic Sr, which point to a depleted to slightly enriched mantle source.Whole rock and mafic phases of diorites have high Mg# values that positively correlate with the incompatible trace element concentrations. The peridotite at the dyke contact is enriched in orthopyroxene, iron and incompatible trace elements with respect to the lherzolites far from dykes. Numerical simulationsindicate that the geochemical characteristics of the diorites can be explained by flow of a hydrous, silicasaturated melt accompanied by reaction with the ambient peridotite and fractional crystallisation. Thecomposition of the more primitive melts calculated in equilibrium with the diorite minerals show tholeiiticto transitional affinity.Internal Sm–Nd, three-point isochrons obtained for two dykes suggest an Upper Triassic-Lower Jurassicemplacement age (from 204±31 to 198±29 Ma). Mesozoic igneous events are unknown in the southernIvrea–Verbano Zone (IVZ), but the intrusion of hydrous melts, mostly silica-saturated, have been welldocumented in the Finero region, i.e. the northernmost part of IVZ and Triassic magmatism with calc-alkalineto shoshonitic affinity is abundant throughout the Central–Eastern Alps. The geochemical and chronological features of the Baldissero diorites shed new light on the geodynamic evolution of the Southern Alps before the opening of the Jurassic Tethys.

2010 - INSIGHTS INTO THE TRIASSIC GEODYNAMIC EVOLUTION OF THE ADRIA PLATE: THE STUDY CASE OF THE MAFIC-ULTRAMAFIC SEQUENCE OF FINERO [Abstract in Rivista]
Giovanardi, T.; Zanetti, A.; Mazzucchelli, Maurizio; Tiepolo, M.; Langone, A.; Vannucci, R.
abstract

A strategic lithologic sequence for the study of the Middle-Upper Trias magmatic events in the South-Alpine domain (Adria plate) is outcropping in the Finero area, located in the north-easternmost sector of the Ivrea-Verbano Zone (Western Alps). Such a sequence is located along the Insubric line and is formed by a strongly-metasomatised mantle body, surrounded by a mafic-ultramafic intrusive sequence [1,2,3], which documents the alternation of mantle and crustal rocks placed at the bottom of the Adria plate before the opening of Ligurian-Piedmontese branch of the Jurassic Tethys.Unlike the intrusive sequences of the central and southern sectors of the Ivrea-Verbano Zone, characterised by Permo-Carboniferous emplacement ages, the Finero massif shows abundant radiometric evidence of intrusion of basic melts at the bottom of the continental crust during Trias, which formed the cumulitic sequences of the so-called Basic Complex of Finero. Besides, in Triassic times, the mantle sequence of Finero suffered a virtually complete metasomatic recrystallisation triggered by several episodes of pervasive to channelled porous flow migration of (mostly hydrous) melts. Later on, but yet in Triassic time, the mantle sequence experienced the intrusion of basic veins-dykes (locally characterised by the presence of sapphirine), which discordantly cut the mantle foliation. Thus, the mafic–ultramafic Finero sequence represents a unique opportunity to characterise the composition of Triassic melts migrating through the Adria realm escaping significant interaction with the continental crust. Notwithstanding that several papers have been devoted to the petrologic investigation of the mafic-ultramafic Finero sequence since the beginning of the seventies, its petrochemical and geodynamic evolution is presently very poorly constrained. Crucial issues still debated are: 1) the sources of the liquids that percolated the mantle sequence, the timing and geodynamic setting of the mantle metasomatism; 2) the age of accretion of the mantle sequence to the bottom of the continental crust; 3) the geochemical composition of the parent melts of the Basic Complex, their differentiation processes, the timing of the different melt injections and their potential relationships with the melt-related events recorded by the associated mantle sequence. In the frame of this contribution, new data about the major and trace mineral chemistry of the three main units of the Basic Complex (i.e Internal Gabbro, Amphibole Peridotite, External Gabbro) and of the various peridotitic (e.g. phlogopite harzburgites, dolomite-apatite-bearing wehrlites, dunites with chromitites bands and/or pyroxenite-hornblendite veins), pyroxenitic (e.g. phlogopite-bearing websterite, orthopyroxenites, clinopyroxenites) and femic (e.g. sapphirine-bearing amphibole gabbros) lithologies of the mantle sequence will be provided, in order to constrain the geodynamic setting of the melt-related processes.References. [1] Siena, F., Coltorti, M. (1989): Jb. Miner. Mh., 6, 255-274; [2] Zanetti, A., Mazzucchelli, M., Rivalenti, G., Vannucci, R. (1999): Contrib. Mineral. Petrol., 134, 107-122.; [3] Morishita, T., Hattori, K.H., Terada, K., Matsumoto, T., Yamamoto, K., Takebe, M., Ishida, Y., Tamura, A., Arai, S. (2008): Chem. Geol., 251, 99-111.

2010 - Nd, Sr and Pb isotopic composition of metasomatised xenoliths from the backarc Patagonian MantleWedge: Insights into the origin of the uprising melts [Abstract in Rivista]
Zanetti, A.; Mazzucchelli, Maurizio; Hémond, C.; Cipriani, Anna; Bertotto, G. W.; Cingolani, C. A.; Vannucci, R.
abstract

Information about the geochemical composition of metasomatic melts migrating through the Patagonian mantle wedge is provided by the ultramafic xenoliths occurrence of Tres Lagos (TL; lat. 49.13°S, long. 71.18°W), Argentina.Such a locality is placed at the eastern border of the Meseta de la Muerte backarc basaltic plateau, wherea post-plateau volcanic diatreme contains mantle xenoliths in both pyroclastites and lavas. Its latitude corresponds with the Northern limit of the Austral Volcanic Arc (AVZ), which is separated from the Southern Volcanic Zone (SVZ) by a gap in the arc magmatism ranging between 49° and 46°300 latitude S. The analysed xenoliths have been distinguished into two groups (Group 1 & 2). Group 1 consists of lherzolites and harzburgites, whereas Group 2 is formed by harzburgites. The texture of the Group 1 lherzolites varies from protogranular to granoblastic to porphyroblastic, whereas Group 1 harzburgites have always granoblastic texture. Group 2 harzburgites have granular texture, which may change to porphyroblastic owing to the random concentration of large olivine and orthopyroxene crystals. The clinopyroxenes (Cpx) from Group 1 lherzolites have PM-normalised REE patterns ranging from LREEdepleted (LaN/SmN= 0.24-0.37), to LREE-enriched (LaN/YbN up to 4.08) and spoon-shaped: the latter have minimum at Pr and Pr-Yb concentrations similar to those shown by the LREE-depleted Cpx.The Cpx from Group 1 harzburgites have lower REE concentrations with respect to the lherzolite ones and their REE patterns vary from HREE-enriched, steadily fractionated, (LaN/YbN = 0.21–0.35, Ybn 1-2) to spoonshaped (LaN/SmN = 2.81; SmN/YbN = 0.89; YbN 3. The Cpx from the Group 2 harzburgites have convex-upward (LaN/SmN = 0.31; SmN/YbN = 1.50) to LREE enriched (LaN/YbN = 2.94) patterns.The Sr, Nd and Pb isotopic compositions of the Group 1 clinopyroxenes form arrays spanning from DM to thefield delimited by the TL basaltic lavas, pointing to EMI end-member. Conversely, Group 2 Cpx have much moreradiogenic Sr and less radiogenic Nd values, approaching more closely the EMI and EMII end-members: these features are associated to unradiogenic lead isotopic compositions (206Pb/204Pb = 17.4-18.1; 207Pb/204Pb = 15.55-15.60; 208Pb/204Pb = 37.3-38.5). The combination of petrographic, trace element and isotopic features indicate that TL harzburgites are likely residua after melt-assisted partial melting triggered by melt/fluid migration in the hottest, and perhaps deeper, parts of the pristine DM lithosphere. The interpretation of the Pb, Sr and Ndisotope composition of Group 2 Cpx is not trivial. In analogy with the interpretation proposed for SWIR, it could unravel the occurrence of mantle sources which incorporated ancient crust and failed to homogenise with the DM mantle. Alternatively, it could be the evidence for ancient continental crust of the South America plate dragged down into the mantle by slab motion.

2010 - PETROGENETIC ASPECTS OF THE CONTAMINATION PROCESSES IN THE MAFIC-ULTRAMAFIC COMPLEX OF NIQUELANDIA (GOIÀS, BRASIL). [Abstract in Rivista]
Giovanardi, T.; Rivalenti, G.; Mazzucchelli, Maurizio; Girardi, V. A. V.; Correia, C. T.
abstract

The mafic-ultramafic complex of Niquelandia is one of the three major layered intrusions occurring in the Goiàs State, Central Brasil.Previous studies on this complex [1] have revealed that the parent melts of the complex have suffered a significant crustal contamination.In a recent field work several lenses or septa of exotic metamorphic rocks were found striking parallel or sub-parallel to the complex layering: the total amount of these rocks has been estimated to represent the 13% of the volume of the whole complex.These lenses or septa were found starting from the intermediate portion of the LGZ unit upwards; however, they are mainly concentrated at the contact between LS and the US. They can be divided in three major lithotypes: quartzites, gneisses and calc-silicate rocks derived from sandstones, pelites and marls, respectively. The petrological and geochemical investigation of transects perpendicular to the complex layering provided the evidences that these rocks are the crustal contaminants of the complex parent melts: as the matter of fact they produced an enrichment in the hybrid melts of incompatible elements (i.e.: LREE, K, Ba and Rb).On the basis of their petrographic and geochemical characters, the contaminants are supposed to belong to the upper stratigraphic unit of the volcano-sedimentary sequence of Indaianopolis. This is also in agreement with geochronological data. In fact Pimentel and coworkers [2] dated one of the septa of contaminants occurring in US with the U/Pb method on zircons: they obtained an age of 1248±23 Ma and suggested that this age is referred as the US intrusion age, proposing that US and LS (their estimate of LS intrusion age is 797±10 Ma) were not coeval. This model is incongruent with the robust evidences that US and LS are cogenetic, as for instance the similare LILE patterns found in rocks of the two different units or the outcropping, in LS, of cumulus rock similar to the US ones.We present a simplified model of assimilation and fractional crystallization which tentatively supports the hypothesis that LS and US are genetically kindred. References. [1] Rivalenti, G., Correia, C.T., Girardi, V.A.V., Mazzuchelli, M., Tassinari, C.C., Bertotto, G.W. (2008): J. South Amer. Earth Sci., 25, 298-312; [2] Pimentel, M.M., Ferreira Filho, C.F., Amstrong, R.A. (2004): Precambrian Research, 132, 132-135.

2010 - SAPPHIRINE-BEARING AMPHIBOLE GABBRO FROM THE MANTLE SEQUENCE OF FINERO (SOUTHERN ALPS): PETROGRAPHY, GEOCHEMISTRY AND GEODYNAMIC CONTEXT [Abstract in Rivista]
Giovanardi, T.; Zanetti, A.; Mazzucchelli, Maurizio; Tiepolo, M.; Vannucci, R.; Morishita, T.
abstract

A late swarm of sapphirine-bearing amphibole gabbroic veins discordantly crosscut the main layering of the Finero phlogopite-peridotite massif, Western Italian Alps. Sapphirine locally occurs in a melanocratic zone placed between a leucocratic gabbroic band, forming the central part of the intrusion, and the host peridotite. The melanocratic zones are observed on both sides of the leucocratic gabbroic vein and consist of (i) an outer orthopyroxene-rich zone along the host peridotite and (ii) an inner amphibole-rich zone placed along the leucocratic gabbroic band side. Sapphirine either overgrows spinel or occurs as isolated inclusion within large amphiboles in the amphibole-rich melanocratic zone. Spinels without sapphirine envelopes also microtexturally co-exist with independent sapphirine grains. EMPA and LA-ICP-MS analyses of minerals from the melanocratic and leucocratic bands evidence significant differences in terms of both major and trace elements in the composition of the parent melts. In particular, the amphiboles in the melanocratic zones show higher TiO2, Na2O and K2O, M-HREE and HFSE than those in the leucocratic ones. The Al2O3 content of amphibole and the Fo in olivine of the host peridotite are significantly higher and lower, respectively, than those in other Finero peridotites far from the amphibole gabbroic veins. Moreover, amphiboles from the host peridotite are characterised by LREE-enriched convex-upward patterns significantly different with respect to those documented in literature for the Finero phlogopite-peridotites [1]. Mineral assemblages and mineral chemistry in both the melanocratic zone and the host peridotites are interpreted as the result of different stages of melt migration associated with melt-rock interaction. In particular, the major and trace element compositions of the amphiboles from the wall peridotite suggest that during an early stage, possibly before the opening of the conduits, the peridotite suffered porous flow migration of a melt more enriched in REE with respect to those forming the gabbroic bands. The crystallisation of the melanocratic bands is related to a second stage, in which the precipitation of large amphiboles was accompanied by a strong reaction between host peridotite and melt flowing into the conduit that determined the complete substitution of peridotite olivine with orthopyroxene at the peridotite-vein contact. A third stage was characterised by with the precipitation of the leucocratic band, associated to a further enlargement of the vein. Petrographic survey highlights that parent melt of leucocratic zone reacted with the minerals of the melanocratic one, inducing sapphirine growth around spinels. The genetic relationships occurring between the parent melts of the melanocratic and leucocratic zones must be yet established. Working hypotheses consider the parent melt of the leucocratic zone either related to a late injection or a residual differentiate after precipitation of melanocratic band in the frame of flow differentiation process. Quantitative considerations suggest that the selective addition of Al-rich phases, like amphiboles and micas, to a basalt can determine the large Al/Si ratios required for sapphirine precipitation. Modelling results indicate that the eutectic T of Finero phlogopite-peridotite is <1000°C and that the first partial melts are saturated in corundum. Thus, it is proposed that injection of basaltic melts triggered the partial melting of limited volumes of phlogopite peridotite producing Al-rich melts: the mixing of such Al-rich components with the migrating basalt is believed to have played a fundamental role in favouring the precipitation of sapphirine.References. [1] Zanetti, A., Mazzucchelli, M., Rivalenti, G., Vannucci, R. (1999): Contrib. Mineral. Petrol., 134, 107-122.

2010 - TUFFACEOUS DEPOSITS IN THE SEDIMENTARY COVER OF THE STRONA-CENERI ZONE AND IN THE VILLAFORTUNA-TRECATE OIL SYSTEM: PETROLOGICAL, GEOCHEMICAL AND GEOCHRONOLOGICAL CHARACTERIZATION [Abstract in Rivista]
Giovanardi, T.; Mazzucchelli, Maurizio; Zanoni, F.; Decarlis, A.; Fantoni, R.; Tiepolo, M.; Vannucci, R.; Zanetti, A.
abstract

In the western sector of the Southern Alps, tuffaceous levels are occasionally found in Triassic sedimentary sequences and the age of the related magmatism is still matter of debate. The few available geochronological data on these tuffites (M. San Giorgio area, Lugano, CH) yield Triassic ages (245±1 Ma; U-Pb zircon [1]), which are slightly older than the stratigraphic position. The precise definition of the tuffite age is, however, important for the Triassic evolution of the western Alps. In particular, if the magmatism is synchronous with their stratigraphic position, tuffites might be linked to the Triassic magmatism documented by the Finero area (NE Ivrea-Verbano Zone [2]). However, it cannot be presently excluded that they are the product of erosion and re-deposition of the Permian acid volcanics located at the base of the Mesozoic sedimentary cover [3].In this work, we have considered the tuffaceous deposits occurring within a late Anisian-Ladinian succession in the Mesozoic sedimentary cover of the Strona-Ceneri Zone in the Borgosesia area [3,4] and the tuffites at the same stratigraphic position found in some wells of the Villafortuna-Trecate oil system field (western Po Plain, Piedmont region, NW Italy) [5]. These tuffaceous deposits are constituted by a variable mixture of magmatic and sedimentary components. Available chamical data on the magmatic component suggests a calc-alkaline affinity. Zircons have been separated with conventional methods from four samples, mounted in epoxy resin and characterised under cathodoluminescence (CL). Based on colour, morphology and internal structure, zircons have been divided in two populations. One group is constituted by light-pink coloured zircons with prismatic habits and tight osciscillatory zoning suggesting growth under magmatic conditions. Zircons from the second group are colourless, rounded in shape and with only relics of magmatic zoning. They most likely suffered metamorphic recrystallization.Ongoing ELA-ICP-MS characterizations of both trace elements composition and U-Pb age of these zircons is expected to provide valuable constraints on the geodynamic evolution of the Southern Alps Domain in Permo-Triassic times.References. [1] Mundil, R., Brack, P., Meier, M., Rieber, H., Oberli, F. (1996): Earth and Planetary Science Letters, 141, 137-151; [2] Peressini, G., Mazzucchelli, M., Rivalenti, G., Hofmann, A.W. (2004): Geophysical Research Abstracts, 6, 05072, SRef-ID: 1607-7962/gra/EGU04-A-05072; [3] Carraro, F., Fiora, L. (1974): Riv. It. Paleont. Strat., 80, 167-191; [4] Fantoni, R., Decarlis, A., Fantoni, E. (2004): Atti Tic. Sc. Terra, 44, 97-110; [5] Fantoni, R., Bello, M., Ronchi, P., Scotti, P. (2002): Extended Abstracts Book EAGE Conference Florence.

2009 - Basaltos del cerro del Mojón, centro-oeste de la provincia de Río Negro, descripción petrográfica y geoquímica [Abstract in Atti di Convegno]
Ponce, A.; Bertotto, G.; Mazzucchelli, Maurizio; Brunelli, Daniele
abstract

abstract

2009 - Dunites in the Balmuccia Peridotite Massif (Western Italian Alps): their origin by focused percolation of pyroxenite-derived melt. [Abstract in Rivista]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; Brunelli, Daniele; Zanetti, A.; Boari, E.
abstract

In the Balmuccia Massif, an Alpine peridotite thought to represent part of the subcontinental mantle, a 50 m thick and 150 m long dunite body, which occurs as a subconcordant, tabular structure, has been recently recognised. The contacts with the host spinel-facies depleted lherzolite are sharp. The dunite body is composed of spinel-rich dunite containing centimetre-size lenses of relict Cr-diopside websterite, spinel-poor granoblastic dunite and virtually monomineralic Cr-spinel layers exhibiting flow structures. Orthopyroxene is a minor, relict phase in all the lithologies; clinopyroxene is intergranular and amphibole is a minor accessory phase. Overall the dunite body is fairly refractory (Fo in olivine: 90.7-93.8). Strontium and neodymium isotope ratios of clinopyroxene separates from the dunitic body resemble those of a Cr-diopside websterite suite that forms a series of dykes cutting the main peridotite host. It is proposed that the dunites were generated in a part of the mantle veined byearly Cr-diopside websterites by a three-stage process involving partial melting of pyroxenite, infiltration of the pyroxenite-derived melt into the depleted lherzolite and its consequent open-system partial melting and focused flow of the resultant partial melts leading to the production of reactive dunite channels through both peridotite and pyroxenite. This process has been simulated using pMELTS assuming that the pyroxenite partially melts at 1.5 GPa and focused melt transport occurs at pressures greater than 0.7 GPa. The results show that, depending on the focusing factor assumed, dunite can form from peridotite at P < 1.2 GPa and from pyroxenite at P < 1.1 GPa, in both cases over a large pressure range. The model accounts for specific characteristics of the dunite, such as its refractory composition, the presence of orthopyroxene relics, the occurrence of relict websterite lenses in the spinel-rich dunites and the flow structures in the Cr-spinel layers. The proposed mechanism allows dunite formation to occur well within the spinel stability field, and therefore at greater depth than dunites in ophiolites,which generally formed within the plagioclase stability field. The aggregated model melts extracted from the segments where dunite forms are high-Mg alkali basalts resembling, after olivine fractionation, the compositions of enriched-type mid-ocean ridge basalt from slow- and ultraslow-spreading ocean ridges.

2009 - Evidence for ultra-depleted Pb isotopic component in the backarc Patagonian mantle wedge. [Abstract in Rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Zanetti, A.; Hémond, C.; Cipriani, Anna
abstract

Inferences on melt and fluid migration trough the Patagonin mantle wedge are provided by the ultramafic xenoliths occurrence of Tres Lagos (TL; lat. 49.13°S, long. 71.18°W). It is placed at the eastern border of the Meseta de la Muerte backarc basaltic plateau, where a post-plateau volcanic diatreme contains mantle xenoliths in both pyroclastites and lavas. Its latitude corresponds with the Northern limit of the Austral Volcanic Arc (AVZ), which is separated from the Southern Volcanic Zone (SVZ) by a gap in the arc magmatism between 49° and 46°30′ latitude S. The selected xenoliths have been distinguished into two groups (Group 1 & 2). Group 1 consists of lherzolites and harzburgites, whereas Group 2 is constituted only by harzburgites. The Sr, Nd and Pb isotopic compositions of the Group I clinopyroxenes (Cpx) form arrays from DM to the TL basaltic lavas, according to the presence of EM1 component. Group 2 Cpx have much more radiogenic Sr and less radiogenic Nd values, approaching those of EM1 and EM2 end-members: these features are associated to very unradiogenic lead isotopic compositions (206Pb/204Pb = 17.4-18.1; 207Pb/204Pb = 15.55-15.60; 208Pb/204Pb = 37.3-38.5). The combination of petrographic, trace element and isotopic fetures indicate that TL harzburgites are residua after melt-assisted partial melting triggered by melt/fluid migration in the hottest, and perhaps deeper, parts of the pristine DM lithosphere. The interpretation of the Pb isotope composition of Group 2 Cpx is not trivial. In analogy with the interpretation proposed for SWIR, it could unravel the occurrence of mantle sources which incorporated ancient crust and failed to homogenise with the DM mantle. Alternatively, it could be the evidence for ancient continental crust of the South America plate dragged down into the mantle by slab motion.

2009 - Evidence for ultra-depleted Pb isotopic metasomatic liquid in the backarc Patagonian Mantle Wedge [Abstract in Rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Zanetti, A; Hémond, C; Cipriani, Anna; Bertotto, Gw; Cingolani, Ca
abstract

Insights into the geochemical composition of melts and fluids migrating trough the Patagonian mantle wedge are provided by the ultramafic xenoliths occurrence of Tres Lagos (TL; lat. 49.13°S, long. 71.18°W). This locality is placed at the eastern border of the Meseta de la Muerte backarc basaltic plateau, where a post-plateau volcanic diatreme contains mantle xenoliths in both pyroclastites and lavas. Its latitude corresponds with the Northern limit of the Austral Volcanic Arc (AVZ), which is separated from the Southern Volcanic Zone (SVZ) by a gap in the arc magmatism ranging between 49° and 46°30′ latitude S. The selected xenoliths have been distinguished into two groups (Group 1 & 2). Group 1 consists of lherzolites and harzburgites, whereas Group 2 is only formed by harzburgites.The texture of the Group 1 lherzolites is protogranular to granoblastic and porphyroblastic, whereas Group 1 harzburgites have always granoblastic texture. Group 2 harzburgites have granular texture, which may fade into porphyroblastic owing to the presence of large olivine and orthopyroxene crystals.The clinopyroxenes (Cpx) from Group 1 lherzolites have PM-normalised REE patterns varying from LREE-depleted (Lan/Smn= 0.24-0.37), to LREE-enriched (Lan/Ybn up to 4.08) and spoon-shaped: the latter have minimum at Pr and Pr-Yb concentrations similar to those shown by the LREE-depleted Cpx.The Cpx from Group 1 harzburgites have lower REE concentrations with respect to the lherzolite ones and their REE patterns vary from HREE-enriched, steadily fractionated, (Lan/Ybn = 0.21–0.35, Ybn ~ 1-2) to spoon-shaped (Lan/Smn = 2.81; Smn/Ybn = 0.89; Ybn ~ 3).The Cpx from the Group 2 harzburgites have convex-upward (Lan/Smn = 0.31; Smn/Ybn = 1.50) to LREE-enriched (Lan/Ybn = 2.94) patterns.The Sr, Nd and Pb isotopic compositions of the Group 1 clinopyroxenes form an array from DM to the TL basaltic lavas, according to the presence of EM1 component. Group 2 Cpx have much more radiogenic Sr and less radiogenic Nd values, approaching those of EM1 and EM2 end-members: these features are associated to very unradiogenic lead isotopic compositions (206Pb/204Pb = 17.4-18.1; 207Pb/204Pb = 15.55-15.60; 208Pb/204Pb = 37.3-38.5). The combination of petrographic, trace element and isotopic fetures indicate that TL harzburgites are residua after melt-assisted partial melting triggered by melt/fluid migration in the hottest, and perhaps deeper, parts of the pristine DM lithosphere. The interpretation of the Pb isotope composition of Group 2 Cpx is not trivial. In analogy with the interpretation proposed for SWIR, it could unravel the occurrence of mantle sources which incorporated ancient crust and failed to homogenise with the DM mantle. Alternatively, it could be the evidence for ancient continental crust of the South America plate dragged down into the mantle by slab motion.

2009 - Formation of Highly-refractory dunite by focused percolation of pyroxenite.derived melt in the Balmuccia spinel peridotite massif (Italy) [Poster]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; Brunelli, Daniele; Alberto, Zanetti; Elena, Boari
abstract

abstract

2009 - Formation of highly-refractory dunite by focused percolation of pyroxenite-derived melt in the Balmuccia peridotite massif (Italy) [Articolo su rivista]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; Brunelli, Daniele; Zanetti, A.; Boari, E.
abstract

A 50 m thick and 150 m long dunite body occurs as a subconcordant, tabular structure in the Balmuccia Massif, an Alpine peridotite thought to represent part of the subcontinental mantle. The contacts with the host spinel-facies depleted lherzolite are sharp. The dunite body is composed of spinel-rich dunite containing centimetre-size lenses of relict Cr-diopside websterite, spinel-poor granoblastic dunite and virtually monomineralic Cr-spinel layers exhibiting flow structures. Orthopyroxene is a minor, relict phase in all the lithologies; clinopyroxene is intergranular and amphibole is a minor accessory phase. Overall the dunite body is fairly refractory (Fo in olivine: 90·7–93·8). Strontium and neodymium isotope ratios of clinopyroxene separates from the dunitic body resemble those of a Cr-diopside websterite suite that forms a series of dykes cutting the main peridotite host. It is proposed that the dunites were generated in a part of the mantle veined by early Cr-diopside websterites by a three-stage process involving partial melting of pyroxenite, infiltration of the pyroxenite-derived melt into the depleted lherzolite and its consequent open-system partial melting and focused flow of the resultant partial melts leading to the production of reactive dunite channels through both peridotite and pyroxenite. This process has been simulated using pMELTS assuming that the pyroxenite partially melts at 1·5 GPa and focused melt transport occurs at pressures greater than 0·7 GPa. The results show that, depending on the focusing factor assumed, dunite can form from peridotite at P < 1·2 GPa and from pyroxenite at P < 1·1 GPa, in both cases over a large pressure range. The model accounts for specific characteristics of the dunite, such as its refractory composition, the presence of orthopyroxene relics, the occurrence of relict websterite lenses in the spinel-rich dunites and the flow structures in the Cr-spinel layers. The proposed mechanism allows dunite formation to occur well within the spinel stability field, and therefore at greater depth than dunites in ophiolites, which generally formed within the plagioclase stability field. The aggregated model melts extracted from the segments where dunite forms are high-Mg alkali basalts resembling, after olivine fractionation, the compositions of enriched-type mid-ocean ridge basalt from slow- and ultraslow-spreading ocean ridges.

2009 - Magmatic Clues on the Triassic evolution of the Western Southern Alps before the opening of the Jurassic Tethys. [Relazione in Atti di Convegno]
Mazzucchelli, Maurizio; Zanetti, A.; CORREIA TEIXEIRA, C.; Tassinari, C. C. G.
abstract

Three-points internal isochrons based on Sm-Nd isotopic data of whole rock, plagioclase and clinopyroxene fractions have been carried out for diorites belonging to a dyke swarm cutting the mantle ultramafic massif of Baldissero (Ivrea-Verbano Zone, IVZ: Western Southern Alps, Italy).The isochron slopes correspond to (i) 19829 Ma, with 143Nd/144Ndi = 0.512669 and ?Ndi = +5.6 (MSWD = 0.63) and of (ii) 204?31 Ma, with 143Nd/144Ndi = 0.512680 and Ndi = +6.0 (MSWD = 1.5), straddling the upper Triassic-lower Jurassic boundary. They are consistent with those obtained by Obermiller (1994) with two-points Sm-Nd internal isochrons. Petrographic and geochemical features of the diorites were not perturbed after the intrusion, and thus such geochronologic data are considered emplacement ages.The dioritic dykes are cumulates and have high Mg# values in bulk-rock and mafic phases that positively correlate with the incompatible trace element concentrations. The geochemical characteristics of the diorites are explained by flow of a hydrous, silica-saturated melt accompanied by reaction with the ambient peridotite and fractional crystallisation, at deep crust condition. The hypothetical parent melts calculated in equilibrium with the diorite minerals show tholeiitic to mild-alkaline affinity. Isotopic composition, characterised by high radiogenic Nd (This Study, Obermiller, 1994) and low radiogenic Sr (Obermiller, 1994), further support a tholeiitic affinity.The strongly LREE-depleted and fractionated compositions of bulk rock and minerals, associated with fertile major element and modal bulk-rock composition, suggest that the mantle lherzolites are refractory residues after low degrees of near-fractional melting. In particular, REE composition of the clinopyroxenes is consistent with that expected after ca. 4% near-fractional melting under garnet-facies. After the melting event, the peridotites experienced subsolidus re-equilibration at spinel-facies conditions and very modest, if any, metasomatism that produced accessory amount of titanian pargasite. Re-Os model age are 376 Ma, straddling Devonian-Carboniferous boundary. It is well in agreement with the age of 378?48 Ma obtained by Obermiller (1994) from the best fit of the Sm-Nd isotopic compositions of clinopyroxenes separated from fourteen Baldissero peridotite samples (Ndi = 0.51247, ?Ndi = +6.2 and MSWD = 26). This age is considered to record a partial melting event related to extensional phases prior the Variscan orogeny.Triassic igneous events are unknown in the southern IVZ, but the intrusion of hydrous melts, mostly silica-saturated and of tholeiitic affinity, have been largely documented in the Finero region, i.e. the northernmost part of IVZ (Gebauer, 1993; Lu et al., 1997; Vavra, 1999; Oppizzi and Schaltegger, 1999; Peressini et al., 2004). Besides, the Finero mantle sequence experienced pervasive, channelled and intrusive migration in Triassic time (Hunziker 1974; Stahle et al., 1990; 2001; Von Quadt et al., 1993; Hartmann and Wedepohl, 1993; Grieco et al., 2001; Morishita et al., 2008) of melts often showing large crustal component (Hartmann and Wedepohl, 1993; Zanetti et al., 1999; Raffone et al., 2006).Triassic magmatism is also abundant throughout the Central-Eastern Southern Alps, where it shows calc-alkaline to shoshonitic affinityThe Middle Triassic tectonic evolution has been interpreted in a number of ways, spanning from aborted rift, possibly related to the development of the Central Atlantic Magmatic Province (Furin et al., 2006), to W-directed subduction with slab-retreating towards E (Doglioni, 2007).Our data represent a further evidence that in the late Trias the westernmost part of the Adria plate experienced the intrusion of melts with tholeiitic affinity s.l.: the eastwards progressive change of the melt affinity to calk-alkaline and shoshonitic could be consistent with a different crustal emplacement level in the westernmost sector, or with different slab contri

2008 - Sr-Nd isotopic evidence for crustal contamination in the Niquelândia Complex, Goiás, Central Brazil. [Articolo su rivista]
Rivalenti, Giorgio; Correia, Ct; Girardi, Vav; Mazzucchelli, Maurizio; Tassinari, Ccg; Bertotto, Gw
abstract

The Niquelandia complex is a Neoproterozoic mafic-ultramafic intrusion resulting from fractional crystallization of primary picriticbasalt intrusions. It consists of two layered sequences: a lower and larger one (LS), where four stratigraphic units exhibit an upward decrease of ultramafic layers and increase of gabbroic layers; an upper, smaller sequence (US), separated from LS by a high-temperature shear zone and consisting of two stratigraphic units (gabbros + anorthosites and amphibolites). Nd and Sr isotopic analyses and rare earth element (REE) profiles provide evidence that the complex suffered important crustal contamination. The LS isotopic array trends from a DM region with positive epsilon Nd and moderately positive epsilon Sr towards a field occupied by crustal xenoliths, especially abundant in the upper LS (negative epsilon Nd and large, positive epsilon Sr). Each LS stratigraphic unit is distinct from the next underlying unit, showing lower epsilon Nd and higher epsilon Sr, suggesting inputs of fresh magma and mixing with the contaminated, residual magma. The US is characterised by a relatively high variation of epsilon Nd and constant epsilon Sr. REE patterns vary within each unit from LREE depleted to LREE enriched in the samples having lower epsilon Nd and higher epsilon Sr. The contamination process has been modelled by using the EC-AFC algorithms from [Spera, F.J., Bohrson, W.A., 2001. Energy-constrained open-system magmatic processes I: general model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation. J. Petrology 42, 999–1018]. The differences between the LS and US isotopic arrays are consistent with contamination by the same crustal component, provided that its melting degree was higher in LS than in US. The different degrees of anatexis are explained by the heat budget released from the magma, higher in LS (because of its larger mass) thanin US. Comparison of the correlations between isotopes and incompatible trace element ratios of the models and of the gabbros shows some differences, which are demonstrably related with the variable amount of cumulus phases and trapped melt in the gabbros.

2007 - Natural stones from sacral buildings of the Marzabotto Etruscan site: Petrochemical characterization and hypothesis of provenance. [Abstract in Rivista]
Lugli, Stefano; Malnati, L; Marrocchino, E; Mazzucchelli, Maurizio; Sighinolfi, F; Vaccaro, C.
abstract

The Etruscan site of Marzabotto (Bologna Italy) was built and developed between V1 and IV sec b.C. In this site several sacral buildings were erect using various natural stone materials. In this study the attention has been focused on the tufa rocks used in some of the sacral buildings of this archaeological site. The samples were characterized from a petrochemical point of view and compared with lithologies outcropping in the surrounding area in order to recognize the ancient quarries. These tufa called "travertini" in the Italian literature, are very porous and are rich of hacrophyte moulds revealing an origin by freshwater carbonate deposition (Ford-Pedley 1996 Facies and thin section analyses show that these rocks were formed in cascade and paludal settings. Hypothesizing that the possible quarry areas of travertine rocks were not far from the site and using the geological ma s of the surrounding of the Etruscan site it was possible to identify few possible supply zones: 1) San Cristoforo di Labante (Castel D'Aiano Bologna) 2) Rio Sponga (Marzabotto, Bologna) and 3) Rio Bucamante (Serramazzoni; Modena). Petrochemical characterization were complemented with a statistical elaboration using the SPSS 14.0 (Copyright @ 2006 SPSS Inc.) method and have highlighted that the San Cristoforo di Labante deposit could be the most suitable quarry area of the travertine used in Marzabotto. The geomorphology of the area suggests that blocks were lowered from the San Cristoforo di Labante deposit trough a "lizza" way (using rollers along a prepared slope) and then shipped along the Reno River to the Etruscan site of Marzabotto.ReferencesT. D. Ford, H. M. Pedley, 1996. A review of tufa and travertine deposits of the world, in: Earth-Science Reviews, 41.

2007 - New constraints on the Mesozoic emplacement of diorite dykes in the Baldissero mantle peridotite massif (Ivrea-Verbano Zone) [Abstract in Rivista]
Mazzucchelli, Maurizio; Zanetti, A; Rivalenti, Giorgio; Vannucci, R; CORREIA TEIXEIRA, C; Tassinari, Ccg
abstract

The Baldissero peridotite is the main mantle massif occurring in the south-western part of the Ivrea zone. It is in contact at the East with the gabbros of the Ivrea-Verbano Mafic Complex and at the West it is sheared by the faults of the Insubric system and in tectonic contact with the terrains of the Austroalpine Domain (Canavese one). Another Alpine shear zone, the Cremosina line, limits at the North-West the Baldissero terrains from the rest of the Ivrea-Verbano Mafic Complex. Dioritic dykes in the Baldissero mantle peridotite have high Mg# values in bulk-rock and mafic phases which positively correlate with incompatile element concentrations. The peridotite at the dyke contact is enriched in orthopyroxene, iron and incompatible trace element with respect to the LREE-depleted lherzolites typical of the Baldissero massif. The geochemical characteristics of the dykes are explained by flow, accompanied by crystallisation and reaction with the ambient peridotite, of a hydrous silica-saturated basaltic melt under crustal condition. The least evolved melts in this process are those that have the lowest incompatible trace element concentration and Mg#. New Internal Sm-Nd isochrons on two dyke samples (198±29, with Ndi = +5.6 and 204±31 Ma with Ndi = +6.0) and Re-Os data on the peridotite at the contact indicate late Triassic - lower Jurassic ages. Recently, Sm-Nd isochron ages published for the Mafic Complex in the Val Sesia and Val Sessera area has been interpreted to date the cooling of mineral assemblages below the blocking temperature for the Sm-Nd system (Peressini et al., 2007). However, the Mafic Complex in the Finero area show differences of about 10 Ma between Sm-Nd internal isochron ages (isochrons with 3 minerals, ranging from 223 to 231 Ma and having positive εNdi from +2.7 to + 5.3, Lu et al, 1997) and U/Pb zircon ages (232 ± 2 Ma; Peressini et al., 2005). Therefore, even if the ages obtained for the diorite dykes in Baldissero may be slightly younger with respect to the age of emplacement, this last should be, anyway, not older than the late Triassic. At this estimated age of dyke intrusion, the Ivrea-Verbano mantle peridotites were already emplaced in the crust. Thus, the low Mg# of the melt intruding the peridotite is explained by previous fractionation in the crust. Mesozoic igneous events are unknown in the southern Ivrea-Verbano region, but only in the northernmost Finero region. The Baldissero dykes, besides for their age, exhibit geochemical and isotopic similarities with the External Gabbro Unit of the Finero Mafic Complex, constituted by amphibole-bearing diorites. The Finero Mafic Complex is in tectonic contact, marked by a high-temperature ENE shear-zone, with the Permian relatively anhydrous mafic-ultramafic sequences occurring in Val Sesia and on the right side of the Val d'Ossola. Therefore, the evidences of Mesozoic magmatism in the westernmost sector of the Southern Alps are, presently, confined by tectonic lineaments to the southernmost and northernmost portion, respectively. Furthermore, the mantle phlogopite-bearing peridotite of Finero, show a metasomatic imprint of late Triassic age, which was attributed to a hydrous silica-saturated component of crustal origin, deriving from a subducting slab (Hartmann & Wedephol, 1993, Zanetti et al. 1999). The evidence of a Mesozoic magmatic activity in the westernmost sector of the Southern Alps, its particular location within the Ivrea-Verbano Zone and the nature of the involved parent melts are a potentially important mark in the geodynamic evolution of the Southern Alps.

2007 - New constraints on the Mesozoic emplacement of dioritic dykes in the Baldissero mantle peridotite massif (Ivrea-Verbano Zone) [Abstract in Rivista]
Mazzucchelli, Maurizio; Zanetti, A; Rivalenti, Giorgio; Vannucci, R; CORREIA TEIXEIRA, C; Tassinari, Ccg
abstract

Dioritic dykes in the Baldissero mantle peridotite have high Mg# values in bulk-rock and mafic phases which positively correlate with incompatile element concentrations. The peridotite at the dyke contact is enriched in orthopyroxene, iron and incompatible trace element with respect to the LREE-depleted lherzolites typical of the Baldissero massif. The geochemical characteristics of the dykes are explained by flow, accompanied by crystallisation and reaction with the ambient peridotite, of a hydrous silica-saturated basaltic melt under crustal condition. The least evolved melts in this process are those that have the lowest incompatible trace element concentration and Mg#. New Internal Sm-Nd isochrons on two dyke samples (198±29, with Ndi = +5.6 and 204±31 Ma with Ndi = +6.0) and Re-Os data on the peridotite at the contact indicate late Triassic - lower Jurassic ages. Recently, Sm-Nd isochron ages published for the Mafic Complex in the Val Sesia and Val Sessera area (from 274±11 to 244±4 Ma, Mayer et al., 2000; Voshage et al. 1987) has been interpreted to date the cooling of mineral assemblages below the blocking temperature for the Sm-Nd system (Peressini et Al., 2007). However, the samples wich give the youngest ages, show negative εNdi values (from -3.8 to -5.3), suggesting substantial crustal contamination and, therefore, open system processes. The sample with the oldest age has, on the contrary, positive εNdi (+9.7) indicating the absence of remarkable crustal contamination. The span between this age (274±11 Ma) and the U/Pb zircon ages reported in Peressini et al. (2007) is around 10 Ma. Moreover, differences of about 10 Ma between Sm-Nd internal isochron ages (isochrons with 3 minerals showing positive εNdi from +2.7 to + 5.3, Lu et al, 1997) and U/Pb zircon ages (Peressini et al. (2005) are reported for the Mafic Complex in the Finero area. Therefore, even if the ages obtained for the diorite dykes in Baldissero may be slightly younger with respect to the age of emplacement, which should be, anyway, not older than the late Triassic. At this estimated age of dyke intrusion, the Ivrea mantle peridotites were already emplaced in the crust. Thus, the low Mg# of the melt intruding the peridotite is explained by previous fractionation in the crust. Mesozoic igneous events are unknown in the southern Ivrea region, but only in the northernmost Finero region. Furthermore, the mantle phlogopite-bearing peridotite of Finero, show a metasomatic imprint of late Triassic age, which was attributed to a hydrous silica-saturated component of crustal origin, deriving from a subducting slab (Hartmann & Wedephol, 1993, Zanetti et al. 1999). The evidence of a Mesozoic magmatic activity in the westernmost sector of the Southern Alps, presently confined by tectonic lineaments to the southernmost and northernmost portion of the Ivrea-Verbano Zone, respectively, and the nature of the involved parent melts are a potentially important mark in the geodynamic evolution of the Southern Alps.

2007 - Petrochemical characterisation of natural stones from the Etruscan site of Marzabotto (Bologna-Italy): Preliminary results [Abstract in Atti di Convegno]
Lugli, Stefano; Marrocchino, E; Mazzucchelli, Maurizio; Sighinolfi, F; Vaccaro, C.
abstract

The tufa rocks used in sacral buildings in the Etruscan site of Marzabotto (Bologna, Italy) were characterised from a petrochemical point of view and compared with lithologies outcropping in the surrounding area in order to recognise the ancient quarries. These tufa, called “travertine” in the Italian literature, are very porous and are rich of macrophyte moulds revealing an origin by freshwater carbonate deposition (Ford-Pedley, 1996). Facies and thin section analyses show that these rocks were formed in cascade and paludal settings. Hypothesising that the possible quarry areas of travertine rocks were not far from the site and using the geological maps of the surrounding of the Etruscan site it was possible to identify few possible supply zones: 1) San Cristoforo di Labante (Castel DAiano, Bologna), 2) Rio Sponga (Marzabotto, Bologna) and 3) Rio Bucamante (Serramazzoni, Modena). Petrochemical characterisation were complemented with a statistical elaboration using the SPSS 14.0 (Copyright © 2006 SPSS Inc.) method and have highlighted that the San Cristoforo di Labante deposit could be the most suitable quarry area of the travertine used in Marzabotto. The geomorphology of the area suggests that blocks were lowered from the San Cristoforo di Labante deposit through a “lizza” way (using rollers along a prepared slope) and thenshipped along the Reno River to the Etruscan site of Marzabotto.ReferencesT. D. Ford, H. M. Pedley, (1996) A review of tufa and travertine deposits of the world, in: Earth-Science Reviews, 41.

2007 - Results of melt migration in the lithospheric mantle and in continental mafic-ultramafic layered complexes. [Abstract in Rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio
abstract

We have studied the following topics: 1) metasomatism of the lithospheric mantle atop a plume (NE Brazil and Fernando de Noronha); 2) metasomatism of the mantle wedge by slab-derived components (Cerro de los Chenques, Patagonia); 3) origin of dunite and wehrlite regions in the subcontinental lherzolitic mantle (Balmuccia); 4) reactions between diorite dikes and subcontinental mantle and consequent geodynamic implications (Baldissero massif).Isotopic and geochemical results indicate that in the case study 1 the mantle lithosphere underwent thermochemical and mechanical erosion by infiltration of alkali basalts during Cenozoic and that this process affected a mantle recording previous, Mesozoic, mantle processes. The similarity of processes and geochemical characteristics in NE Brazil and Fernando de Noronha supports the hypothesis that the lithosphere under the Island is a detached portion of the continental one and that the respective lithospheres interfered with the same plume.Geochemical and isotope evidence indicate that the wedge beneath Chenques (case 2) interfered with slab-derived fluids rather than melts. Since the study locality is 400 km east from the trench and the slab beneath Chenques is about 200 km deep, a main problem is how to explain the long travel of slab-released fluids in order to reach the observed spinel-facies region of the wedge. We have modelled this process as break-down of phengite in the slab and percolation of the derived fluids in the wedge under increasing temperature and water activity. Reaction during the long fluid migration produces phlogopite, K-richterite, garnet, and, in the lower-P region, amphibole. The geochemical effects of these reaction products are recorded in the fluids reaching the observed spinel-facies region. Besides already known decimetric dunite at websterite dyke contacts, decametric dunites regions have been discovered into the subcontinental Balmuccia Peridotite (case 3). The latter dunites (± wehrlites) are of two types: a) very high Mg# values in bulk-rock and minerals and characterised by massive Cr-spinel layers; b) Mg# values in bulk rock and minerals lower than the ambient lherzolite and characterised by amphibole, phlogopite and plagioclase pods. As discussed in more detail in a specific abstract of this session, these two dunite types are related with infiltration of different melts and different processes, of increasing and decreasing melt mass, respectively, during reactive flow.The reaction between diorite dykes and the Baldissero peridotite (topic 3) has already been previously discussed. However, new isotopic data provide constraints on the age of dyke emplacement which have important geodynamic implications, specifically illustrated in a work presented in this session. Finally, the mafic-ultramafic Niquelandia complex have been re-interpreted in the light of isotopic evidence in terms of energy-controlled assimilation and fractional crystallisation. Thermal balance indicate that the complex underwent large crystallisation before heating the crust to melting. Heat production was related to the intrusion volume and resulted higher in the largest lower sequence with respect to the smaller upper sequence. The results were production of different anatectic melts, different residual metasediments, and different interference results between anatectic melts and mafic magmas.

2007 - The Dunites in the Balmuccia peridotite massif. [Abstract in Rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Brunelli, Daniele; Zanetti, A; Tommasini, S; Boari, E.
abstract

The Balmuccia massif is dominantly lherzolitic. Dunites are a minor lithotype occurring: a) at the contacts of websterite dykes; b) as tabular bodies associated with layers of Cr-rich spinel; c) as large, decametric lenses typically associated with hornblendite pockets and veins. Type a dunites are <20-30 cm thick and were previously studied by Rivalenti et al. (1995, Contrib. Mineral. Petrol. 121, 275-288), who inferred they were related to local peridotite depletion induced by the melt which fractionated the websterites.Type b dunites are up to 14 m thick and olivine is accompanied by clinopyroxene (fading into wehrlite) and Cr-rich spinel. Clinopyroxene clusters occur in specific horizons and they represent previous, disrupted Cr-diopside dykes. Massive Cr-spinel layers, up to ~8 cm thick, are concordant with the dunite foliation and fade into dunite by decreasing modal spinel. These dunites are discordantly cut by later Cr-diopside websterite dykes and have the highest Mg# values in their bulk rock and silicate phase (Mg# range 0.90 – 0.95 and 0.91 – 0.94 in clinopyroxene and olivine, respectively) and the highest Cr# values of spinel (up to 0.55) so far observed at Balmuccia. In the spinel layers and at their contacts the silicate phases are highly zoned, with increase of the Mg# value at their rim. A similar zoning also occurs, but is less extreme, in the dunites farther from spinel layers.By contrast, type c dunites have the lowest Mg# values found in the Balmuccia peridotites (Mg# down to 0.89 and 0.88 in clinopyroxene and olivine, respectively). In these dunites, the minerals accompanying olivine vary approaching hornblendite pockets. Far from pockets (> 15 cm), olivine coexists with clinopyroxene. At a distance of 10-13 cm, clinopyroxene coexists with a “pyribole” (a mineral constituted by a few microns-large lamellae of clinopyroxene and amphibole). At 7-10 cm distance, clinopyroxene disappears and the pyribole coexists with discrete amphibole (kaersutite) crystals. Closer to the pocket, pyribole disappears and only amphibole and spinel (high Ti, low Cr) are present along with olivine. Similar variations occur in thin veins cutting the dunite and ending at the pocket. The pockets, up to 40 cm large, are constituted by giant kaersutitic amphibole, phlogopite, plagioclase, rutile, Al-Mg spinel. Towards the ambient lherzolites, the dunite fades into wehrlite, where orthopyroxene abundance increases outwards and the outer belt is constituted by a granular, undeformed, opx-rich lherzolite.On the basis of petrography, Sr and Nd isotopes and trace element characteristics of clinopyroxene we preliminarily hypothesise that type b dunites are the result of Cr-diopside dykes melting, eventually triggered by percolation of deeper-derived melts, and reactive percolation of these melts into the ambient lherzolite under increasing melt mass conditions. Type c dunites may represent a relatively shallow event (as indicated by the plagioclase stability) of reactive percolation of an hydrous, possibly alkaline, melt under decreasing melt mass.

2007 - The dunites of the Balmuccia peridotitic massif [Abstract in Rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Brunelli, Daniele; Zanetti, Alberto; Tommasini, Simone; Boari, Elena
abstract

abs

2007 - The effect of the Fernando de Noronha plume on the mantle lithosphere in north-eastern Brazil [Articolo su rivista]
Rivalenti, Giorgio; Zanetti, A; Girardi, Vav; Mazzucchelli, Maurizio; Tassinari, Ccg; Bertotto, Gw
abstract

AbstractNew xenolith occurrences in the Cenozoic alkali basalts of north-eastern Brazil have been studied in order to constrain the possible imprint on the continental mantle lithosphere of its passage over the Fernando de Noronha plume and the regional mantle processes. Texturally, the lherzolite and harzburgite xenoliths define three groups: group 1, porphyroclastic; group 2, protogranular; group 3, transitional between groups 1 and 2. Equilibrium temperatures are highest for group 1 and lowest for group 2. Clinopyroxenes from group 1 peridotites have Primitive Mantle (PM)-normalised REE patterns varying from L-MREE-enriched convex-upward, typical of phases in equilibrium with alkaline melts, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. Group 2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. The M-HREE are at 3–5 ×PM concentration level, as typical in fertile lithospheric lherzolites. Most of group 3 clinopyroxenes show LREE-depleted patterns similar to the group 2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the group 1 clinopyroxenes form an array between DM and EMI-like components, both of them are also present in the host basalts. Melts estimated to be in equilibrium with the group 1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The groups 2 and 3 clinopyroxenes define two distinct compositional fields at higher 143Nd/144Nd values, correlated with their LREE composition. The isotopes of the groups 2 and 3 LREE-depleted clinopyroxenes form an array from DM towards the isotopic composition of Mesozoic tholeiitic basalts from north-eastern Brazil. Melts in equilibrium with these clinopyroxenes are similar to these basalts, thus suggesting that such xenoliths record geochemical imprint from older melt-related processes.The LREE-enriched spoon-shaped group 3 clinopyroxenes are characterised by the highest 143Nd/144Nd values at any given 87Sr/86Sr composition. These results are interpreted in terms of a lithospheric mantle section which underwent thermo-chemical and mechanical erosion by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics during Cenozoic time. At that time, the lithospheric mantle consisted of fertile lherzolites and harzburgites recording the geochemical imprint of Mesozoic mantle processes. The onset of the interaction between lithospheric peridotites and alkaline melts was characterised by the porous flow percolation of small melt volumes that induced chromatographic enrichments in highly incompatible elements and the isotope signature of the spoon-shaped, group 3 clinopyroxenes. Group 1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the group 2 documents the shallower lithospheric section, with group 3 being the transition. The similarity of processes and isotope components in the protogranular xenoliths from Fernando de Noronha area and north-eastern Brazil supports the hypothesis that the lithosphere beneath Fernando de Noronha is a detached portion of the continental one. Furthermore, the similarity in terms of textural and geochemical features documented by the mantle samples coming from the two different regions seems to confirm the interference of the two regions with the same plume.

2007 - XENOLITHS FROM EL CERRO DE LOS CHENQUES (PATAGONIA): AN EXAMPLE OF SLAB-RELATED METASOMATISM IN THE BACKARC LITHOSPHERIC MANTLE. [Articolo su rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Zanetti, A; Vannucci, R; Bollinger, C; C., Hemond; Bertotto, Gw
abstract

The alkali basalts of the Cerro de los Chenques quaternary backarc volcano (200 km E of the volcanic arc) carry to the surface abundant spinel-facies mantle xenoliths (lherzolites, harzburgites and dunites). The clinopyroxene geochemistry indicates that the pristine mantle was a lherzolite with Depleted Mantle (DM) composition, recording either melting episodes triggered by infiltration of a metasomatic agent or only enrichment of highly incompatible elements in those sectors where percolation occurred under decreasing fluid volume. Metasomatism was operated by a fluid originated in garnet-bearing assemblages that induced olivine and clinopyroxenedissolution, variations in the Sr and Nd isotopic signatures (i.e. pre-metasomatic mantle: 87Sr/86Sr = 0.702712 and 143Nd/144Nd = 0.513495; xenoliths recording the highest metasomatism: 87Sr/86Sr = 0.704234 and 143Nd/144Nd = 0.512870), and increase in LILE and LREE, but not in Nb and Ti. The resulting geochemical characteristics are similar to those observed in arc magmas and are consistent with the signatures of sediments and basalts forming the upper part of the subducted Nazca plate. Evidence for element fractionation operated by infiltration of the host alkali basalts and related reactive porous-flow processes are lacking or weak. The granoblastic texture of the xenoliths, bearing no evidence of deformation, rules out the hypothesis that the Chenques mantle formed in the subarc and was transported 200 km to the east by reverse corner flow.Alternatively, it is suggested that the slab components were released as supercritical fluids beneath Chenques (at ∼6 GPa) by phengite breakdown. Fluids flowed through, and reacted with, the hidden garnet-facies mantleand reached the observed spinel-facies level. Reactions in the hidden mantle region led the supercritical fluid to increase (La/Sm)N, Th,U, Nb, Pb and Sr while decreasing its initial (Sm/Yb)N and Ti.

2006 - Crustal contamination in the Niquelandia Complex, Central Goias, Brazil: Sr-Nd isotopes and trace-element geochemistry. [Relazione in Atti di Convegno]
Girardi, Vav; Rivalenti, Giorgio; Correia, Ct; Tassinari, Ccg; Munha, Jg; Mazzucchelli, Maurizio; Bertotto, Gw
abstract

The Niquelândia stratiform complex is constituted by two main superposed sequences, separated by a high T shear zone. The lower unit (LS), at the eastern, includes gabbros, peridotites and pyroxenites, and the upper unit (US), at the western part of the body comprises gabro-norites and amphibolites. US is bordered by the volcano-sedimentary Proterozoic Indaianopolis sequence, and LS is in tectonic contact with the terrains of the Archaean basement. The 143Nd/144Nd – 147Sm/144Nd plot do not provide good geochronological data. However LS and US arrays suggest two parallel 870 Ma reference lines, which were used for correlations between Sm-Nd isotopic data and trace-element geochemistry. These diagrams indicate that the Niquelândia complex originates from a depleted mantle source and experienced crustal contamination. The variation fields of US and LS in the 143Nd/144Nd – 87Sr/86Sr diagram, using the least contaminated gabbro as a proxy of the parental melt, indicate distinct arrays for the units. AFC modeling using the Juscelândia volcano-sedimentary sequence as the enriched end-member reproduces successfully the LS variation, whereas the US arrays is compatible with a contaminant characterized by low to moderate 87Sr/86Sr and low 147Nd/144Nd values. This study shows that LS and US may be cogenetic and coeval, if melts underwent contaminations from different components.

2006 - Geochemical and isotope evidence of Nazca Plate-derived components in the Backarc Mantle of Central Patagonia: The Cerro de los Chenques case [Abstract in Atti di Convegno]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Hemond, C.; Zanetti, A.; Vannucci, R.; Bertotto, G. W.
abstract

Abundant spinel-facies mantle xenoliths (dominantly lherzolites and subordinate harzburgites and dunites) occur in the alkaline lavas of the Quaternary Cerro de los Chenques volcano, in the Andean backarc ~200 km east from the Southernmost Southern Volcanic Zone. Major element variation trends in clinopyroxene show decreasing fertility with increasing Mg#. Clinopyroxene REE patterns are: 1) LREE-depleted and flat from Sm to Yb at 4 x PM (Primitive Mantle) concentration (in lherzolites) passing to LEE-enriched clinopyroxenes with spoon-shaped profile; 2) only slightly LREE-depleted or enriched and flat from Sm to Yb, but at concentrations ranging from ~2 to ~4 x PM (in cpx-poor lherzolites); 3) LREE-enriched, steadily fractionated (in cpx-poor lherzolites and harzburgites). All the clinopyroxenes exhibit Th, U and Sr enrichment (increasing in the LREE-enriched samples) and Nb, Ta, Pb and Ti depletion. These features are consistent with a variety of metasomatic processes ranging from complete re-equilibration with the metasomatic agent, to fluid-assisted melting, to percolation into a fertile mantle under decreasing melt mass and consequent chromatographic-like trace element fractionation. Isotopically, the clinopyroxenes form a tight array from a DM region representing the premetasomatic lithosphere, towards a high-Sr, low-Nd region, overlapping that of the Patagonia adakites and arc basalts. This array differs from that of the backarc basalts for having higher Sr at a given Nd isotope value and trends towards the composition of the Nazca sediments. Isotope enrichment positively correlates with the abundance of LILE and LREE and negatively correlates with the Mg# value and Ti, Y and HREE abundances of clinopyroxene, thus indicating that the metasomatic end-member triggering melting of the pristine spinel-facies lithospheric mantle was enriched in LILE and LREE. The better candidate as metasomatic agent, suggested both by isotopes and trace elements, is a slab-derived component containing a large contribution from sediments. However, the slab depth beneath C. Chenques is probably in excess of 180 Km and any component released from the slab had to cross a very thick mantle segment before reaching the spinel-facies level sampled by the xenoliths. Experimental work indicates that at the depth of interest the main water carrier is phengite both in basalts and sediments and that the slab component was probably released as a supercritical fluid having a K-rich, granitic composition and leaving a garnet-dominated residuum. Crystallisation of phlogopite during the upwards travel depletes the fluid in K and Rb and “freezes” its Sr isotope signature. The fluid reaching the spinel-facies has probably an adakitic composition and it is strikingly similar to the potential melts estimated from clinopyroxenes having the highest radiogenic isotopes.

2006 - THE REGIONAL MANTLE PROCESSES IN NORTH-EASTERN BRAZIL: EVIDENCE OF INTERACTION BETWEEN THE CONTINENTAL MANTLE LITHOSPHERE AND THE FERNANDO DE NORONHA PLUME [Abstract in Atti di Convegno]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; Girardi, Vav; Tassinari, Ccg; Zanetti, A; Bertotto, Gw
abstract

We investigated new occurrences of mantle xenolith in the Cenozoic alkali basalts of north-eastern Brazil in order to constrain the regional mantle processes. Xenoliths have been divided in three groups, on the basis of their textural characteristics: G1, porphyroclastic; G2, protogranular; G3, transitional between G1 and G2. Clinopyroxenes from G1 peridotites have REE patterns varying from L-MREE-enriched convex-upward, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. G2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. Most of G3 clinopyroxenes show LREE-depleted patterns similar to the G2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the G1 clinopyroxenes form an array between DM and EMI-like components, both of them also present in the host basalts. Melts estimated to be in equilibrium with the G1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The G2 and G3 clinopyroxenes define two distinct compositional fields at higher 143Nd/144Nd values, correlated with their LREE composition. The isotopes of the G2 and G3 LREE-depleted clinopyroxenes form an array from DM towards the isotopic composition of Mesozoic tholeiitic basalts from north-eastern Brazil. Melts in equilibrium with these clinopyroxenes are similar to these basalts, thus suggesting that such xenoliths record geochemical imprint from older melt-related processes. The LREE-enriched, spoon-shaped G3 clinopyroxenes are characterised by the highest 143Nd/144Nd values at any given 87Sr/86Sr composition. These results are interpreted in terms of a lithospheric mantle section which underwent thermo-chemical and mechanical erosion by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics during Cenozoic time. At that time, the lithospheric mantle consisted of fertile lherzolites and harzburgites recording the geochemical imprint of Mesozoic mantle processes. The onset of the interaction between lithospheric peridotites and alkaline melts was characterised by the porous flow percolation of small melt volumes that induced chromatographic enrichments in highly incompatible elements and the isotope signature of the spoon-shaped, G3 clinopyroxenes. G1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the G2 documents the shallower lithospheric section, with G3 being the transition. The similarity of processes and isotope components in the protogranular xenoliths from Fernando de Noronha area and north-eastern Brazil supports the hypothesis that the lithosphere beneath Fernando de Noronha is a detached portion of the continental one. Furthermore, the similarity in terms of textural and geochemical features documented by the mantle samples coming from the two different regions seems to confirm the interference of the two regions with the same plume.

2006 - Variacao do processo de contaminacao na estratigrafia do Complexo de Niquelandia, Goias, e sua relacao com a cristallizacao ignea: dados geoquimicos e isotopicos [Abstract in Atti di Convegno]
Girardi, Vav; Rivalenti, Giorgio; Correia, Ct; Mazzucchelli, Maurizio; Tassinari, Ccg; Munha, Jm
abstract

Mostrou-se recentemente que a variação isotópica de Sr e Nd, recalculados para 830 Ma, a provável idade de cristalização do Complexo, foi influenciada por processos de contamnaçãocmstal, através da ação de diferentes contaminantes nas suas duas unidades: a superior (US) e a inferior (LS). Esta conclusão 6 reforçada através do exame da variação isotópica e geoquímica das zonas estratigráficas do Complexo. O uso de razões entre elementos traços permite melhor avaliação das variações geoquímicas, face ao fato de serem as mhas do Complexo de Niquelândia produto da mistura de cumulatos e líquidos residuais. A variação máxima dessas razões pode ser controlada por modelos que levam em consideração sistemas fechados (fracionamento) ou abertos ("boundary layercrystallization"). Tais cálculos mostraram que a variação de várias razões entre elementos traços incompatíveis, tais como BaN, LaN, Ba/Zr, NdSr, NdIZr e ZrMf, são muito mais altas que os valores determinados pelos citados modelos. Em particular, na zona basal (BGZ). as razões possuem valores paroxísticos. As zonas situadas estratigraficamente acima desta exibem desde variações de razóes regulares e sinusoidais, até variações explicáveis somente abavbs da adição de material contamnante. Na escala atual de detalhes, diversas variações de razões podem ser detectadas, sendo os picos destas relacionadas à presença de horizontes de material cmstal. Tal ocorre, por exemplo, no topo da Unidade Inferior do Complexo (LS), na sua transição para a Unidade Superior (US), onde as razões de elementos traços de LGZ (Zona Superior de LS) e de UGAZ (Zona Inferior de US) convergem para a composição do material cmstal .As variações isotópicas de 87Sr/86Sr e I43Nd/144Nd, recalculadas a 830 Ma são semelhantes às exibidas pelas razões de elementostraços, embora documentadas por número menor de amostras. Embora as razões dos elementos traços e os valores isotópicos das amostras contaminadas convirjam para a amostra representativa do material crustal, esta situa-se fora da curva de variação das rochas do Complexo, e, portanto não pode ser o representante indubitável do contaminante de seu magma original. Tal se daria somente se o processo fosse do tipo ACF. Os dados presentes esclarecem o processo de crescimento do Complexo de Niquelândia, que consiste em sucessivas injeções de magmas derivados de um manto empobrecido, cada umadelas contaminadas por uma mistura de fusos residuais da cristalização magmática com os provenientes de rochas crustais. Neste cenário alterações geoquímicas potenciais provocada por metamorfismo parecem irrelevantes em relação aos elementos traços e isótopos considerados.

2005 - Bronze age axes from the “Terramare” area (Modena – Emilia Romagna, Italy). [Abstract in Rivista]
Giovannini, Sonia; Cardarelli, Andrea; Mazzucchelli, Maurizio
abstract

abs. 10.1474, 01-0437.We have undertaken a systematic study of Bronze axes found in some Early to Middle Age sites of the Modena province conserved at the Modena Archaeological and Ethnological Museum of Municipality. The axes show different forms, reflecting an evolution in the metallurgic craft process and in the raw material used.Aim of this study is to identify a suitable method010 y in order to obtain the most reliable data by collecting the minor amount of fragments from the archaeological remains. Analyses at first were performed on two axes reproduced by using experimental archaeometallurgic techniques and one remain of Early Bronze Age (Savignano 994). Due to their state of conservation, it was possible onthese objects to obtain polished thin sections normally to the elongation axis of the manufact from.Analytical techniques were Electron Microprobe (EMPA), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and a microstructural characterisation using light microscopy and Scanning Electron Mlcroscopy (SEM). A comparison of the chemical analyses from polished thin sections with those from fragments of the same objects permits the assess of the minimum number of the micro-cores sampling needed to obtain unbiased estimates of the mean composition.Successively, the study was extended to about twenty archaeological finds. From this items, some fragments were took out by using a pillar micro-drill with a tip of one millimetre diameter. EMPA analyses were performed on these fragments in order to determine the major and minor element concentration (Cu; Sn; Pb). Concentration of minor and trace element was estimated by ICP-OES (Sn; Ag; Au; As; Sb, Ni; Bi; CO; Fe; Ge; Ga; In; Sr; Cr; Se; V; Mn; Cd; Zn). Within the limits of the present restricted sample population the analytical results show the following characteristics: a) a confirmation of the well known trend of increasing Sn content from the Early Bronze to the Middle-Recent Bronze Age finds; b) an abrupt decrease of the Sn content in the alloys of the Recent Bronze Items. Concomitantly with the latter variation, the artefacts show an increase in In (from negligible amounts to 40-200 ppm) and Zn, and a decrease in As, Sb, Ni, Cr. These variations may be caused by: a) employment of different raw material; b) different sites of purchasing of raw material; c) variation of procedures of copper and tin mining (new scorifier used); d) other processes. Major and trace element analyses of other artefacts and various possiible raw material are currently in progress, in order to better constrain the above possibilities. In this context, of crucial interest for archaeometric studies is: a) the determination of the trace element partition coefficients between the metal and the slag phases during the smelting of the raw material; b) the factors controlling their variations; c) the behaviour of trace elements during the ingot melting and artefact production.

2005 - Geochemical variations in the back-arc, South Patagonian lithospheric mantle beneath Patagonia (South America): towards a geodynamic model [Abstract in Rivista]
Vannucci, R; Zanetti, A; Rivalenti, Giorgio; Mazzucchelli, Maurizio; Cingolani, Ca
abstract

Samples of the mantle wedge of convergent plate regions are rare and direct evidence of the interaction between slab-derived components and wedge is therefore unusual. Abundant occurrences in Patagonia of mantle xenoliths in alkaline volcanic centres located in the backarc region of the Andean subduction zone (Stern et al., 1990; Laurora et al., 2001; Rivalenti et al., 2004a,b), 300 – 600 km east from the Chile trench provide the possibility to study these processes. Research on Patagonia arc and back-arc magmas provides evidence that the mantle wedge, fertilised by slab components, played an important role either as the source of the arc magmas or as a component of the plateau and post-plateau back-arc basalts. These latter consist of older (12-5 Ma) tholeiitic sequences that formed voluminous main-plateau sequences and younger (7-2 Ma), less voluminous alkaline lavas with OIB-like characteristics erupted mostly from monogenetic volcanic centres. Stern et al. (1990) recognised an eastward decrease of slab-related geochemical features in back-arc basalts and proposed a supra-slab model for their origin. More recently, a sub-slab model has been envisaged with partial melting of sub-slab sources and migration of magmas to the surface through slab-windows (D’Orazio et al., 2001; Gorring and Kay, 2001). Abundant occurrences of mantle xenoliths in alkaline volcanic centres located in the back-arc South Patagonian provide a general overview of the lithospheric mantle composition and a reconstruction of its depletion and metasomatic enrichment processes. Combined petrological and texture-related geochemical investigations allowed us to address some important and still debated questions: • which is the role of the lithospheric mantle in the determination of the East-West geochemical variability of the lavas? • is the continental lithospheric mantle a potential reservoir for the Southern Patagonia magmas? • is the eastward fading of the slab signatures in the basalts recorded also in the lithospheric mantle? • which is the geodynamic mechanism determining the upwelling of the asthenosphere in the Patagonian back-arc area (extensional tectonic stress, mantle plume, slab window)? • how can the geochemical composition of the mantle ultramafics be used to infer the composition of migrating melts ? In particular, did the mantle wedge interact with carbonatitic or adakitic metasomatic agents as it has been inferred for Gobernador Gregores (Gorring and Kay, 2000) or some westernmost occurrences (Kilian and Stern, 2002), respectively? Xenoliths from ten localities from the Rio Negro to the Santa Cruz provinces were examined to address the important issues. Major results based on the trace element signatures of minerals and glasses determined in-situ by SIMS and LA ICP-MS techniques as well as isotopic signatures of mineral separates from Pali-Aike and Gobernador Gregores xenoliths can be summarised in the following: • the compositional regional variations observed in Patagonia volcanics (transitional to OIB-like basalts) are much weaker in basalt-borne xenoliths; nevertheless, an eastwards fading of possible slab-related geochemical signatures is observed; • only few spinel-bearing xenoliths document the presence of highly depleted lithospheric mantle domains related to old partial melting episodes;• the majority of sp-bearing xenoliths provide textural and chemical evidence of significant re-crystallisation due to interaction with melts; in westernmost and northernmost localities cpxcompositions suggest either equilibrium or reaction with arc derived magmas and transitional basalts; • in southernmost and easternmost occurrences lithospheric mantle has been extensively percolated via reactive porous flow by uprising OIB-like asthenospheric melts, which probably erased at various extent the signatures of previous metasomatic events; • the thermo-chemical erosion of the lithosphere is particularl

2005 - Melt percolation and reaction in the Balmuccia peridotite. [Abstract in Rivista]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; Brunelli, Daniele; Paglioli, S; Zanetti, A.
abstract

abs. 10.1474, 01-1081.The Balmuccia lherzolite massif is traditionally considered as a document of the continental lithospheric mantle variably depleted by melting and melt extraction processes. Interaction with percolating melts were only described in the peridotite, over a maximum thickness of a few decimetres, at the contact with websteritic dikes belonging to the Cr-diopside and to the Al-Augite suites, which are the result of focused melt flow and fractionation into fractures or ductile extension zones. Recent detailed mapping of the peridotite heterogeneity, however, revealed the existence of peridotite domains which have petrochemical features ascribable to reactive porous flow of melts of various compositions. These domains can be distinguished into three types:1) regions constituted by cpx-bearing dunite or wehrlite, which contains chromitite horizons and pods up to 7 cm thick. The dunite-wehrlite bands are subconcordant to slightly discordant with the lherzolite foliation, reaching a thickness of about 6 metres and fade into zones with porphyroblastic opx. The process producing chromitite and dunite-wehrlite predated the emplacement of the oldest Cr-diopside dikes. The infiltrated melts, estimated from clinopyroxene trace element characteristics, had alkali-tholeiitic transitional affinity. 2) regions of porphyroclastic lherzolite-harzburgite, in which the porphyroclasts are represented by up to 1.5 cm large orthopyroxenes. These domains, about 10 metres large, are apparently concordant with the protogranular foliated lherzolites. The process producing the porphyroclast was broadly synchronous with the emplacement of the Cr-diopside dykes. These porphyroclastic lherzolites and harzburgites, however, contains boudinaged lenses of Cr-diopside websterite and orthopyroxenite, on their turn crossed by later Cr-diopside dykes. Texturally evident substitution of olivine by orthopyroxene indicates that the percolating melts were silica saturated.3) Regions of dunite, up to 20 metres large, containing pockets and veins constituted by large amphibole crystals, plagioclase, spinel and rutile. Dunite fades into lherzolite containing orthopyroxene porphyroblasts, suggesting the infiltration of hydrous basalt and of SiO2-saturated melts. Field relationships reveals that the process producing dunite post-dates the emplacement of all the Cr-diopside dykes and the early Al-Augite dykes.Studies on the processes, nature of the infiltrated melt and its variations during flow are in progress.

2005 - Mesozoic Magmatism in the Ivrea-Verbano Zone and its geodynamic implications. [Abstract in Rivista]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; Peressini, G.
abstract

abs. 10.1474, 01-0560.In the Southern Alps, Mesozoic magmatism is well documented in the Dolomite region and in the Lombard Pre-Alps. In the western portion of the Southern Alps, the Mesozoic magmatic activity has received much less attention.In the present study we report a review of geochronological evidences of the existence of Mesozoic magmatism in the portion of the Southern Alps west of the Lugano lake. The Mesozoic events are exclusively located: a) south of the Cremosina fault system; b) in the northeasternmost portion of the Ivrea-Verbano Zone.a) A suite of diorite-norite dykes occurring into the Baldissero mantle peridotite (Southern Ivrea-Verbano Zone) has been recently recognized. The age of dyke intrusion is currently constrained by: a) a two point mineral (plagioclase + clinopyroxene) best fit calculated from Sm-Nd isotopic data which yields a slope corresponding to an age of 18026 Ma, with a Ndi = 0.512804 and Ndi = 7.8 (Obermiller, 1984, PhD Thesis Un. Mainz, Germany); b) Re-Os model ages (Re depletion model age) between 140 and 190 Ma calculated on whole rock samples of the ambient peridotite (Mazzucchelli et al., 2004, EGU04 Geophys. Res. Abs., 6, 03966). Moreover a number of acid tuff layers from mm to several meters in thickness are present in the Mesozoic sedimentary cover of the Southern Alps from the Lugano to Biella area. They occur in the Crevacuore and Sostegno sedimentary succession, in the Villafortuna-Trecate oil field, and in the sedimentary cover of Monte San Giorgio (Ticino, Switzerland). In the Monte San Giorgio occurrence, high-resolution U-Pb zircon age gives 241±0.8 Ma (Mundil et al., 1996, Earth Planet. Sci. Letters, 141, 137-151). b) In the Basic Complex cropping out in the Finero and Val Grande area, most of the isotopic data invariably give Triassic or Early Jurassic ages. The Basic Complex in this area shows an antiformal structure, constituted by various cumulus rocks and gabbroic lithotypes (Internal Gabbro, Hornblende Peridotite and External Gabbro Units). At the core of the antiform, in tectonic contact with the rocks of the Complex, a mantle phlogopite-bearing peridotite occurs, whose metasomatic imprint was attributed to crustal components, deriving from a subducting slab. The age of metasomatism is Mesozoic [207 Ma - U/Pb on zircons from chromitites (Von Quadt et al., 1992, Ivrea-Verbano Zone Workshop, U.S. Geol. Survey Circular 1089, Abs., 20.); 226-177 Ma - Rb/Sr internal isochrons on amphibole and phlogopite pairs (Hartmann & Wedephol, 1993, Geochim. Cosmochim. Acta, 57, 1761-1782); 220 Ma - Ar/Ar on phlogopite (Hartmann & Wedephol, 1993)]. Magmatic, subeuhedral, pink crystals with oscillatory zoning in CathodoLuminescence (CL) from the External Gabbro Unit has been recently dated with SHRIMP (Peressini et al., 2005, this session). Magmatic growth of the zircons took place at 232±2 Ma and was overprinted at 214±5 Ma by a second event, dated by the rim-recrystallization ages. These ages are well in accordance with the literature data reported for the Basic Complex. In spite of the similar ages, the Basic Complex does not record any evidence of the metasomatic agent which affected the mantle peridotite. The Finero Basic Complex is in tectonic contact, marked by a high-temperature ENE shear-zone, with the Permian (Peressini et al., 2005, this session) relatively anhydrous mafic-ultramafic sequences occurring in Val Sesia and on the right side of the Val d'Ossola.Presently, the evidences of Mesozoic magmatism in the westernmost sector of the Southern Alps are confined by tectonic lineaments to the southernmost and northernmost portion, respectively. This put new constraints for the comprehension of the geodynamic reconstruction of the whole Southern Alps.

2005 - Slab-related mantle metasomatism: A SIMS investigation of LLE composition of Finero Phlogopite-peridotite (W Alps). [Abstract in Rivista]
Raffone, N; Lefevre, B; Zanetti, A; Ottolini, L; Vannucci, R; Rivalenti, Giorgio; Mazzucchelli, Maurizio
abstract

The Finero mafic-ultramafic complex (Ivrea-Verbano Zone, Western Italian Alps) offers a unique opportunity to study km-scale hydrous melt migration on a outcropping mantle body. The ultramafic mantle section is mainly formed by spinel harzburgites, which contain hydrous mineral assemblages (amphibole + phlogopite ± apatite) and show LILE (Cs, Rb, Ba, K, Sr, LREE) enrichments. Moreover, these peridotites are characterised by large radiogenic Sr and Pb and low radiogenic Nd (Voshage et al., 1987; Cummings et al., 1987; Hartmann and Wedepohl, 1993). Such petrochemical features have been supposed to be related to pervasive migration of slab-derived melts through a supra-subduction mantle wedge (e.g. Zanetti et al. 1999; Prouteau et al., 2001; Morishita et al., 2003). Alternative hypotheses involve the upward migration of different kinds of alkaline melts (from K-rich differentiates to carbonatites), in the frame of variable geodynamic environments (Grieco et al., 2001, 2004; Garuti et al., 2001; Zaccarini et al, 2004). A new SIMS investigation has been accomplished on clinopyroxenes (Cpx) from the mantle sequence of the Finero peridotite, in order to place further constraints on the nature of the ascending melt(s) and on the genetic and temporal relationships occurring among the different ultramafic units so far recognised. Micro-analytical characterisation of selected LILE, HFSE and REE was combined with the in-situ determination of Li and B concentration (Light Lithophile Elements: LLE), because the fractionation of these elements represents a valuable geochemical marker about the presence of slab-derived components in uprising magmas (Ottolini et al., 2004). Five spinel-facies samples, namely one phlogopite pyroxenite, two apatite-free phlogopite harzburgites, one apatite-bearing phlogopite wehrlite and one chromitite, representative of the main Finero ultramafics were selected for this study. Pyroxenite is a pegmatitic websterite, in which clinopyroxene (the dominant phase), amphibole and orthopyroxene are up to 16, 13 and 6 mm long, respectively. Phlogopite is also present with crystals up to 3 cm long. Minerals do not show deformation. Amphibole and phlogopite normally occur as interstitial phases. Unlike pyroxenite, the peridotites show amphibole modal concentration exceeding that of clinopyroxene. The apatite-free phlogopite harzburgites (the dominant mantle lithology) have porphyroclastic textures. Olivines, and sometimes orthopyroxenes, show evidence of ductile deformation, which is absent in most of clinopyroxenes and in the hydrous minerals. In the apatite-bearing wehrlites, orthopyroxene is lacking or occurs only in trace amounts. They have a mosaic equigranular texture, consisting of crystals of equant dimension often with 120° triple junctions. Modal abundance of amphibole and phlogopite increases up to 25 and 4% by volume, respectively. Apatite occurs as interstitial grains, sometimes associated with dolomite, and within fractures cross-cutting large olivine. Dolomite is also present as inclusion in spinel. Dunite-chromitite bands have secondary coarse granular texture, modally dominated by very large olivine and spinel grains. Clinopyroxene and amphibole are randomly present as strongly anhedral interstitial phases. Locally, phlogopite-rich layers occur. The B content is 3.9-9.7 ppm and 1.3-1.6 ppm in Cpx from phlogopite harzburgites and pegmatitic websterites, respectively. Such concentrations are rather large for mantle minerals and lie in the range of the lower continental crust (Leeman and Sisson, 1996). Conversely, Cpx from apatite-bearing wehrlite and dunite-chromititebands have low B concentrations (~0.8 and ~0.6 ppm, respectively), which are consistent with those of the upper mantle (<1 ppm) (Chaussidon and Jambon, 1994). Li contents show a narrow range. However, Cpx from phlogopite harzburgites have lower Li concentrations (0.5-1.1 ppm) than pegmatitic web

2005 - Slab-related metasomatism in the Patagonia backarc lithosphere of the Andean subduction: the Cerro de los Chenques case. [Abstract in Rivista]
Rivalenti, Giorgio; Zanetti, A; Mazzucchelli, Maurizio; BERTOTTO G., W; Hemond, C.
abstract

abs. 10.1474, 01-1080.Cerro de los Chenques is a monogenic quaternary volcano containing spinel facies mantle xenoliths and occurring just North of the latitude of the triple point between the Nazca, Antarctic and South America plates. It overlies the subducted Nazca Plate and occurs at the margin of the supposed slab window induced by the impact of the Chile ridge against South America (Ramos and Kay 1992, Tectonophysics 205, 261-282; Gorring et al. 1997, Tectonics 16, 1-17). Xenoliths are anhydrous lherzolites and less frequently harzburgites and dunites having porphyroblastic to granoblastic texture. All the xenoliths contain glass veins that represent infiltration of the host basalts. The interaction of these glasses with the matrix mineral phases resulted in a very large compositional range that overlaps the composition of the glasses often considered as representative of specific, different, metasomatic agents. For this reason the mantle processes and the nature of the metasomatic agent are constrained only on the basis of matrix mantle clinopyroxene. Clinopyroxenes from the lherzolites are predominately LREE–depleted (Cen/Ybn = 0.26 - 0.91), being the occurrence of LREE-enriched (Cen/Ybn = 1.34 - 2.60) composition limited to very few samples (Cen/Ybn = 1.34 - 2.60). Clinopyroxene compositions in harzburgites vary from steadily LREE-enriched (Cen/Ybn = 14.4) to U-shaped, whereas in dunite clinopyroxene has a slightly concave-upward profile, which mimics in the range MREE-HREE (although at a lower concentration level) that observed in lherzolites.All the pyroxenes are enriched in Th, U and La with respect to Nb and Ta. Th and U enrichment and Nb relative depletion increases in the LREE-enriched lherzolites, in the harzburgites and in the dunite. In general, U/Th is > 1. With the exception of harzburgite clinopyroxenes, Zr and Hf are depleted relative to REE. Similarly, Ti usually defines a negative anomaly and its depletion increases in the LREE-enriched lherzolites and harburgites. The clinopyroxene 143Nd/144Nd and the 87Sr/86Sr value are in range of 0.513495(±7) – 0.512789(±4) and 0.702712(±8) – 0.704234(±8), respectively. Sr-Nd isotopes form an array from depleted mantle to an enriched component, which overlaps that of the Nazca plate MORB with a small contribution of oceanic sediments. The 143Nd/144Nd values increase with increasing MgO, Na2O and TiO2 and decreases with increasing La, Ce, Sr, Zr, U and Th concentrations and the Cen/Ybn and Zr/Hf values of clinopyroxene. Sr isotopes display complementary correlations with the geochemical parameters. These compositional relationships permit to constrain the characteristics of the metasomatic end-member, which has low Nd and high Sr isotope values and is enriched in incompatible element. It is proposed that this agent interacted with a depleted lithosphere having low Sr and high Nd values by reactive porous flow processes under decreasing porosity. Further support to contributions from the subducted Nazca plate are also indicated by the similarity of the potential melts in equilibrium with clinopyroxene with subduction-related arc magmas from Patagonia.

2005 - Slab-related metasomatism in the Patagonia backarc lithosphere of the Andean subduction: the Cerro de los Chenques case. [Abstract in Rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Zanetti, A; Vannucci, R; Bertotto, Gw; Bollinger, C; Cingolani, Ca
abstract

Recent research on the backarc mantle lithosphere in Patagonia has shown that it suffered a regional, pervasive, re-crystallisation and that the regions to the north and south of the latitude of the actual triple point (40°30’) between the Antartctica, Nazca and South America plate present important differences in the regional distribution of metasomatic components. Subduction of the Chile ridge is believed to have induced an interruption in the subducting plate (slab window, D’0razio et al. 2001, Gorring and Kay, 2001), upraise and melting of the sub-slab astenosphere, producing the abundant plateau and postplateu OIB like lavas, which record slab component only in the westernmost occurrences (Stern et al., 1990). The Cerro de los Chenques locality occurs immediately to the north of the latitude of the Triple point, it overlays the subducted Nazca plate and occurs at the edge of the putative slab window. It is, therefore, of especial importance for revealing the processes and constraining the metasomatic components of the mantle wedge in a complex geotectonic situation.Xenoliths occurring in alkaline lavas are predominately granular to porphyroblastic spinel-facies lherzolites, being harzburgites and dunites less abundant. They do not contain hydrous phases, but may contain trace amounts of glass derived from infiltration of the host basalt. They record equilibrium temperature in the range of 900-1100°C. Bulk-rock and mineral phases exhibit major element variation trends apparently consistent with melting and melt removal processes. This is also the case for moderately incompatible elements (e.g. HREE) in bulk rock and clinopyroxene, but not for highly incompatible elements.In lherzolites, clinopyroxene varies from moderately LREE depleted (Cen/Ybn 0.26 – 0.91) to moderately LREE-enriched (Cen/Ybn up to 2.6), being the REE pattern virtually flat in the range Eu – Lu at a 2 to 5 xPM concentration level. Harzburgitic clinopyroxene is LREE-enriched and the whole REE pattern varies from U-shaped to steadily fractionated (Cen/Ybn 14.4) In dunite, cpx is slightly LREE enriched. All the pyroxenes are enriched in Th, U and La with respect to Nb and Ta and depleted in Ti and Pb with respect to Gd and Ce, respectively. The 87Sr/86Sr value in clinopyroxene ranges 0.70279 – 0.70423 and 143Nd/144Nd 0.51279 – 0.51349. In the Sr-Nd isotope diagram, clinopyroxenes constitute a tight array (R2 = 0.79) between a highly depleted end-member and a component enriched in 87Sr/86Sr with respect to BE. The C. Chenques array runs at higher 87Sr/86Sr than the field of the Gobernador Gregores xenoliths and it is followed also by the xenoliths from P. Indios and E. Alvares (northern Patagonia) and Fraile and Cumbres (south of the volcanic gap). It is also on line at the enriched side with the isotope values of the C. Pampa, Lautaro and Aguilera adakites (those closest to the present occurrence).Isotope variations correlate with bulk-rock and cpx composition, so that the increase of 87Sr/86Sr correlates in cpx with an increase of the Mg# value and Sr, Zr, LREE, Pb, Th and U concentrations and with the decrease of Na, Fe, Ti, V, Y, Nb and HREE concentrations. Obviously the 143Nd/144Nd variation with respect to the geochemical parameters has the opposite behaviour of Sr. Assuming higher porosity in depleted peridotites, these data may indicate that a metasomatic agent affected a mantle that suffered previous variable depletion, being the amount of the metasomatic component proportional to the depletion. Alternatively, the variation trends may indicate melting triggered by the addition of a metasomatic agent and removal of the resulting hybrid melt. The increase of the highly incompatible element concentrations with increasing isotope fertility favours the first possibility. The trace element - isotope relationships reveal that the added component was enriched in LILE, LREE, Zr and depleted in Nb and Ti. T

2005 - The mantle lithosphere in north-eastern Brazil and the records of the Fernando de Noronha plume. [Abstract in Rivista]
Rivalenti, Giorgio; Zanetti, A; Mazzucchelli, Maurizio; GIRARDI V. A., V; Tassinari, C. C. G.; Bertotto, G. W.
abstract

The Cenozoic alkali basalt centres in north-eastern Brazil are believed to represent the surface track of the passage of the craton over the Fernando de Noronha plume (O’Connor and Duncan, 1990). This hypothesis is at present largely unconstrained because the only one previously studied xenolith occurrence, Pico Cabuji, showed large isotopic differences with xenoliths from the Fernando de Noronha island (Rivalenti et al., 2000). We revisit this hypothesis on the basis of a larger geochemical and isotope data set on the xenoliths of unstudied Brazilian localities (Fig. 1). Texturally, the xenoliths define three group: 1) porphyroclastic (lherzolites, harzburgites and one wehrlite); 2) protogranular (lherzolites and harzburgites); 3) transitional between groups 1 and 2 (lherzolites and harzburgites). Equilibrium temperature and pressure ranges 950 – 1280°C, 1.9 – 2.5 GPa in group 1, 830 – 990°C, 1.3 – 1.8 GPa in group 2, 890 – 980°C, 1.5 – 2 GPa in group 3. . The three textural groups have a similar mg# range in clinopyroxene, but the porphyroclastic clinopyroxenes have, at any given mg# value, lower Ti, Al, Ca and Na and higher Fe concentrations (a.f.u.), with respect to the protogranular ones, and the cpx of the transitional xenoliths define an array intermediate between the others. Orthopyroxenes have opposite characteristics with respect to cpx, so that the porphyroclastic ones are the richest in Ti, Al, Ca, Na and the lowest in Fe. In group 1 the REE patterns of clinopyroxenes vary from convex upward, typical of phases in equilibrium with alkaline melts, to spoon shaped, to steadily LREE-enriched in wehrlite (Fig 2). Group 2 clinopyroxene are predominately LREE—depleted and enriched only in Th and U and group 3 clinopyroxenes are similar to those of group 2, but have higher LREE concentrations and in some cases spoon-shaped profiles. Sr and Nd isotopes of the group 1 clinopyroxenes form an array between DM and an EMI-like component, present also in the host basalts, whereas the protogranular xenoliths exhibit large spreading of the Sr-isotopic values (Fig. 3). The samples with higher 87Sr/86Sr trend towards EMII. Potential melts in equilibrium with the group 1 clinopyroxenes are similar to the alkali basalts hosting the xenoliths and those estimated from protogranular clinopyroxenes are similar, except for a higher concentration of the highly incompatible elements Th and U, to Mesozoic tholeiitic basalts of north-eastern Brazil. These results are consistent with the following conclusions: a) the xenoliths sampled a mantle section whose deepest and shallowest regions are represented by the group 1 and 2 xenoliths, respectively, being those of group 3 derived from an intermediate region; b) the geothermal gradient of the section was as high as 15°C/Km; c) geochemical modelling supports that the deepest region underwent reactive porous flow of alkali basalt, isotopically closer to EMI, whereas metasomatism was controlled by percolation and crystallisation under decreasing melt mass in the regions represented by the group 3 and 2 xenoliths; d) lithosphere predating metasomatism is largely represented by the group 2 xenoliths, which isotopically plot in between DM and an EMII component, thus considered as lithospheric. Similarity with Mesozoic basalts of the group 2 and 3 potential melts calculated from clinopyroxene and high 87Sr/86Sr in clinopyroxene suggests that these xenoliths predominately record mantle processes predating the Cenozoic event. A comparison of the characteristics of north-eastern Brazil and Fernando de Noronha shows that the respective lithospheres record similar stratigraphy, interaction with an asthenospheric EMI component and also similar heterogeneity in the cooler lithosphere section documented by the protogranular xenoliths. This similarity lends support to the notion that the lithosphere beneath Fernando de Noronha is a detached piece of the conti

2005 - U-Pb SHRIMP and REEs - LA-ICPMS zircon data constrain the Anisian emplacement of the External Gabbro unit of the Finero Mafic Complex in the Ivrea-Verbano Zone (South-Alpine, NW-Italian Alps): new hints on the Triassic evolution of Adria. [Abstract in Rivista]
Peressini, G; Rivalenti, Giorgio; Mazzucchelli, Maurizio; HOFMANN A., W; Zanetti, A; Fanning, M.
abstract

abs. 10.1474, 01-0560.The northern part of the mafic-ultramafic Ivrea-Verbano (IV) complex, the Finero region, differs from the rest of the pluton in terms of petrology and geochemistry of mantle peridotites (e.g. the phlogoplte-bearing metasomatism shown by Flnero harzburgite is absent in the other mantle massifs of IV zone), as well as for the lithoiogy and geochemistry of mafic intrusions. Besides, the Finero Mafic Complex shows different relationship with the Kinzigite Formation, the metamorphic sequence into which it intruded. The emplacement age of the Mafic Complex in the Balmuccia area has been proved to be homogeneously between 282 and 189 Ma (Peressini & al, this congress).Conversely, in this contribution we provide evidence for a substantially different age of emplacement of the External Gabbro unit of the Finero Mafic Complex. We performed a detailed zircon study on 5 samples from the External Gabbro, most of which yielded a homogeneous population of colourless zircons. Only one sample showed a composite population, comprised of:a) maqmatic, subeuhedral, pink crystals with oscillatory zoning in cathodoluminescence (CL);b) detrital zircons, rounded, small, colourless grains with blurred, "granulitic" CL-patterns.Three events, clearly recognczable in CL, were dated with SHRIMP U-Pb analyses:1) primary magmatic crystallization of the pink zircons took place at 232 +/- 3 Ma;2) the pink zircon rims suffered a distinct recrystallization event at 215 +/- 4 Ma;3) a metamorphic age-peak at 280-310 Ma is clearly, although imprecisely (U below: 20 ppm), constrained by the colourless zircons.The U-Pb data of the colourless zircons are consistent with the age of the major event (ca. 300 Ma) that resets the U-Pb system of the zircons from the Kinzigite Formations, being compatible with both magmatic and metamorphic ages in othe sectors of the IV-zone (Vavra & al., Contr. Min. Pet., 134, 1999). This observation together with the degree of crustal contamination revealed by Nd and Sr isotopes on the same samples (Lu & al, Chem.Geol. 40, 1997), suggests that colourless zircons and zirconium were inherited/digested ffom the country rock. No magmatic Paleozoic zircon was found, as opposed to the predominant Permo-Carboniferous magmatism in the Baimuccia sector of the IV-Zone.Trace elements were measured on all zircon domains, and confirm the occurrence of a geochemical difference between the two age-groups. Triassic zircons have typical HREE-enriched strongly fractionated patterns from which those of the Permo-Carboniferous ones differr for the slightly more flattened HREE. Moreover, the Nb/Ta is ca. 1 for Mesozoic zircons, but ca. 10 for the Paleozoic ones. SHRIMP (U-Pb) and LA-ICP-MS (traces) zircon data presented in this study allow us to recognize an Anisian emplacement of the External Gabbro unit. A separate thermal event took place in Norian time at 215 Ma (see also Stähle & al, SMPM 70, 1990; von Quadt & al, SMPM 73, 1993, Vavra & Schaltegger, Contr. Min. Pet. 134, 1999). The Anisian emplacement of tholeiitic rocks in the western South-Alpine, and the possibly Triassic age of some events of mantle metasomatism in Finero (Grieco & al. 2001, J. Pet. 52, Zaccarini, in prep.), constitute importantinformationf or the interpretation of Mesozoic magmatism in the Southern Alps, and can have regional-scale implications for the geodynamic events preceding the opening of the Neo-Tethys.In the IV zone, magmatic ages cluster in two ranges:1. Permo-Carboniferous (280-315 MA, Peressini $ al., this congress);2. Early Mesozoic (Zircon U-Pb: Finero 210-235 Ma, this study, but also: Rimella 200-250 Ma Surface Controlled Alteration, and Val Fiorina, 215-245 Ma Vavra $ al., 1999; Sm/Nd ages Baldissero 160-200 Ma, Mazzucchelli $ al., 2005), an age distribution similar to that of most gabbroic intrusions in the Alpine-Appenine system (Rampone & al, J. Pet. 46, 2005).In Finero, the Paleozoic and the t

2004 - Equivocal carbonatite markers in the Mantle xenoliths of the Patagonia Backarc: the Gobernador Gregores case (Santa Cruz province, Argentina). [Abstract in Rivista]
Rivalenti, Giorgio; Zanetti, A.; Mazzucchelli, Maurizio; Vannucci, R.; Cingolani, C. A.
abstract

SRef-ID: 1607-7962/gra/EGU04-A-03921.Mantle xenoliths at Gobernador Gregores consist of spinel-facies anhydrous, depleted lherzolites and harzburgites and, more abundantly, of hydrous (amphibole +/- phlogopite +/- apatite-bearing) lherzolites, harzburgites and wehrlites. The hydrous xenoliths display modal and geochemical bulk-rock and mineral phase characteristics held as indicators of carbonatitic metasomatism. However, several lines of evidence do not support carbonatitic metasomatism. For instance, a modal trend towards wehrlite is shown by hydrous xenoliths having high TiO2/Al2O3, whereas the samples with low TiO2/Al2O3 modally trend towards harzburgite. Carbonatitic geochemical features characterize both groups, but are more marked in the harzburgites of the second group. The entity of the Zr, Hf and Ti negative anomalies in bulk-rock, clinopyroxene and amphibole steadily increase at decreasing Na2O and HFSE concentrations. Steady variation trends between xenoliths do not having and having carbonatitic characteristics rules out the possibility that carbonatitic metasomatism overprinted on, or was followed by, a basaltic metasomatism that affected a previously depleted mantle lithosphere. They, rather, suggest a control by reactive porous flow of only one agent, inferred to be initially a ne-normative hydrous basalt (because of the presence of wehrlites) evolving towards silica saturation. Variation trends exhibit cusps when amphibole appears in the mode. Appearance of amphibole may explain the Ti anomaly variations, but not those of Zr and Hf. Numerical modelling (Plate Model; Vernières et al., 1997) gives results consistent with the observed geochemical features by assuming the presence of loveringite. ModestHFSE anomalies in the infiltrating melt may also be acquired during percolation in the garnet-facies peridotites beneath the spinel-facies level documented by the xenoliths sampled by the basalts, or directly from the asthenospheric melt source fertilised by component derived from the eclogitic subducted slab.ReferenceVernières J., Godard M., Bodinier J-L. (1997) J. Geophys. Res. 102, 24771-24784.

2004 - Equivocal carbonatite markers in the mantle xenoliths of the Patagonia backarc: the obernador Gregores case (Santa Cruz Province, Argentina) [Articolo su rivista]
Rivalenti, Giorgio; A., Zanetti; Mazzucchelli, Maurizio; R., Vannucci; Ca, Cingolani
abstract

Amphibole +/- phlogopite +/- apatite-bearing mantle xenoliths at Gobernador Gregores display modal, bulk-rock and phase geochemical characteristics held as indicators of carbonatitic metasomatism. However, part of these xenoliths has high TiO2/Al2O3 and those displaying the most pronounced carbonatitic geochemical markers modally trend towards harzburgite. Bulk-rock, clinopyroxene and amphibole show Zr, Hf and Ti negative anomalies, which increase at decreasing Na2O and high field strength elements (HFSE) concentrations. Steady variation trends between xenoliths which have and do not have carbonatitic characteristics suggest a control by reactive porous flow of only one agent, inferred to be initially a ne-normative hydrous basalt (because of the presence of wehrlites) evolving towards silica saturation. Variation trends exhibit cusps when amphibole appears in the mode. Appearance of amphibole may explain the Ti anomaly variations, but not those of Zr and Hf. Numerical modelling [Plate Model (Vernieres et al. in J Geophys Res 102:24771-24784, 1997)] gives results consistent with the observed geochemical features by assuming the presence of loveringite. Modest HFSE anomalies in the infiltrating melt may be acquired during percolation in the garnet-facies.

2004 - Mesozoic Igneous event into the Baldissero mantle peridotite (Ivrea-Verbano Zone, NW Italy)? [Abstract in Rivista]
Mazzucchelli, Maurizio; Correia, C. T.; Rivalenti, Giorgio; Peressini, G.; Zanetti, A.; Tassinari, C. C. G.; Girardi, V. A. V.
abstract

SRef-ID: 1607-7962/gra/EGU04-A-03966A suite of diorite dykes occurring into the Baldissero mantle peridotite (Southern Ivrea-Verbano Zone) shows an unusual geochemistry, characterised by high mg# values in bulk rock and mineral phases and a positive correlation of this parameter with incompatible element concentration. The dykes, in spite of their fine-grained texture, contain large amounts of cumulus phases. Their geochemical characteristics may be explained by melt flow, accompanied by crystallisation and reaction with the ambient peridotite, of a hydrous basaltic melt. The least evolved melts in this process are those that have the lowest incompatible element concentration and mg#. Reactionswith the ambient peridotite are very limited and consists of olivine dissolution and orthopyroxene crystallisation, so that an orthopyroxenite (orthopyroxene, spinel, minor olivine and clinopyroxene) rim, less than 0.5 cm thick, may appear at the contact. Very smooth geochemical gradients, which extend for a few cm inside the ambient peridotite, may be present. The age of dyke intrusion is currently constrained by: a) a two point mineral (plagioclase + clinopyroxene) best fit calculated from Sm-Nd isotopic data which yelds a slope corresponding to an age of 180 +/- 26 Ma, with a Ndi = 0.512804 and epsilon Ndi = 7.8 (Obermiller, 1994); b) Re-Os model ages (Re depletion model age) between 140 and 190 Ma calculated on whole rock samples of the ambient peridotite. Other isotopic determinations are in progress. More indications of Mesozoic ages, obtained in other parts of the Ivrea-Verbano Zone, are reported in Lu et al. (1997) and Peressini et al. (2004).The presence of remarkable episodes of Mesozoic magmatism in the Ivrea-Verbano Zone furnishes new constraints for the understanding the post-Variscan geodynamic evolution of the Europa-Adria lithosphere.ReferencesLu M., Hofmann A.W., Mazzucchelli M., Rivalenti G. (1997) Chem. Geol. 140, 223-235.Obermiller W.A. (1994) Unpub. PhD thesis, J. Gutenberg-Universität Mainz, 192 pp.Peressini G., Mazzucchelli M., Rivalenti G., Hofmann A.W. (2004). This Congress.

2004 - Metasomatism by OIB-like alakaline melts in the back-arc Patagonian lithospheric mantle: the Gobernador Gregores case (Santa Cruz Province, Argentina). [Abstract in Atti di Convegno]
Zanetti, A; Rivalenti, Giorgio; Mazzucchelli, Maurizio; Vannucci, R; Cingolani, Ca
abstract

Abstract: S07C_O_16Spinel-facies mantle xenoliths occurring in pyroclastic deposits at Gobernador Gregores in the backarc southern Patagonia, ~400 km E of the actual Chile trench, display geochemical signatures ascribed to either carbonatite metasomatism (Gorring and Kay, 2000) or hydrous silicate melts (Laurora et al., 2001). Carbonatite metasomatism in this region is unexpected because subduction of young carbonated oceanic crust (Nazca Plate) occurred during mid-Miocene. Alternatively, introduction of carbonatitic melts/fluids could be related to either the heating of the lithospheric mantle by upwelling slab-window asthenosphere or a rising plume, as suggested for Pragauniyeu xenolithoccurrence (Ntaflos et al., 2001). The xenoliths show both anhydrous and hydrous mineral assemblages. The former (lherzolites and harzburgites) have geochemical composition consistent with melt–assisted partial melting. The latter (amphibole ± phlogopite ± apatite–bearing) display modal and chemical characteristics held as indicators of carbonatitic metasomatism. However, part of these xenoliths has high TiO2/Al2O3 and those with the most pronounced carbonatitic signatures modally trend towards harzburgite. Bulk-rock, clinopyroxene and amphibole show Zr, Hf and Ti negative anomalies, which increase at decreasing Na2O and HFSE concentrations. Steady variation trends between xenoliths with and without carbonatitic characteristics suggest a control by reactive porous flow of a unique agent, inferred to be initially a ne-normative hydrous basalt. Appearance of amphibole may explain the variation of Ti anomaly, whereas the presence of Ti-Zr-oxides is required to explain Zr and Hf anomalies. Modest HFSE anomalies in the infiltrating melt may be acquired during percolation of garnet-facies peridotite.

2004 - The backarc mantle lithosphere in Patagonia, South America [Articolo su rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Laurora, Angela; Ciuffi, Sia; Zanetti, A; Vannucci, R; Cingolani, Ca
abstract

The backarc Patagonia mantle underwent pervasive regional re-crystallisation that left only remnants of the pre-existing mantle. Anhydrous xenoliths (dominantly lherzolites and harzburgites, few dunites) are predominant. Xenoliths containing hydrous phases (Iherzolites and harzburgites, rare wehrlites) abundantly occur only at G. Gregores. Bulk-rock and clinopyroxene trace element patterns range from predominant LREE-depleted, LILE-enriched and variably Nb-depleted, to LREE-enriched patterns, with negative Zr, Hf and Ti spikes when amphibole is present. V-shaped trace element patterns are found only in rare non-recrystallised xenoliths. Trace element abundances in potential melts calculated from clinopyroxene are similar to arc magmas, but in southern Patagonia fade eastwards to EMORB-like compositions. Trace element characteristics of bulk-rock and pyroxenes are interpreted as the consequence of two processes: (1) melting in the region of thermal inversion of the wedge, triggered by infiltration of hydrous components; (2) reactive porous flow of the melts into the overlaying mantle. The component triggering melting is inferred to be slab-derived in the western occurrences and a garnet-facies, asthenosphere-derived melt in the eastern occurrences as a consequence of wedge thickening. Differences between northern and southern Patagonia are interpreted to be due to variable contribution of slab components to the wedge. Compared to the southern region, slab-derived melts are tentatively attributed to the subduction of older and colder segments of the Nazca plate in the North.

2004 - The backarc mantle lithosphere in Patagonia, South America. [Abstract in Rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Laurora, Angela; Ciuffi, S. I. A.; Zanetti, A.; Vannucci, R.; Cingolani, C. A.
abstract

The backarc Patagonia mantle underwent pervasive regional re-crystallisation that left only remnants of the pre-existing mantle. Anhydrous xenoliths (dominantly lherzolites and harzburgites, few dunites) are predominant. Xenoliths containing hydrous phases (lherzolites and harburgites, rare wehrlites) abundantly occur only at G. Gregores. Bulk-rock and clinopyroxene trace element patterns range from predominant LREE-depleted, LILE-enriched and variably Nb-depleted, to LREE-enriched patterns, with negative Zr, Hf and Ti spikes when amphibole is present. V-shaped trace element patterns are found only in rare non-recrystallised xenoliths. Trace element abundances in potential melts calculated from clinopyroxene are similar to arc-magmas, but in southern Patagonia fade eastwards to E-MORB-like compositions. Trace element characteristics of bulk-rock and pyroxenes are interpreted as the consequence of two processes: 1) melting in the region of thermal inversion of the wedge, triggered by infiltration of hydrous components; 2) reactive porous flow of the melts into the overlaying mantle. The component triggering melting is inferred to be slab-derived in the western occurrences and a garnet-facies, asthenosphere-derived melt in the eastern occurrences as a consequence of wedge thickening. Differences between northern and southern Patagonia are interpreted to be due to variable contribution of slab components to the wedge. Compared to the southern region, slab-derived melts are tentatively attributed to the subduction of older and colder segments of the Nazca plate in the North.

2004 - The mantle lithosphere in the Rio Grande do Norte state, North-Eastern Brazil. [Abstract in Atti di Convegno]
Girardi, V. A. V.; Mazzucchelli, Maurizio; Bertotto, G. W.; Tassinari, C. G. C.
abstract

abs. 255-28The composition and variability of the mantle lithosphere in NE Brazil and the effects of the Fernando de Noronha Plume track are constrained by studying the petrology and Sr and Nd isotope characteristics of spinel-facies mantle xenoliths occurring in Cenozoic alkali basalts in five localities of the the Rio Grande do Norte State. Previous work on Pico Cabuji, showed that the litosphere was affected by an OIB-like component. This process induced isotope variation arrays between DMM and EM1. The new data on the other localities support this metasomatic scenario, but show that the EM1 component only sets out clearly at Pico Cabuji. In all the other localities the Sr and Nd isotope arrays suggest that an EM2-like component affected a depleted mantle. Bulk-rock geochemical characteristics are the result of three main processes: 1) ancient melting and basalt extraction; 2) metasomatism occurring during lithosphere erosion; 2) chromatographic enrichment processes. The first and the third processes are mainly recorded in xenoliths having equilibrium temperatures < 1000°C (Bray and Kohler, 1990, geothermometer). In this group, melting and basalt extraction processes are documented by major element geochemical variations in bulk rock and mineral phases. Bulk-rock REE profiles vary from spoon-shaped to flat and extended trace element patterns exhibit marked Rb enrichment and positive Zr,Hf spikes. 87Sr/86Sr30Ma and 143Nd/144Nd30Ma range 0.7023 - 0.7038 and 0.51329 - 0.51283, respectively. The second process is mainly documented in xenoliths having equilibrium temperature > 1000°C. Major element variation trends in bulk rock and cpx resemble those modelled for fractional melting residua of a Primitive Mantle source, but are enriched in TiO2, Al2O3 and Na2O.The bulk-rock REE patterns vary from flat to slightly LREE enriched or, rarely, depleted and steadily fractionated to HREE. REE concentration is 0.8 - 0.4 x PM. Rb is enriched and Ba depleted with respect to LREE. Zr and Hf have positive spikes. 87Sr/86Sr30Ma and 143Nd/144Nd30Ma range 0.7033 - 0.7044 and 0.51288 - 0.51264, respectively. In agreement with addition of a metasomatic component 143Nd/144Nd30Ma decreases, and 87Sr/86Sr30Ma, Mg#, Cen/Ybn and La/Nb increase, with increasing equilibrium temperature. The metasomatic component is geochemically and isotopically consistent with the host basalt composition, thus suggesting that metasomatism is chronologically related with the basalts.

2004 - Triassic U-Pb SHRIMP Ages on magmatic Zircons from the External Gabbro unit of the Finero mafic complex, Ivrea Zone, Western Italian Alps. [Abstract in Atti di Convegno]
Peressini, G.; Mazzucchelli, Maurizio; Rivalenti, Giorgio; Hofmann, A. W.
abstract

abs. 236-38The northern part of the mafic-ultramafic Ivrea Verbano complex, the Finero region, differs from the rest of the complex in many features: petrology and geochemistry of mantle peridotites, stratigraphy, lithology and geochemistry of igneous bodies, relationships with the metamorphic sequence (Kinzigite Formation) into which the complex intruded. We provide evidence for a substantial difference also in the age of emplacement. The Mafic Complex (MC) in the Val Sesia area has been recently proved to have intruded between 283 and 289 Ma. At Finero, published Sm-Nd isochrones span 203-533 Ma, zircon ages span 208-549 Ma (Lu et al, 1997, Chem.Geol.140, 223-235, and ref. therein).We performed a zircon study on 5 samples from the External Gabbro unit (EG) of the complex. Three events are revealed by the SHRIMP U-Pb results on the 2 most representative samples, one of which has a composite population of magmatic and detrital zircons, clearly distinguished for grain-morphology, color and CL-pattern. Primary crystallization of the pink magmatic zircons was dated at 232±2 Ma; these were overprinted at 214±5 Ma by a second event, dated by the rim-recrystallization ages. A 280-to-310 Ma age peak is clearly, but poorly constrained by the colorless zircons (U<20 ppm), proving that older ages are preserved, but must be considered detrital. This is further confirmed by the ages recorded in the second sample, which yielded no magmatic zircons, but only metamorphic grains. As for the significance of the older age peak, zircon yield and degree of crustal contamination (Nd-Sr isotopic compositions) of the studied samples suggest that zircons and zirconium were inherited/digested from the country rock. This latter underwent a major event that opened and reset the U-Pb system in zircon around 300 Ma, compatible with ages in the MC of the Balmuccia sector.In the EG, though, no magmatic Carboniferous or Permian zircon was found: the EG was emplaced in Anisian-Ladinian time at 232Ma, and a separate thermal event took place in Norian time at 214 Ma (Staehle et al., SMPM 70, 1990; von Quadt et al., SMPM 73, 1993). The clearly documented Mesozoic igneous activity, distinct from the predominant Permo-Carboniferous magmatism in the Ivrea-Verbano, and the possibly Mesozoic age of mantle metasomatism at Finero (Grieco et al, 2001, J. Pet.52, 89-101) suggest revisiting the current interpretations of Mesozoic magmatism in the Southern Alps of Lombardy and Trentino regions.

2004 - Triassic emplacement of the External Gabbro unit of the Finero mafic complex: U-Pb SHRIMP zircon ages and their implications for the Ivrea-Verbano Zone, Western Italian Alps. [Abstract in Rivista]
Peressini, G.; Mazzucchelli, Maurizio; Rivalenti, Giorgio; Hofmann, A. W.
abstract

SRef-ID: 1607-7962/gra/EGU04-A-05072The Mafic Complex (MC) of the Ivrea Zone displays distinctive features in different sectors. Apart from petrology and geochemistry of the mantle peridotites (e.g. the strong metasomatism of Finero is absent in Balmuccia), the Finero region differs from the rest of the complex in stratigraphy, lithology and geochemistry of the mafic intrusives, and their relationships with the Kinzigite Formation, the metamorphic sequence into which the MC intruded. We give new evidence for a substantial difference also in the age of magmatism. The age of emplacement of the MC in the Val Sesia area has been recently proved to be homogeneous, between 283 and 289 Ma, after a thermal, upper-amphibolite event that affected the country rock at 320-to-310 Ma. At Finero, published Sm-Nd isochrones span 203-533 Ma, zircon ages span 208-549 Ma (Lu et al, 1997, Chem.Geol. 140, 223-235, Grieco et al, 2001, J. Pet. 52, 89-101). The more composite nature of the Finero sector is possibly hiding a more articulated geological evolution of the MC, as suggested by the presence of numerous high-T shear-zones (e.g., Kenkmann, 2000, J.Struc.Geol. 22, 471-487, Manckeltow etal., 2002, J.Struc.Geol. 24, 567-585, and ref. therein). We performed a detailed zircon study on samples from the External Gabbro unit of the complex, most of which have a homogeneous population of colorless zircons. The key for interpretation is one sample, bearing both magmatic, subeuhedral, pink crystals with oscillatory zoning in CL, and detrital zircons, rounded, small, colorless grains with blurred CL-patterns. The three events recorded by these zircons are clearly recognizable in CL, and were dated with SHRIMP U-Pb analyses. Magmatic growth of the zircons took place at 232+/-2 Ma and was overprinted at 214+/-5 Ma by a second event, dated by the rim-recrystallization ages. A 280-to-310 Ma age peak is clearly, though imprecisely constrained by the colorless zircons, proving that older ages are preserved, but must be considered detrital. Two problems arise: reason and significance of the older age peak. Together with the positive correlation between zircon yield and degree of crustal contamination (Nd-Sr isotopic compositions) of the studied samples, the U-Pb data allow to conclude that zircons and zirconium were inherited/digested from the country rock. The latter underwent a major event that reset the U-Pb system in zircon around 300 Ma, compatible with both magmatic and metamorphic ages in the MC of the Balmuccia sector. In the External Gabbro, though, no magmatic Carboniferous or Permian zircon was found: emplacement took place at 232 Ma, and a separate thermal event is dated at 214 Ma. The new data extend as far west as the Ivrea zone the evidence of the important Mesozoic magmatism of the central and eastern South Alpine. Mesozoic igneous activity in the Finero region is clearly distinct from the predominating Permo-Carboniferous magmatism in the Ivrea-Verbano Zone. Also, as for most gabbroic intrusions in the alpine-appenine system, ages in the Ivrea Zone cluster in two ranges, at either 285-310 Ma, or at 200-250 Ma. This distribution is thoroughly represented in the zircons from one sample only, the ages of which represent distinct episodes of heating, melting and metasomatism.

2003 - Mobilizzazione di Piombo e Zinco dal sito minerario di Viarago (Pergine Valsugana, Trento): valutazione di impatto sulla qualità ambientale [Relazione in Atti di Convegno]
Corsini, Alessandro; Matteucci, C.; Mazzucchelli, Maurizio; Piacentini, D.
abstract

Oggetto del presente studio è la valutazione dell’impatto sulla qualità ambientale connesso con lo sfruttamento, in epoca storica e fino al 1940, di manifestazioni filoniane a solfuri misti nei pressi di Viarago, nel comune di Pergine Valsugana (Trento). In particolare, lo studio analizza gli effetti connessi con la deposizione nei suoli sottostanti il sito minerario di metalli pesanti, soprattutto Pb e Zn, che vengono rilasciati dall’accumulo di materiale di scarto che ricopre il versante oggetto dell’attività estrattiva. L’approccio adottato per la valutazione della qualità ambientale dell’area di studio è stato mutuato dal progetto europeo ELANEM (Euro-Latin American Network on Enviromental Assessment and Monitoring) in cui il presente lavoro si inquadra. Questo approccio prevede l’analisi di indicatori di pressione, stato e risposta relativi a varie componenti ambientali (biota, risorse fisiche, clima, aria, pericolosità geologica, etc.) e la loro combinazione, relativamente alle funzioni ambientali di cui sono rappresentativi (Naturalità, Risorse, Smaltimento Rifiuti, Supporto e Servizi), al fine di ottenere valori indice per le diverse funzioni ambientali stesse. Gli indici sono poi, a loro volta utilizzati per ricavare un valore indice integrato di qualità ambientale. Lo studio si è avvalso di dati ricavati da rilievi diretti sul terreno (relativamente a geomorfologia, pedologia, vegetazione), campionamenti di suoli, analisi di laboratorio e ricerche presso enti pubblici e privati. Tutta la procedura di elaborazione dati è stata effettuata in ambiente GIS. E stato in primo luogo evidenziato che i microelementi considerati, ovvero Pb e Zn, assumono concentrazioni elevate nella zona dell’accumulo detritico e in quelle ad esso direttamente sottostanti, raggiungendo valori di gran lunga superiori al limite di attenzione indicati dalla Legge 478/82. I vari elaborati cartografici prodotti e rappresentativi di indici di funzioni e qualità ambientale hanno permesso di evidenziare che l’area di studio presenta valori assoluti di qualità ambientale che si attestano su classi assolute medie, differenziate però in maniera più evidente in termini relativi specifici per l’area. L’impatto della miniera sulla qualità ambientale dell’area è stato valutato effettuando un confronto tra le carte di indici di funzioni e di qualità ambientale ottenute nelle condizioni reali e carte analoghe ottenute simulando l’assenza di tutte le pressioni e variazioni di stato connesse con la miniera stessa. Rispetto la funzione ambientale “naturalità”, si è notato che in presenza della miniera un’ampia zona ad essa sottostante presenta valori relativi dell’indice di Naturalità da bassi a medi con piccole unità con valori molto bassi. Nell’analoga carta relativa alla condizione ipotetica tale zona è assente. Situazione pressoché analoga si è riscontrata per le altre funzioni ambientali e quindi, di conseguenza per i valori dell’indice integrato di qualità ambientale, che risultano bassi e medi nella zona limitrofa alla miniera, ma che sarebbero medi o alti senza la miniera. Da tali risultati ottenuti è evidente che un eventuale intervento di bonifica dell’area dovrebbe essere mirato a limitare l’alterazione del detrito di scarto e quindi la mobilizzazione di metalli pesanti, per esempio tramite la messa in posto di una copertura di materiali terrigeni, su cui far sviluppare un suolo e, in fase successiva, attecchire la vegetazione.

2001 - Metasomatism and melting in carbonated peridotite xenoliths from the mantle wedge: The Gobernador Gregores case (Southern Patagonia) [Articolo su rivista]
Laurora, Angela; Mazzucchelli, Maurizio; Rivalenti, Giorgio; R., Vannucci; A., Zanetti; Ma, Barbieri; Ca, Cingolani
abstract

Spinel-facies mantle xenoliths occur in a diatreme cutting through the Neogene Southern Patagonia Plateau at Gobernador Gregores (Santa Cruz Province, Argentina). This plateau is in a back-arc position with respect to the Chile trench. Xenoliths differ in their whole-rock composition from other South America occurrences, having higher CaO/Al2O3 ratios and in some samples, TiO2 enrichment, whereas the Na2O/Al2O3 variation range is similar. Three assemblages can be distinguished. Assemblage 1, in anhydrous protogranular lherzolites and harzburgites, contains clinopyroxene with a depleted major and trace element composition, indicating premetasomatic depletion processes. This assemblage fully recrystallized to Assemblage 2 (amphibole +/- phlogopite +/- Cl-apatite-bearing) during a metasomatic episode. This causes clinopyroxene to acquire geochemical characteristics often attributed to carbonate-melt metasomatism. Noticeably, amphibole is markedly enriched in Nb (up to 298 ppm), especially when depleted in Ti. A further event, related to decompression during xenolith uplift to the surface, induces closed-system (perhaps with the exception of CO2 addition) disequilibrium melting of Assemblage 2, dominantly of amphibole. It is found in pockets (where amphibole is a residual phase) consisting of Na Si-rich glass and carbonate (Mg-rich calcite) drops, and in veins originating from the pockets (Assemblage 3). Euhedral olivine, clinopyroxene and spinel crystallize only in the silicate glass. So do new, euhedral apatite crystals when glass is in contact with previous Assemblage 2 apatite. Textural evidence and comparison with experimental work suggest that silicate glass and carbonates are the result of unmixing of a former homogeneous melt. Because of the different flow rates of carbonate and silicate melt, the xenoliths become enriched in carbonate, which is found in the veins, during their migration. Thus, the high CaO/Al2O3 ratio of whole rocks provides inconclusive evidence of carbonatite metasomatism. This factor, and other minor deviations from the expected results of carbonatite metasomatism, lead us to hypothesize an aqueous, Cl-rich fluid, possibly slab derived, as an alternative agent. Amphibole, resulting from reactive porous flow of this agent in the mantle, could fully explain the observed geochemical features, as indicated by estimates of its partition coefficients.

2001 - Petrology of metabasaltic dykes in the Diamantina region, Minas Gerais, Brazil. [Articolo su rivista]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; AB MENEZES, Leal; Vav, Girardi; BB BRITO, Neves; W., Teixeira
abstract

A sequence of mafic dykes is found in the Diamantina region (Minas Gerais, Brazil) at the eastern border of the southern São Francisco Craton. The dykes have been distinguished into four groups on the basis of petrography and geochemical characteristics. Groups 1 to 3 cut the rock sequence older than Meso-Proterozoic, but do not cut the Neo-Proterozoic rocks, thus constraining their emplacement age. Group 1 is constituted by fine grained, sometimes foliated metabasites, which only exceptionally preserve relics of the primary mineral assemblage, and are located only in the lowest stratigraphic units of the Espinhaço Supergroup, suggesting an older emplacement age with respect to the other groups. Group 2 and 3 metabasites are better preserved than those of Group 1. Group 4 represents a single igneous body virtually non metamorphic. Its age is not constrained, but it is similar to Mesozoic dykes occurring further south in this region. In all groups, composition is basaltic with tholeiitic affinity. Metamorphic element mobility substantially affected only the LILE, whereas igneous variation trends are preserved for all the other elements. The various groups differ for their incompatible trace element composition and ratios. These ratios are more similar to OIB (Ocean Island Basalt) values rather than to any other magma type. Geochemical evidence rules out any important influence of crustal contamination, fractional crystallisation, or variable degrees of melting of a common source material as an explanation for the inter-group variability. It is inferred that the geochemistry of the different groups reflects complementary characteristics and differences of their mantle sources. These latter are attributed either to a metasomatic enrichment of a variably depleted premetasomatic mantle by a component with OIB characteristics, or, alternatively (our preferred interpretation), to the melting of the metasomatised mantle at different depths. The metasomatised sources underwent extensive melting, producing tholeiitic melts retaining OIB-like geochemical characteristics. By analogy with OIB, the metasomatic component may be plume-related. Dyke emplacement may be controlled by passive crustal rifting induced by plume-related mantle diapirs. In such a scenario, the Group 1 samples could be related to the initial rifting phase and plume impingement in the lithosphere, whereas Groups 2 and 3 could represent advanced stages of crustal thinning and melting of the plume head source. The youngest Group 4 dykes presumably represent a limited and local occurrence of the Mesozoic mafic magmatism which affects the area of the Serra do Espinhaçho.

2000 - Composition and processes of the mantle lithosphere in northeastern Brazil and Fernando de Noronha: evidence from mantle xenoliths [Articolo su rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Vav, Girardi; R., Vannucci; Ma, Barbieri; A., Zanetti; Sl, Goldstein
abstract

Spinel-peridotite facies mantle xenoliths in Cenozoic alkali basalts of the Pico Cabuji volcano (Rio Grande do Norte State. Northeast Brazil) and the adjacent South Atlantic oceanic island of Fernando de Noronha are studied for: (1) the information they provide on the composition of the lithospheric component in the erupted basalt geochemistry, and (2) to check the effects of the Fernando de Noronha plume track on the mantle lithosphere. Xenoliths from Pico Cnbuji are protogranular lherzolites and porphyroclastic harzburgites recording average equilibrium temperatures of 825 +/- 116 and 1248 +/- 19 degrees C, respectively. Pressure in the porphyroclastic xenoliths ranges from 1.9 to 2.7 GPa (Ca-in-olivine geobarometer), Both groups show major element chemical variation trends in whole-rock and Ti and HREE (Er. Yb) variations in clinopyroxene consistent with fractional melting and basalt extraction. REE (rare earth element) profiles of clinopyroxenes vary from LREE (La, Ce) enriched (spoon shaped) to LREE depleted in the protogranular group, whereas they are slightly convex upward in most porphyroclastic clinopyroxenes, HFSE (Ti and Zr) negative anomalies are in general modest in the clinopyroxenes of both groups. Xenoliths from Fernando de Noronha have textural variations similar to those of Pico Cabuji. Protogranular and porphyroclastic samples have similar temperature (1035 +/- 80 degrees C) and thr pressure is 1-1.9 and 2.3 GPa, respectively. Whole-rock chemical variation trends overlap and extend further than those of Pico Cabuji. The trace element profiles of the clinopyroxenes of the porphyroclastic xenoliths are enriched in La up to 30 X PM and are smoothly fractionated from LREE to HREE. with deep, negative, Zr and Ti anomalies, The geochemical heterogeneities of the xenoliths: from both localities are interpreted in terms of reactive porous percolation. The porphyroclastic xenoliths from Pico Cabuji represent the lower Dart of a mantle column (the head of a mantle diapir. at the transition conductive-adiabatic mantle), where OIB infiltration triggers melting, and the protogranular xenoliths the top of the mantle column: chromatographically enriched by percolation at a low melt/rock ratio. This interpretation may also apply for Fernando de Noronha, but the different geochemical signature recorded by the clinopyroxenes requires a different composition of the infiltrated melt. Nd and Sr isotopes of the: Pico Cabuji porphyroclastic clinopyroxenes (Nd-143/Nd-144 = (0.51339-0.51255. Sr-87/Sr-86 = 0.70275-0.70319) and of Fernando de Noronha (Nd-143/Nd-144 = 0.51323-0.51285, Sr-87/Sr-86 = 0.70323-0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to low Nd-low Sr (EMI) and low Nd-high Sr (EMII), respectively. Correlation of the isotope variation with geochemical parameters indicates that the isotopic variation was induced by the metasomatic component, of EMI type at Pico Cabuji and of EMII type at Fernando de Noronha. These different components, enriched a lithosphere isotopically similar to DMM (depleted MORE mantle) at both localities, At Fernando de Noronha, the isotopic signature of the metasomatic component is similar to that of the similar to 8 Ma old lavas of the Remedios Formation suggesting that this is the age of metasomatism, At Pico Cabuji, the mantle xenoliths do not record the high Sr-87/Sr-86 component present in the basalts. We speculate that the EMII component derives from a lithospheric reservoir, which was not thermally affected during mantle metasomatism at Pico Cabuji, but was mobilized by the hotspot thermal influence at Fernando de Noronha. This interpretation provides a plausible explanation for the presence of distinct metasomatic components at the two localities, which would be difficult to reconcile with their genetic relationship with the same plume.

1999 - Carbonated peridotite xenoliths from the mantle wedge: The Patagonia case [Abstract in Atti di Convegno]
Laurora, A.; Rivalenti, G.; Mazzucchelli, M.; Bottazzi, P.; Barbieri, M. A.; Cingolani, C. A.; Vannucci, R.
abstract

Carbonated mantle xenoliths are mainly documented in intraplate environments. Here we provide evidence that carbonated mantle also occurs in the wedge of the back-arc region related with the Andean subduction. Along the whole length of the Argentinean Andes, this region is characterised by alkali basalt volcanic centres containing abundant mantle xenoliths. The xenoliths of the easternmost volcanoes bear evidence of mantle interaction with a CO2-rich component. In the volcanic centre of Gobernador Gregores (Santa Cruz Province, Southern Patagonia) this interaction produces abundant carbonates. Here a large diatreme constituted by pyroclastic deposits contains mantle xenoliths up to 60 cm. The peridotites are dominantly lherzolites and subordinate harzburgites, wherlites and dunites, whose texture varies from secondary-protogranular (recrystallized) to weakly foliated. The relationships between the forming phases indicate the following events: an original mineral assemblage of olivine (ol1), clinopyroxene (cpx1), orthopyroxene and spinel (spl1) is overprinted by a subsequent mineral assemblage, constituted by new clinopyroxene (cpx2), pargasite and sometimes phlogopite; in turn, this mineral assemblage reacts with a further metasomatic agent which causes instability of cpx2 and of the hydrous phases. As an effect of the latter episode, cpx2 and hydrous phases are surrounded by pockets of carbonate plus silicate glass. Carbonate also occurs in veins at the crystal boundaries, as blebs in the silicate glass. Euhedral olivine (ol2), clinopyroxene (cpx3), spinel (spl2) and rarely rutile crystallise from the silicate glass. Apatite occurs both in the silicate glass and in the carbonates. Forsterite concentration in olivine varies from 87/91 in ol1 to 89/93 in ol2, which also contains high CaO (0.15/0.80 wt%). Cpx3 is characterised by higher Al2O3, CaO, TiO2 and lower Na2O with respect to cpx2. Spl2 contains higher Al2O3 and TiO2 with respect to spl1. Carbonate is calcite with MgCO3 concentrations up to 4%. Glass composition varies from trachiandesitic to tefriphonolitic. Trace element characteristics of clinopyroxenes vary markedly from cpx1 to cpx3 (Figs. 1a, b, c). Cpx1 is LREE depleted, whereas cpx2 is the richest in REE and has a fractionated REE pattern (La(N)/Yb(N) = 9.6/14.8) with a maximum at Ce. Cpx3 has a lower REE concentration with respect to cpx2; its REE pattern (La(N)/Yb(N) = 1.8/2.9) shows a maximum at MREE. HFSE anomaly are negative and variable in cpx2 and cpx3 (La(N)/Nb(N), Ti/Ti(*) and Zr/Zr(*) range 6.77/52.13, 0.02/0.24, 0.08/0.34 respectively in cpx2 and 0.76/0.88, 0.10/0.16, 0.14/0.22 respectively in cpx3). Amphibole has REE patterns (Fig. 1d) similar to cpx2 ((parg/cpx2)D(REE) ~1), but with marked positive Nb spikes ((parg/cpx2)D(Nb) = 42.3/148.4) and positive Ti and Sr anomalies ((parg/cpx2)D(Ti) = 5.4/8.2; (parg/cpx2)D(Sr) = 1.7/1.9). Zr is preferentially partitioned into amphibole ((parg/cpx2)D(Zr) = 1/1.5). Glass is LREE enriched and has smoothly fractionated patterns from La to Yb (La(N)/Yb(N) = 10.0/29.6). It has remarkable positive Nb anomalies (La(N)/Nb(N) = 0.1/0.6) - much higher respect to the associated amphibole ((parg/glass)D(Nb) = 0.4/0.7) - and generally negative Zr and Ti spikes (Fig. 1e). Apatite has a very high LREE concentration (La(N) ~ 4300) and La(N)/Yb(N) ~3500 (Fig. 1f). The last metasomatic episode is mass balanced by the following reaction: 1.00anf + 0.26opx + 0.02carb = 0.41cpx3 + 0.05spl2 + 0.36ol2 + 0.46glass. The relationships above described indicate that litospheric spinel facies mantle was affected by metasomatic episodes consistent with the former addition of a hydrous component followed by a CO2-rich component. The P-T range of these metasomatic events is 8.3 - 22.4 kbar and 890 - 1230°C (Fig. 2). Amphibole and cpx2 instability increases with increasing T and P, so that the more marked carbonatation occurs in the higher T-P

1999 - Garnet to Spinel facies transition and related metasomatism in the mantle wedge of Southern Patagonia (Pali Aike). [Abstract in Atti di Convegno]
Barbieri, Ma; Zanetti, A.; Mazzucchelli, Maurizio; Cingolani, C. A.; Kempton, P. D.; Vannucci, R.
abstract

The Pali Aike volcanic field (longitude 67.7°W, latitude 52.1°S) is the only occurrence in extra-Andean Patagonia (back-arc) recording garnet- to spinel-peridotite facies transition.Garnet-facies peridotites are lherzolites with dominantly protogranular texture. In some cases garnet is accompanied by Cr-rich spinel. Spinel-facies peridotites are distinguished into: i) harzburgites with Cr-rich spinel and secondary recrystallised texture and ii) harzburgites and lherzolites with protogranular texture. Hydrous phases (phlogopite and pargasite) and glass may occur in all the peridotite facies. In the garnet-facies samples, garnet is always surrounded by a kelyphitic rim of minute crystals of opx, cpx and brown, euhedral Al-rich spinel, representing the breakdown reaction ol + gnt = sp + opx + cpx.The garnet peridotites equilibrated in the range 985 - 1190 °C and 2.2 - 2.7 GPa. When they contain Cr-spinel, equilibration range is 895 - 1165 °C and 1.7 - 2.3 GPa. Protogranular spinel peridotites have significantly lower temperatures of equilibration than the secondary recrystallised, but similar pressure (T = 730 - 970 °C and P = 1.43 - 1.9 GPa as compared with 1010 - 1143 °C and P = 1.2 -1.6 GPa) (Fig. 1).Major element variations with respect to fertility indexes display correlations consistent with melting and basalt extraction. Samples containing metasomatic phases deviate from the main trend toward lower CaO and higher TiO2 concentrations. The REE patterns vary from LREE depleted and flat from MREE to HREE, to variably LREE enriched. The LREE-enriched profiles vary from spoon-shaped, with inflections at Nd or Eu, to extremely fractionate. These latter predominate in the spinel-facies peridotites. In bulk rock Sr and HFSE (Nb, Hf, Ti, Zr) concentrations increase at decreasing MgO concentration, as expected from a depletion process caused by basalt removal. In most cases the HFSE anomalies are modest and vary from positive to negative. Only the samples which contain metasomatic phases show marked positive anomalies in HFSE (Ti/Ti* = 1.30 - 10.91, Zr/Zr* = 2.51 - 7.89). Sr and Nd isotopic composition of whole rocks, clinopyroxene and phlogopite separates show a variation range of Nd = 3.12 - 20.03 and Sr = -2.83 - -23.65, respectively (Fig. 2). Most of the samples plot within or close to the mantle array. The field of the host basalt is contained in the xenolith range but it is much less variable. Two samples deviate from the variation trend toward high 87Sr/86Sr values. Lead isotopic determinations on bulk rock and clinopyroxenes define an array overlapping the field of the host basalt, the Nazca sediment and the Pacific MORB.The observed REE profiles are traditionally explained by a chromatographic fractionation of fluid or melt percolating a previously depleted mantle. The LREE enrichment and the absence of significant HFSE anomalies in most peridotites suggest that these features are characteristics of the metasomatic component. They are typical of OIB melts and characterise also the host basalt, which has, furthermore, similar incompatible trace element ratios and isotope composition overlapping part of the peridotites. These evidences strongly support the hypothesis that the host basalt was an important metasomatic component. However, the different behaviour of some elements (mainly LILE) in modally metasomatised samples and the isotopic signatures suggest a contribution from slab-derived components.We suggest that the Pali Aike mantle column (straddling the garnet- spinel-facies boundary) has been repeatedly infiltrated by metasomatic agents, consisting of slab-derived components and, later on, alkali basalts. These latter underwent chromatographic fractionation under the observed T, P gradient and variously overprinted previous metasomatic signatures.

1999 - Origin and significance of late noritic dykes in the Baldissero Peridotite Massif (Ivrea-Verbano zone) [Abstract in Atti di Convegno]
Mazzucchelli, M.; Rivalenti, G.; Zanetti, A.; Vannucci, R.; Cavazzini, G.
abstract

The Baldissero Peridotite Massif (Southern Ivrea-Verbano Zone) is a depleted lherzolite (ol 65/70, opx 16/20, cpx 10/11, sp 2/2.5, La(N)/Yb(N) 0.003/0.26, ε(Nd)270 = 6.4/14, 87Sr/86Sr = 0.7021/0.7035). It is cross-cutted by a swarm of 5/15 cm thick, fine grained (&lt;1mm), NNE trending noritic dykes. Contacts peridotite-dyke are sharp. Dyke texture is hypidiomorphic and their mode consists of dominant plagioclase (31/45 %, An 40/48), orthopyroxene (25/37%, En = 78.9/87.8, Wo = 0.6/1.1), subordinate clinopyroxene (14/18%, En = 42.7/44.0, Wo = 47.9/52.4), interstitial pargasitic amphibole (10/15%) and opaques (1/2%, Fe-Ni sulfides). Re-equilibration conditions are 0.8/1 GPa and 800/900°C, like those of the ambient peridotite. Major element (wt%) dyke composition is characterized by high MgO (10/16) and high Mg values (0.81/0.89). In the observed MgO range, SiO2, CaO and K2O vary negligibly (52/53, 9/10 and &lt;0.2, respectively). With decreasing MgO, TiO2 varies randomly from 0.8 to 0.4, Al2O3 increases from 15 to 21, Na2O from 3 to 4.5 and FeO decreases from 5 to 2. Bulk rock incompatible trace element patterns (Fig. 1) vary from weakly LREE depleted to weakly LREE enriched (La(N)/Yb(N) = 0.5/2.4) and have marked positive Sr anomaly (Sr/Sr(*) = 5.6/10.4), smaller Eu anomaly (Eu/Eu(*) 1.3/1.5) and negative Nb anomaly (La(N)/Nb(N) = 1.2/2.2). Incompatible element concentration increases, and the magnitude of the negative Nb and of the positive Sr anomalies decrease, with increasing MgO or Mg. Main incompatible trace element carriers are clinopyroxene and hornblende, which display patterns varying from slightly LREE depleted to enriched. Orthopyroxene is LREE depleted and plagioclase is LREE and Sr enriched. Clinopyroxene is Nb-depleted (La(N)/Nb(N) = 1.0/5.9) and amphibole is Nb enriched (La(N)/Nb(N) = 0.09/0.35). The latter phases have negative Zr spikes. Incompatible element concentration in the mafic phases show correlations with Mg similar to those of bulk rock (Fig. 2). The only available isotope determination refers to a Sm-Nd internal isochron which results in an age of 180Ma and ε(Nd) = 7.8. In spite of the contacts being sharp, peridotite mineral assemblage and composition varies over a distance of 7 cm. In the peridotite, toward the dyke contact, modal orthopyroxene increases and clinopyroxene decreases, whereas trace element concentration and La(N)/Yb(N) increase (from 0.003/0.017 to 0.1/0.26) and Mg decreases from 0.9 to 0.8. The dyke at the peridotite contact does not show sizeable chemical or modal variations. There is no clear relationship between the Mg variation of the dyke and the chemical trends observed in the contact peridotite. The high Mg numbers of the dykes and the Mg value of orthopyroxene, lower than that of bulk-rock, indicate that these norites are not melts, but rather represent mineral segregates possibly retaining some interstitial liquid. Estimates of the Mg values of the parental melts, made assuming (opx/liq)D(Mg) = 1.2, indicates a Mg range from 0.66 to 0.74, which is consistent with that of primary mantle-derived magmas. Since olivine does not occur even in the more MgO-rich dyke composition, the parent melt had to be, furthermore, SiO2-rich and hydrous, as indicated by the amphibole abundance. Whatever the set of solid/melt partition coefficient used for estimating trace elements in the parent melts, the calculated REE profiles would vary from slightly LREE depleted to variably LREE enriched. The HFSE characteristics of the primary melts cannot be precisely constrained because of the strong dependence of their partition coefficient on the melt composition. For example, using available partition coefficients for basaltic compositions, the estimated parental melts would have a weak Nb and Zr anomaly, which disappears using D sets for intermediate compositions and become positive if the D's are referred to high-silica compositions. In any case, t

1999 - The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism [Articolo su rivista]
A., Zanetti; Mazzucchelli, Maurizio; Rivalenti, Giorgio; R., Vannucci
abstract

The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (La-N/Yb-N = 8-17) and depleted in Nb. It has high radiogenic Sr (Sr-87/Sr-86((270)) = 0.7055-0.7093), low radiogenic Nd (epsilon Nd-(270) = -1 to -3) and EMII-like Pb isotopes. Two pyroxenite - peridotite sections examined in detail show the virtual absence of major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes exhibit LREE-enriched patterns (La-N/Yb-N similar to 16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative Sr anomaly, but incompatible element concentration similar to 1.9 (Sr) to similar to 7.9 (Ti) times higher than that of coexisting clinopyroxene. Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg# and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical signatures of carbonatite metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and Sr concentrations than amphibole (D-amph/cpx(REE),(Sr) = 0.7-0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process, which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and with the occurrence of modal metasomatism before peridotite incorporation in the crust.

1999 - The mantle lithosphere in Northeastern Brazil and Fernando de Noronha. Plume-related mantle metasomatism? [Abstract in Atti di Convegno]
Rivalenti, G.; Mazzucchelli, M.; Girardi, V. A. V.; Barbieri, M. A.; Zanetti, A.; Goldstein, S. L.
abstract

Cenozoic alkali basalts in the Rio Grande do Norte State and the Fernando de Noronha Island contain abundant spinel facies mantle xenoliths. At Pico Cabuji (NE Brazil) both protogranular and porphyroclastic xenoliths occur. The former mainly consists of lherzolites and the latter of harzburgites, with average equilibrium temperatures of 1064±6°C and 1238±7°C, respectively. Pressure is assumed essentially equal in the two groups (~1.8 GPa). Porphyroclastic xenoliths have more refractory bulk rock and mineral phase compositions than the progranular ones, in keeping with their mode. Both groups show chemical variation trends consistent with fractional melting and basalt extraction. REE profiles of clinopyroxenes vary from LREE enriched (spoon shaped) to LREE depleted in the progranular group, whereas they are dominantly slightly convex upwards in the porphyroclastic clinopyroxenes. HFSE (Ti and Zr) anomalies are in general modest in the clinopyroxenes of both groups. At Fernando de Noronha, xenoliths are variably-textured lherzolites and harzburgites. Their chemical variation trends overlap and extend those of the Pico Cabuji samples. Average temperature is 1056±87°C and pressure 1.6±0.2 GPa. Clinopyroxenes have spoon-shaped to LREE depleted profiles similar to those of the Pico Cabuji protogranular type, but have higher REE concentrations and more marked negative HFSE spikes (Fig. 1). Nd and Sr isotopes of the Pico Cabuji porphyroclastic clinopyroxenes (143Nd/144Nd = 0.51339-0.51255, 87Sr/86Sr = 0.70275-0.70319) and of Fernando de Noronha (143Nd/144Nd = 0.51323-051285, 87Sr/86Sr = 0.70323-0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to lowNd-lowSr and lowNd-highSr, respectively. 143Nd/144Nd decreases, and 87Sr/86Sr increases, with increasing Mg of clinopyroxene and with all the parameters indicative of previous melting episodes (modal clinopyroxene, bulk rock MgO). The opposite correlations are observed with LREE concentration, La(N)/Sm(N), and with all the parameters reflecting metasomatic enrichment indicating different isotope and geochemical characteristics of the Pico Cabuji and Fernando de Noronha metasomatic agents. In both occurrences the premetasomatic lithosphere was isotopically similar to DMM (depleted MORB mantle) whereas the metasomatic component was EMI-like and EMII-like at Pico Cabuji and Fernando de Noronha respectively. The EMII component of Fernando de Noronha may be related with a crustal reservoir present at Fernando de Noronha and not at Pico Cabuji. The diversity of the metasomatic components between Pico Cabuji and Fernando de Noronha indicates considerable heterogeneity in their respective sources, difficult to reconcile with their supposed genetic relationship with the same plume.

1998 - Petrogenesis of the Paleoproterozoic basalt-andesite-rhyolite dyke association in the Carajas region, Amazonian craton [Articolo su rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Vav, Girardi; G., Cavazzini; C., Finatti; Ma, Barbieri; W., Teixeira
abstract

Paleoproterozoic basaltic, andesitic and rhyolitic dykes crosscut the Archaean Carajas basement. Basalts are distinguished into a high and a low TiO2 group (HTi and LTi), each group consisting of geochemically distinct NE- and NW-trending swarms. The HTi dykes are evolved transitional basalts having essentially EMORB-type geochemistry. The LTi basalts are tholeiites (NE-trending swarm) and high-Al basalts (MY-trending swarm) displaying incompatible trace elements patterns with variably negative Nb anomaly, enrichment in Rb, Ba, K (LILE) and La, Ce and Nd (LREE) and positive Sr anomaly. With respect to orogenic analogues, andesites have lower Al2O3, CaO and Ni, higher FeO, LILE, LREE, Nb, Zr and Ti and negative Sr anomaly. Rhyolites have geochemical characteristics comparable with those of A-type granites. At 1.8 Ga, Sr-87/Sr-86 ranges from 0.700 to 0.705 in the HTi basalts and from 0.700 to 0.704 in the LTi group. Andesites define an isochron of 1874 +/- 110 Ma (Sr-o = 0.7038 +/- 0.0010). Rhyolites from Southern and Northern Carajas define two isochrons of 1802 +/- 130 Ma (Sr-o = 0.7062 +/- 0.0046) and 1535 +/- 82 Ga (Sr-o = 0.7625) respectively, the younger date being interpreted as resetting of the Rb-Sr isotopic system. We propose a petrogenetic model relating LTi basalts with melting of lithospheric mantle metasomatized by acid melts derived from incipient melting of eclogites, representing in turn the subsolidus product of basaltic batches trapped in the mantle. The HTi basalts are explained by melting of the lithospheric mantle containing the complementary residual eclogite, Andesite petrogenesis is consistent with crystal fractionation from a high-Mg andesite parent derived from a mantle source more extensively metasomatized by eclogite-derived melts. Rhyolite composition is consistent with low melting degree of the basement rocks. The basalt-andesite-rhyolite dykes may represent the effects of crustal extension and arching in Carajas, which produced the anorogenic acid to intermediate magmatism (Uatuma group) and affecting a large part of the Amazon craton between 1.85 and 1.7 Ga. (C) 1998 Elsevier Science B.V. All rights reserved.

1997 - Caratteri geochimici ed isotopici delle rocce basiche della Zona Ivrea-Verbano Nord-Orientale: Il Gabbro di Finero e le Anfiboliti. [Articolo su rivista]
Mazzucchelli, Maurizio; Barbieri, M. A.
abstract

Vengono messe in evidenza le differenze geochimiche tra l'Unità del Gabbro Esterno del complesso intrusivo di Finero (Alpi Meridionali) e le anfiboliti con affinità di tipo N-MORB dcll’Unità Kinzigitica. In particolare le anfiboliti mostrano arricchimenti in LREE (La, Ce, Nd) e LILE rispetto al Gabbro Esterno. I rapporti Ba/Nb risultano profondamente diversi tra il Gabbro Esterno (valori << 1) e le anfiboliti (valori > 1). I dati radiometrici a disposizione sono contrastanti, indicando due possibili scenari alternativi per l'evoluzione del complesso basico di Finero. II primo prevede che il complesso di Finero sia circa coevo o posteriore a quello della Val Sesia (- 270Ma). mentre il secondo prevede che il complesso si sia intruso circa 530 Ma fa in crosta oceanica, contemporaneamente ai protoliti delle anfiboliti della Unità Kinzigitica e che il complesso basico e l'Unità Kinzigitica costituiscano parti di un unico prisma di accrezione. In questa breve nota vengono evidenziati dati a sostegno dell'ipotesi che il complesso si sia intruso posteriormente all'evento termico che ha prodotto la facies granulitica nell' Unità Kinzigitica, e che quindi sia circa coevo con quello della Val Sesia.

1997 - Garnetite-forming processes in the deep crust: The Val Fiorina case study (Ivrea-Verbano Zone, NW Alps) [Articolo su rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Ma, Barbieri; M., Parenti; R., Schmid; A., Zanetti
abstract

Garnetite (garnet > 60 %) is a prominent rock type in Val Fiorina (northern Ivrea Zone), where it occurs as layers up to 10 m thick between granulite-facies metapelites and mafic-ultramafic lithologies. The contacts are sharp between garnetite and metapelite and gradational towards the mafic-ultramafic rocks. Biotite occurs together with garnet in streaky garnetite domains, while both hornblende and clinopyroxene increase in garnetite towards the mafic-ultramafic layers. The grossular content of garnet increases from the metasediment contact towards the mafic rocks, while almandine decreases, bulk-rock Y/Sc and Zr/Ti as well as initial Sr-87/Sr-86 decreases and K/Rb increases. Garnets have a negative Eu anomaly, but Eu/Eu* increases smoothly in the garnets with increasing distance from the metasediment contact. Two hypotheses are discussed for garnetite petrogenesis: 1) metasomatism taking place by interstitial melt retention in amphibolites and metapelites undergoing synchronous anatexis; 2) reaction between metapelite-derived anatectic melt and phases segregated from a basaltic sill during intrusion which induced partial melting of the country rocks. An objection to hypothesis 1 is the improbability of interstitial melts being retained until completion of the metasomatism. The second hypothesis may explain the observed composition and structures and has been proposed for other Ivrea Zone sectors such as Val Sesia. However, this process only produced extensive garnetites in Val Fiorina. The special conditions required in this case are tentatively ascribed to the flow characteristics and/or to differences in the anatectic component determined by a more fertile metapelite source in Val Fiorina with respect to Val Sesia. Both processes led to geological scenarios consistent with events affecting the Ivrea Zone.

1997 - The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone) .1. Chemistry of MORB-like magmas [Articolo su rivista]
Mh, Lu; Aw, Hofmann; Mazzucchelli, Maurizio; Rivalenti, Giorgio
abstract

Geochemical data are presented for the meta-igneous, mafic-ultramafic complex near Finero. This complex is in contact with a phlogopite-bearing mantle peridotite and is subdivided into the Internal Gabbro unit, the Amphibole Peridotite unit, and the External Gabbro unit. The Internal Gabbro and the Amphibole Peridotite units consist of coarse-grained, chemically heterogeneous cumulates, whereas the External Gabbro unit is generally massive, chemically more uniform and approximately representative of the residual melt with MgO contents between 6.6 and 9.1% and Mg numbers between 38 and 58. Both whole-rock and mineral contents of Ni and Cr are significantly higher (at similar Mg numbers) in the Amphibole Peridotite unit than in the Internal Gabbro, unit. The most straightforward interpretation of this is that the Amphibole Peridotite unit accumulated after the influx of fresh mafic (or ultramafic) magma into the magma chamber. Major-element chemical trends are continuous from the Amphibole Peridotite unit to the External Gabbro unit and are consistent with closed-system fractionation with no further addition of magma or contamination by wall or roof rock assimilation. In the External Gabbro unit, total FeO and TiO2 contents are strongly correlated with each other (and with P2O5 and Zr) and reach values as high as 19 and 4%, respectively, indicating an advanced degree of crystal fractionation along a tholeiitic trend. The External Gabbro samples have generally smooth normalized trace element patterns, which are consistent with being representative of a liquid composition. The residual nature of the External Gabbro magma is also indicated by negative Eu and Sr anomalies, clear evidence for prior feldspar-fractionation. REE patterns are otherwise indistinguishable from N-type MORE, but Th and U an significantly more depleted than in MORE. This Th and U depletion is similar to that found in olivine basalts and picrites on Iceland and Hawaii; its origin is not well understood. No evidence is seen for any assimilation of crustal material, in sharp contrast with the situation of the igneous complex in Val Sesia near Balmuccia, where the magma composition is dominated by assimilation of crust. We suggest that the heat provided by at most two injections of magma near Finero was insufficient to induce crustal anatexis, in contrast with the excess heat supplied by multiple magma injections at Balmuccia.

1997 - The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone). 2. Geochronology and isotope geochemistry [Articolo su rivista]
Mh, Lu; Aw, Hofmann; Mazzucchelli, Maurizio; Rivalenti, Giorgio
abstract

Whole-rock Nd and Sr isotopic compositions of the mafic-ultramafic complex near Finero demonstrate that the magma was derived from a depleted, perhaps MORE-type mantle reservoir. The Sm-Nd data for the Amphibole Peridotite unit can be interpreted as an isochron with an apparent age of 533 +/- 20 Ma, which is consistent with a Pb-207/Pb-206 evaporation age of 549 +/- 12 Ma of a single zircon grain from the Internal Gabbro unit. However, the interpretation of these apparent ages remains open to question. We therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 Ma ago, the latter bring the most likely age of emplacement of the much larger magma body near Balmuccia (Val Sesia). The implication of the older emplacement age (if correct) would be that the igneous complex may be related to the numerous amphibolite units, which are intercalated with the metapelites of the overlying Kinzigite Formation, and together with them may constitute an accretionary complex. In this ease, the mafic-ultramafic complex itself might also be part of such an accretionary complex (as has been proposed for the Balmuccia peridotite). Internal Sm-Nd isochrons involving grt, cpx, plag and amph from the Internal Gabbro unit yield concordant ages of 231 +/- 23, 226 +/- 7,223 +/- 10, 214 +/- 17, and 203 +/- 13 Ma. These results confirm published evidence for a separate, regional heating event about 215 +/- 15 Ma ago. Initial epsilon(Nd)(533) values average + 6.3 +/- 0.4 for six samples of the Amphibole Peridotite unit and + 6.0 +/- 1.2 for ten samples of the External Gabbro unit. Sr-87/Sr-86 ratios require little or no age correction and range from 0.7026 to 0.7047 (with two outliers at 0.7053 and 0.7071). Strong correlations between Sr-87/Sr-86 and K2O and weaker correlations between initial epsilon(Nd) and K2O imply a comparatively minor (less than or equal to 10%) contamination of the External Gabbro magma by crustal material and a later alteration by a crustal or seawater-derived fluid. These results contrast sharply with the isotopic composition (negative epsilon(Nd) and high Sr-87/Sr-86 values) of the associated mantle rocks, the Phlogopite Peridotite unit, which has been pervasively metasomatized by crustal fluids. This type of metasomatism and its isotopic signature are never seen in the magmatic complex. This evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite. The crust-mantle interaction is the opposite of that seen at Balmuccia, where the mantle peridotite is essentially ´pristine´ and the magmatic body has been extensively contaminated by assimilation of crustal rocks.

1996 - La geochimica del granato nelle granatiti della Val Fiorina (Val Grande, Ivrea-Verbano). [Articolo su rivista]
BARBIERI M., A; Mazzucchelli, Maurizio
abstract

In this note we consider the relationship between gabbroic sills and metasediment in a peculiar area of the Ivrea Zone, Val Fiorina, where the dominant rock type is garnetite (modal garnet > 60%). Garnetite layers, which reach a tnickness up to 15 m, invariably separate melasediment and gabbroic silIs. Garnets of these layers are almandine-rich pyralspites. The main variation of the garnet composition which occur trough tne sections is a grossular concentration increase and a decrease of the pyrope-almandine component, away from the metasediments. Garnets are LREE depleted and in general have flat or slightly increasing MREE-HREE patterns. The most striking feature of the REE profiles is the negative Eu anomaly, which decreases away from the metasediment contact. A garnetite petrogenesis related with interaction of basaltic and anatectic melts is here favoured wilh respect to processes of subsolidus element diffusion and a formation of refractory residua after advanced anatexis of metasediments.

1996 - Peridotite clinopyroxene chemistry reflects mantle processes rather than continental versus oceanic settings [Articolo su rivista]
Rivalenti, Giorgio; R., Vannucci; E., Rampone; Mazzucchelli, Maurizio; Gb, Piccardo; Em, Piccirillo; P., Bottazzi; L., Ottolini
abstract

Comparison of 360 trace element analyses of clinopyroxenes from peridotites indicates that clinopyroxene composition cannot discriminate between suboceanic and subcontinental mantle. Clinopyroxenes from abyssal and fore-arc peridotites have the lowest incompatible element concentrations and record melting and basalt extraction as the dominant process. Clinopyroxenes from continental peridotite massifs partially overlap the composition of clinopyroxenes from abyssal peridotites and, in general, behave as a less depleted component in the chemical variation trends defined by the latter. In the peridotite massifs, subsolidus re-equilibration involving phase transitions (spinel to plagioclase facies) may cause significant element variations in clinopyroxene (e.g. Sr and Eu/Eu* decrease, REE, Ti increase). Metasomatic processes cause only comparatively minor trace element variations. Clinopyroxenes from mantle xenoliths in OIB and continental alkali basalts have the highest incompatible trace element concentrations and the largest compositional variations, They primarily record metasomatic enrichment processes, which are similar in suboceanic and subcontinental mantle. These processes induced the highest incompatible element enrichment in the clinopyroxenes from the most refractory peridotites, supporting the chromatographic nature of infiltration metasomatism. This enrichment, however, does not affect Ti, which is in the same concentration range in clinopyroxenes from both xenoliths and abyssal peridotites. The apparent Ti immobility may be due to several causes, such as reduced Ti solubility in hydrous fluids, fractionation of Ti-rich phases from percolating silicate melts, reaction with carbonatite melts formerly equilibrated with amphibole-peridotite. In general, clinopyroxene geochemistry does not allow a clear distinction between different metasomatic agents. The similarity between the geochemical characteristics of xenoliths from continental and oceanic environments supports previous results that the compositions of the pre-metasomatic lithosphere and of the asthenosphere, from where metasomatic agents derive, do not differ in the two environments.

1996 - Petrology and geochemistry of the mafic dyke swarm of the Treinta Y Tres Region, Northeast Uruguay. [Articolo su rivista]
Girardi, V. A. V.; Mazzucchelli, Maurizio; Civetta, L.; Bossi, J.; Campal, N.; Teixeira, W.; Teixeira, C. T.
abstract

Transitional basalts of the Treinta y Tres region intrude late Proterozoic granites in northeast Uruguay. The age of the swarm is assumed to be close to 750 Ma. Major and trace geochemical diagrams indicate gabbro type fractionation, which is supported by MPR plots. In comparison to the Florida and Nico Perez suites, the Treinta y Tres swarm is enriched in Ti, P, Zr and Sr and depleted in LILE and REE. These features and the Sro and Ndo isotope values are discussed in terms of source heterogeneity and crustal contamination. The geochemical characteristics do not support crustal contamination. The Treinta y Tres suite is better explained by a source affected by two melting episodes. The dikes were originated from the latter melting, which occurred in a an isotopically heterogeneous source. Diagrarns related to several incompatible elements ratios indicate that the mantle source of Uruguaian dikes and of continental basalts of Brazil have similar geochemical features since Early Proterozoic up to Mesozoic.

1995 - Composizione dei Granati in esperimenti di anatessi di rocce crostali. [Articolo su rivista]
Cavallini, M; Vielzeuf, D; Mazzucchelli, Maurizio
abstract

An experimental study on melting of pelitic rocks shows that garnet is an important residual phase coexisting with peraluminous granitic liquid. At 0.5 GPa garnet is present, but it is less abundant than cordierite. At 0.7 and 0.9 GPa garnet is the prevailing ferromagnesian phase and coexists with biotite and/or spinel, depending on temperature. Garnet compositions vary markedly with temperature and to a smaller extent with pressure. The almandine content decreases with increasing temperature, while pyrope content decreases. The grossular content changes little with temperature and pressure. The spessartine content is pressure-dependent, increasing with pressure decrease. As a consequence, spessartine rich-garnet is stable at 0.5 GPa, within the cordierite stability field. In coexisting garnets and cordierites, XMg of the two phases [XMg = Mg/(Mg+ Fe2+) molar) increases with increasing temperature and pressure, being XMgCordierite > XMgGarnet. Moreover, XMgGarnet is constantly higher than XMgLiquid, with the increasing temperature. This is in contrast with the observation of Ellis D.J. (1986), who finds XMgGarnet < XMgLiquid.

1995 - ORIGIN OF LREE-DEPLETED AMPHIBOLES IN THE SUBCONTINENTAL MANTLE [Articolo su rivista]
R., Vannucci; G. B., Piccardo; Rivalenti, Giorgio; A., Zanetti; E., Rampone; L., Ottolini; R., Oberti; Mazzucchelli, Maurizio; P., Bottazzi
abstract

Ion-microprobe analyses of interstitial kaersutite and Ti-pargasite grains from orogenic peridotites and Iherzolite xenoliths reveal that LREE-depleted amphiboles are common in the subcontinental mantle samples. Incompatibility diagrams for the investigated amphiboles show that REEs almost parallel those of coexisting clinopyroxenes, whereas Sr, Zr, and Ti show variable anomalies (i.e., Sr/Sr* and Ti/Ti* > 1 and Zr/Zr* < 1). In the chondrite-normalized incompatibility diagrams, Sr lies almost a factor of two above Ce and Nd and is usually depleted relative to HREEs. Average amphibole/clinopyroxene partition coefficients for spinel-bearing assemblages range from 1.4-1.8 for LREEs and from 1.8-2.2 for HREEs. Corresponding D values for Zr, Sr, and Ti are about 1, 3, and 5, respectively. Present data apparently contrast with the conventional wisdom that the presence of amphibole in mantle rocks is related to the introduction of melt or fluids enriched in incompatible elements. In the absence of experimental evidence that aqueous fluids in equilibrium with deep mantle are LREE-, Sr-depleted, we propose either a diffusive redistribution (near solidus or at subsolidus) or a chromatographic process to account for the formation of depleted amphibole from LREE-, Sr-enriched fluids. The crystallization of LREE-, Sr-depleted kaersutite and Ti-pargasite has important geodynamic implications, since it refers, at least for some peridotite massifs (i.e., Zabargad, External Ligurides, Eastern Pyrenees) to the steady-state equilibration under spinel facies conditions and is related to the early evolution of peridotites. This stage is broadly related to the timing of lithospheric accretion.

1995 - Petrology of the Proterozoic mafic dike swarms of Uruguay and constraints on their mantle source composition. [Articolo su rivista]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; Em, Piccirillo; Vav, Girardi; L., Civetta; R., Petrini
abstract

Three dyke swarms in Uruguay range in age from Palaeoproterozoic (1.86 Ga, Florida region) to Neoproterozoic (similar to 0.7 Ga, Nice Perez and Treinta y Tres regions). The Florida and Nice Perez swarms are basalts, basaltic andesites and andesites with tholeiitic affinity, characterized by LILE and LREE enrichment with respect to HFSE, Nb depletion with respect to K and La, K/Rb < 260, Ba/Rb < 19, Zr/Nb > 13, (La/Yb)(n)> 4.5. Both swarms have positive epsilon(Sr) and negative epsilon Nd, but the Florida isotopic array is dominated by epsilon(Nd) variations (EM1 type) and that of Nice Perez by epsilon(Sr) variation (EM2 type). The Treinta y Tres swarm consists of slightly ne-normative transitional or alkaline basalts with geochemical characteristics resembling those of OIB (e.g., Gough Island),but suggestive of a less enriched OIB source, and have slightly positive epsilon(Nd) and epsilon(Sr). Crustal contamination does not appear to be important in the petrogenesis of the swarms. It is proposed that the geochemical characteristics of the Florida swarm derive from melting of lithospheric mantle infiltrated by hydrous fluids released from the thermal breakdown of hydrous phases, either contained in underplated oceanic crust or formed by interaction of asthenospheric fluids with the lithospheric mantle. The EM1-type isotopic features are considered as unrelated with possible fluid addition, but as a pre-existing feature. The geochemical and isotope characteristics of the Nice Perez swarm are attributed to time-integrated enrichment processes, which affected the Nice Perez lithospheric mantle during the Palaeoproterozoic Florida episode, The geochemistry of the Treinta y Tres swarm may be explained by partial melting of a lithospheric mantle which did not suffer Palaeoproterozoic LILE and LREE enrichment but which was isotopically reset in Palaeoproterozoic times. Alternatively, a residual mantle related to the Florida event is required. Mantle melting and dyke intrusion probably occurred in an ensialic environment. The geochemical and isotopic characteristics of the Uruguay dyke swarms are similar to those of the Mesozoic basalts related to the Gondwana break-up, supporting the possibility that the latter derived from a heterogeneous lithospheric mantle source which recorded Proterozoic enrichment processes.

1995 - Petrology of the upper proterozoic mafic dikes in the Nico Perez region, Central Uruguay [Articolo su rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; M., Molesini; R., Petrini; Vav, Girardi; J., Bossi; N., Campal
abstract

Tholeiitic basaltic and basaltic andesite dikes of Brasiliano (or PanAfrican) age (similar to 600 Ma) intrude the basement of the Nice Perez region, Uruguay. Major and trace element geochemistry of the basalts indicates that they suffered fractionation in shallow magma chambers. The variation in element ratios, which remain virtually unchanged during fractionation (K/Rb, Rb/Ba, Ba/Nb, La/Nb, Zr/Nb and Ti/Zr), indicate that the dikes are not strictly comagmatic. However, they have certain features in common: LILE and LREE enrichment with respect to HFSE and HREE; high Rb/Ba(> 0.9) and Rb/Sr (> 0.08); low K/Rb (< 214); negative Nb and Ti anomalies (La-n/Nb-n > 2; Ba/Nb > 22, Ti/Zr < 60). Sr-87/Sr-86 and Nd-143/Nd-144 at 665 Ma are in the range 0.7052 - 0.7119 and 0.51158 - 0.51177, respectively. The lack of correlations between isotope and trace element variations indicate that these characteristics are not controlled by crustal contamination of the melts. They are interpreted as being due to the melting of an enriched mantle (C1) under the influence of a fluid-rich component (C2) which stabilized a Nb-retaining titanate phase in the residuum. Although this process may be related to a subduction environment, it is also possible that it occurred in an ensialic region by the interaction of deep mantle fluids with the lithospheric continental mantle. The parent mantle underlying the early Proterozoic (1.8 Ga) Florida region had isotope and geochemical characteristics which could evolve to the values observed in the adjacent Nice Perez region. It is therefore proposed that mantle enrichment took place in the early Proterozoic and that this mantle melted under the influence of fluids in the late Proterozoic to derive the Nice Perez dikes.

1995 - Processi di anatessi in crosta profonda: Un esempio nella Zona Ivrea-Verbano (Val d'Ossola, Italia Nord-Occidentale). [Articolo su rivista]
Parenti, M; BARBIERI M., A; Mazzucchelli, Maurizio
abstract

The Ivrea-Verbano zone, Southern Alps, is a section of lower crust intruded by basic magmas, both later reequilibrated at high P-T conditions. In the Val Fiorina, north of the middle Val d'Ossola, these concordant intrusions are constituted by sills of differentiated basic rock (gabbro, amphibolic pyroxenite and peridotite) that are bounded by very restitic crustal rocks (garnetites) from former felsic granulites. The whole rock major element trends, mineral associations, phase crystalchemistry and patterns of minor and trace elements from these crustal rocks are consistent with anatectic processes. These processes were supposedly generated by the intrusion of basic sills, produced in the surrounding metasediments a restitic portion (garnetites) and a granitic melt, the latter presently recorded in the area by some leucocratic dykes.

1995 - The relationship between websterite and peridotite in the Balmuccia massif as revealed by trace element variations in clinopyroxene [Articolo su rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; R., Vannucci; Aw, Hofmann; L., Ottolini; P., Bottazzi; W., Obermiller
abstract

Clinopyroxenes from peridotite, Cr-diopside and Al-augite websterites of the Balmuccia massif have been analyzed by ion-probe for REE, Sr, Zr, Ti, V, Y, Sc and Cr. In the peridotites, the REE pattern of clinopyroxene varies from markedly LREE depleted [(Ce/Yb)(n)=0.03] to moderately LREE enriched [(Ce/Yb)(n)=1.96]. Clinopyroxenes of the Cr-Diopside suite vary from moderately LREE depleted to enriched [(Ce/Yb)(n)=0.53-1.36], but HREE are always depleted with respect to MREE [(Sm/Yb)(n) = 1.19-2.06]. The clinopyroxenes of the Al-Augite dikes have higher trace element concentrations and are relatively depeleted in LREE. Two types of peridotite to websterite transitions have been documented. One type characterizes the transitions from peridotite to Cr-diopside websterite. The depletion of peridotite increases, and cpx ''fertility'' decreases, towards the contact; i.e. REE from Sm-Eu to Yb and Ti, V, Y, Sc and Cr in cpx define abundance trends consistent with melt extraction. LREE, Sr, and perhaps Zr, on the contrary, increase in the contact region over a distance of 0-40 cm. The variation of the latter elements and of CaO and Al2O3 from peridotite to websterite is smooth, whereas it is steep for the other elements. Alumina, HREE, Y and Sc are lower, Ca is higher, and the MREE/HREE pattern is steeper in the clinopyroxene of the websterite with respect to peridotite. In the second type of transition, as the contact is approached, there is an increase in modal cpx and decrease of the opx/cpx ratio. Cpx trace element patterns are similar in peridotite and websterite. This transition is generally at the Al-augite websterite contacts, but also was found at two Cr-diopside contacts. The wall rock depletion of the first transition type is interpreted as caused by melt extraction induced by intrusion of melts derived from deeper mantle (garnet-facies), whereas the LREE and Sr variation is interpreted in terms of diffusive redistribution of trace elements after emplacement of the dikes and after the modal clinopyroxene has been removed from the peridotite. Diffusive exchange occurred under stagnant conditions but in the presence of a partial melt or a ''metamorphic fluid'', rather than under dry subsolidus conditions. There is no conclusive evidence favouring either a partial melt or an aqueous fluid as a medium for diffusive transport. However, we favour transport via a fluid phase because the presence of such a fluid is indicated by ubiquitous traces of amphibole in the peridotite. The second transition type is interpreted in terms of melt infiltration and re-equilibration of the melt with the peridotite matrix.

1994 - Chemical evolution of a large mafic intrusion in the lower crust, Ivrea-Verbano Zone, northern Italy. [Articolo su rivista]
Sinigoi, S.; Quick, J. E.; CLEMENS KNOTT, D.; Mayer, A.; Demarchi, G.; Mazzucchelli, Maurizio; Negrini, L.; Rivalenti, Giorgio
abstract

The Ivrea-Verbano and adjacent Strona-Ceneri zones have been described collectively as a section through the continental crust. While resident in the lower crust, amphibolite- to granulite-facies paragneiss of the Ivrea-Verbano Zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the Mafic Complex. Growth of the Mafic Complex involved hypersolidus deformation in an extensional environment. Isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle-derived melts that were extensively contaminated by crustal material. Previous investigations determined that the contaminant was fingerprinted by 87Sr/86Sr > 0.71, δ18O = 10-12.5‰, and a positive Eu anomaly. In the present study, the contaminant is also shown to have been enriched in Ba with respect to Rb and K. Charnockites associated with paragneiss septa in the lower part of the Mafic Complex have the appropriate chemistry to be samples of the contaminating material. These chemical features can be explained by melting of granulite facies paragneiss, which had previously been depleted in K and Rb by an earlier melting event. The Ba enrichment in the core of the Mafic Complex can be modeled by a replenishment-tapping-fractional-crystallization (RTF) process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by Ba-rich charnockite. Very high Ba/K in the lower part or the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa, in contrast to the Mafic Complex, the chemistry of coeval granites in the adjacent Strona-Ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event. Significantly, the incompatible trace element abundances in the Mafic Complex and Strona-Ceneri granites are similar to model compositions for the lower and upper crust, respectively.

1994 - Determinazione di Zr e Y in peridotiti mediante fluorescenza a raggi X con il metodo delle aggiunte. [Articolo su rivista]
Buffagni, S.; Mazzucchelli, Maurizio
abstract

A “spiking method” in X-Ray Fluorescence (XRF) has been developed in order to analyze Zr and Y at very low concentration level in peridotitic rocks. The “spike” mixture is constituted by pure Zr element and Y oxide (Y2O3) diluted in a quartz powder (SiO2 > 99.9 %). For each sample, 4 different pressed pellets spiked with 0, 3.3, 6.7 and 10 ppm for Zr and 0, 5, 10, 15 ppm for Y, respectively, have been prepared. The correlations obtained between XRF intensities and spiked concentration are very good (R2 >= 0.97) and allows the actual Zr and Y concentration of each sample to be calculated. Precision and accuracy are believed to be <± 30% for Zr and < 20% for Y respectively. When the intensities of Zr and Y are plotted versus the concentrations of all the pressed pellets, good calibration curves in the range of 0¸15 ppm are obtained (uncertainties <±10%). The international standards available in this range of concentrations do not deviate appreciably from the regression lines, with the exception of NIM standards.

1994 - Magmatismo mafico nella regione di Carajàs (Parà Meridionale, Brasile). [Articolo su rivista]
Molesini, M.; Mazzucchelli, Maurizio
abstract

This study examines the petrological characteristics of the basaltic dykes of the Carajás region and of a few samples of the older Archean flood basalts (Grão Pará). The age of the mafic dykes is unknow, although they are Proterozoic. They have predominant trends NE-NNE (NED) and NW-NNW (NWD); more rare is N-S (NSD). Moreover, basaltic sills (SB) occur at the Salobo mine in drill holes. NED and SB dykes are tholeiites and andesite basalts respectively; NWD and NSD dykes are principally transitional basalts, two NWD dykes are trachyandesites. Alltogheter, the dykes span a fractionation range between mg = 0.63 and mg = 0.36, dominated by clinopyroxene, plagioclase and olivine. Consistent compositional differences between various dykes groups not caused by fractionation as shown by incompatible element ratios (e.g. Ba/Nb ane Ba/Rb) and element patterns. These differences derive in part from different melting degrees of a garnet peridotite source and in part reflect complementars processes of crustal contamination and/or mantle metasomatism. The Grão Pará basalts are intercalated with rhyolites, dacites and metasediments and have an U/Pb age of 2758±39 Ma (WIRTH, GiBBS & OLSZEWSKI, 1986) on a rhyolites zircon. They have suffered green-schist metamorphism, which has partially trasformed the mafic minerals into chlorite and actinolite. Compositionally they are latibasalts and andesite-basalts and have suffered fractionation of plagioclase and clinopyroxene; their mg number varys from 0.61 to 0.55. They are LILE and LREE enriched, expecially in Ba (2850 ppm) and show negative Nb anomalies. Their overall chemical trends and characteristics indicate calcalcaline affinity.

1993 - Crystal chemical variations in Ba-rich biotites from gabbroic rocks of lower crust (Ivrea Zone, NW Italy) [Articolo su rivista]
Bigi, Simona; Brigatti, Maria Franca; Mazzucchelli, Maurizio; Rivalenti, Giorgio
abstract

Biotites from mafic rocks occurring at different stratigraphic levels of the Ivrea-Verbano Mafic Complex are studied. The rocks are gabbros and diorites. All the biotites are intermediate between phlogopite and annite [0.28<Fe/(Mg+Fe)<0.66]. Biotites from gabbros are enriched in BaO and TiO2 (up to 7.14 and 9.32 wt%, respectively) with respect to those of the diorites (up to 1.26 and 6.26 wt%, respectively). Systematic compositional variations support the substitution model 2(IV)Si + ((VI)R2+) half arrow left over half arrow right 2(IV)Al + (VI)Ti (R2+ = Fe + Mg + Mn) in gabbros and (IV)Si + (VI)Al half arrow right over half arrow left (IV)Al + (VI)Ti in diorites. A predominance of disordered stacking sequences, coexisting with 1M, 2M1 and 3T polytypes was observed in all biotites. It was possible to carry out structural refinements only on three biotites-2M1 from diorites (R-values between 2.68 and 3.77) 'and one biotite-1M from gabbros (R-value=3.09). It was shown that: (1) the reduced thickness of the tetrahedral sheet in Ba-rich biotites supports the coupled substitution (IV)Si + (XII)K half arrow left over half arrow right (IV)Al + (XII)Ba; (2) the interlayer site geometry is affected by the whole layer chemistry and does not reflect only local chemical variations; (3) in two samples of the 2M1 polytype, the M(1) octahedral site is larger and more distorted than the M(2) sites because of the preferential ordering of Fe2+ in the M(1) site, whereas one sample shows complete cation disorder in the octahedral sites. Biotite-1M shows that Fe2+ can also be located in the M(2) site. Some of the differences between the biotites of gabbros and diorites (e.g. Ba concentration and exchange vectors) may be linked to the host rock composition and to its crystallization process. Biotite occurs in trace amounts in gabbros and its crystallization is related to the interstitial melt which contributed to the adcumulus growth of the main rock forming phases and became highly enriched in K, Ba and Ti. Diorites are the result of equilibrium crystallization of a residual melt rich in incompatible elements, where biotite is a major constituent.

1993 - Early Proterozoic dike swarms from western Uruguay: Geochemistry, Sr-Nd isotopes and petrogenesis [Articolo su rivista]
J., Bossi; N., Campal; L., Civetta; G., Demarchi; Vav, Girardi; Mazzucchelli, Maurizio; L., Negrini; Rivalenti, Giorgio; Arsf, Cesar; S., Sinigoi; W., Teixeira; Em, Piccirillo; M., Molesini
abstract

Early Proterozoic (1.86 +/- 0.12 Ga), unmetamorphosed basic dikes intrude the Precambrian Rio de la Plata Craton of western Uruguay. The dikes define two distinct swarms of tholeiitic andesitic basalt and andesite composition, respectively. Major- and trace-element geochemistry indicates that these two magma types are related via gabbro fractionation. The incompatible trace-element patterns favour a derivation of the parent melts from melting of a garnet peridotite source. The high LILE and LREE content is discussed in terms of crustal contamination of the melts or of the source, or of mantle metasomatism. Mixing calculations do not support important crustal contamination of the melts or of the source. Mantle metasomatism is a preferred process, which operated slightly before or during melting. An anorogenic ensialic environment of emplacement is inferred.

1993 - THE TCHIVIRA-BONGA ALKALINE-CARBONATITE COMPLEX (ANGOLA) - PETROLOGICAL STUDY AND COMPARISON WITH SOME BRAZILIAN ANALOGS [Articolo su rivista]
Coltorti, M; Alberti, A; Beccaluva, L; Dossantos, Ab; Mazzucchelli, Maurizio; Morais, E; Rivalenti, Giorgio; Siena, F.
abstract

The Tchivira alkaline-carbonatite complex, of Lower Cretaceous age (130-138 Ma), crops out in the Precambrian crystalline basement of southwestern Angola as a circular intrusion covering an area of about 100 km(2). The plutonic complex forms a roughly circular concentric pattern and comprises, from the outside inward, nepheline syenites, ijolites/urtites and carbonatites. The western side of the complex is cut by a second intrusion of alkali gabbros and syenites. Bonga, an isolated plug a few kilometres northeast of Tchivira, consists exclusively of carbonatites with an area of about 12 km(2). A number of dykes, ranging in composition from tephrite to alkali basalt, hawaiite, mugearite, phonolite and trachy-phonolite, are widespread in the complex and in the basement country rocks. Field relationships and petrological data can be used to define two magmatic suites: 1) nepheline syenite-ijolite-carbonatite, and 2) alkali gabbro-syenite, both of which evolved essentially by fractional crystallization under nearly closed system conditions. Mass balance calculations account for the generation of the various rock-types in both suites, starting from basanitic and alkali basaltic parental magmas, respectively. Ca-carbonatite melts may be genetically linked to nepheline syenite by liquid immiscibility processes, and may themselves undergo fractional crystallization -mostly of calcite- leading to differentiated Mg-carbonatites. Initial Sr-87/Sr-86 isotopic ratios mostly range from 0.7038 to 0.7050 in the nepheline syenite-ijolite suite, and from 0.7046 to 0.7059 in the alkali gabbro-syenite suite. Significantly higher isotopic ratios (0.7073-0.7255), which are observed in some dykes and in the border facies of the intrusion, may be related to contamination by the granitic basement. The age relations, petrology, Sr isotopes, rock associations and spatial arrangements of the Tchivira-Bonga complex are comparable with those of the south Brazilian alkaline-carbonatite complexes, particularly Juquia and Jacupiranga. Such a comparison suggests a common geodynamic and petrogenetic setting for the South Brazilian and Angolan alkaline magmatic suites just before the opening of the South Atlantic Ocean.

1992 - I Metasedimenti della Zona Ivrea-Verbano nell'area di Premosello (Val d'Ossola). [Articolo su rivista]
Cavallini, M.; Mazzucchelli, Maurizio
abstract

In the Ossola Valley the exposed lower continental crust (i.e. the Ivrea Zone) is made of a metasedimentary sequence intruded by sills of mafic and ultramafic rocks. The metasediments studied here are located in the westernmost part of the Ivrea Zone, close to the Insubric line. They are metapelites, with minor meta-arenites and meta-greywackes, metamorphosed under granulite facies conditions. In the granulites five types of mineral assemblages can be distinguished. Whole-rock chemistry indicate that associations with very abundant grt+sil are strongly SiO2-depleted and enriched in relatively refractory elements (Mg, Fe, Al, Ti) and garnet-compatible elements (Y, Nb). These rocks are believed to be the residue left after the extraction of large amount of granitoid melts. Recent experimental results are used to constrain our residual mineral assemblages.

1992 - Il corpo peridotitico di Premosello Chiovenda in Val d'Ossola (Zona Ivrea-Verbano). [Articolo su rivista]
Mazzucchelli, Maurizio; Marchesi, S.; Bottazzi, P.; Ottolini, L.; Vannucci, R.
abstract

The geological, petrographic and geochemical characteristics of the Premosello Chiovenda peridotitic body in the Ivrea-Verbano zone are here discussed. The small outcrop (around 300 m2) occurs in the Ossola valley (NW Italy) close to the Insubric line, between the Finero body to the NE and the Balmuccia body to the S, and is constituted by clinopyroxene-poor lherzolites and harzburgites. Bulk rock, mineral compositions and Rare Earth Element patterns are closely similar to those from the Balmuccia body.

1992 - Primary positive Eu anomaly in clinopyroxenes of low-crust gabbroic rocks [Articolo su rivista]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; R., Vannucci; P., Bottazzi; L., Ottolini; Aw, Hofmann; M., Parenti
abstract

Positive Eu and Sr anomalies have been found in whole rocks and clinopyroxenes, as well as feldspars, in gabbros of the deep crustal mafic intrusion of the Ivrea Zone (northwestern Italy). These anomalies are particularly pronounced at one stratigraphic level of the intrusion, the upper part of the Upper Zone of the Layered Series (HUZ). The Eu/Eu* ratios range from 19 to 40 in plagioclases and from 3 to 14 in the coexisting clinopyroxenes. Mass balance calculations show that metamorphic redistribution of Eu from plagioclase cannot account for the positive Eu anomalies of the clinopyroxenes and that a corresponding anomaly in the parent melt is required. Speculatively, this feature is attributed to mixing of a mantle component with crustal, highly polymerized, melts deriving from advanced anatexis of granulite-facies metasediments. The existence in the deep crust of hybrid melts having a positive Eu anomaly may be important for the Eu balance between the lower and upper crust.

1992 - Rocce subcrostali nella zona d'Ivrea (Italia): I "Corpi minori" della Val Strona". [Articolo su rivista]
Marchesi, S.; Mazzucchelli, Maurizio; Bottazzi, P.; Ottolini, L.; Vannucci, R.
abstract

Besides the main peridotitic massifs (from SW to NE: Baldissero, Balmuccia and Finero), in The Ivrea Zone other “minor” ultramafic lenses occur completely embedded in the metamorphic Kinzigitic Formation. They are mainly located in the Strona valley. The Alpe Francesca, Alpe Piumero and Alpe Crotta define a discontinuous horizon in correspondence of the shear zone, which characterizes the transition, in the Kinzigitic Formation, from the amphibolite to the granulite facies.The Alpe Morello is the stratigraphically highest massif in Strona valley.It occurs at the boundary between the Ivrea Zone and the Serie dei Laghi unit.The petrological and geochemical characterization of all these occurrence of subcrustal rocks is here presented.

1992 - Trace element distribution between clinopyroxene and garnet in gabbroic rocks of deep crust: an ion microprobe study. [Articolo su rivista]
Mazzucchelli, Maurizio; Rivalenti, Giorgio; R., Vannucci; P., Bottazzi; L., Ottolini; A. W., Hofmann; S., Sinigoi; G., Demarchi
abstract

Clinopyroxenes and garnets from gabbroic rocks of the Ivrea Verbano mafic complex have been analyzed by electron microprobe for major elements and by ion microprobe for REE, Sc, Cr, Ti, V, Zr, Na, and Sr content. The samples represent two petrographic types: in the first, garnet is formed by subsolidus reaction and occurs in coronas (c-type); in the other, garnet occurs as large porphyroblasts (p-type) and may have been a phase on the liquidus. Clinopyroxenes and garnets are unzoned (with one exception) for major and trace elements, suggesting that, in general. equilibrium has been attained under granulite facies conditions as indicated by the geothermometers. Clinopyroxene, although affected in its HREE and Sc content by the coexistence with garnet, has REE patterns which vary, along with the bulk sock patterns, stratigraphically upwards from LREE-depleted to LREE-enriched. Trace element distribution coefficients (D) between clinopyroxene and garnet, as measured in the p-type assemblages, vary systematically with major-element compositional parameters such as FeO, MgO, FeO/MgO, Al2O3, Na20, and apparent equilibration temperature. In addition, the overall pattern of REE partitioning, D(Ce) to D(Yb), is significantly steeper than those found in previously published estimates, except when they were determined on exceptionally carefully prepared mineral separates. The D values determined on c-type assemblages are comparatively erratic and appear to depend on the modal gnt/cpx ratio. This feature is tentatively attributed to failure to achieve complete equilibrium during slow cooling when the corona structures were formed. Subsolidus reequilibration between phases has generally obliterated the igneous phase chemistry of the rocks sufficiently so that the composition of the parent liquid cannot be determined from those of the constituent minerals even when these represent original "phenocrysts."

1991 - Biotites from Ivrea-Verbano basic complex [Abstract in Rivista]
Brigatti, Maria Franca; Bigi, Simona; Mazzucchelli, Maurizio; Rivalenti, Giorgio
abstract

nd

1991 - Crystal chemical variations in biotites from lower crust gabbroic rocks (Ivrea zone, NW Italy) [Abstract in Rivista]
Bigi, Simona; Brigatti, Maria Franca; Mazzucchelli, Maurizio; Rivalenti, Giorgio
abstract

nd

1991 - Interactions of mantle and crustal magmas in the southern part of the Ivrea Zone (Italy) [Articolo su rivista]
Sinigoi, S.; Antonini, P.; Demarchi, G.; Longinelli, A.; Mazzucchelli, Maurizio; Negrini, L.; Rivalenti, Giorgio
abstract

In the southern part of the Ivrea Zone (Italy), the majority of the Mafic Formation is composed of: 1. amphibole-bearing gabbro; 2. a series of rocks ranging from norites to charnockites; 3. leucocratic charnockites. In the proximity of metasedimentary septa within the Mafic Formation, the igneous lithologies are in many places intimately and chaotically intermingled, giving rise to a marble-cake structure. Whole-rock chemistry, and oxygen and strontium isotopic compositions indicate that the mafic and felsic rocks are dominated by mantle and crustal sources respectively. The norite-charnockite suite may be modelled as the mixing product of basic and acid melts. Abundant plastic deformation structures suggest that mafic and hybrid rocks experienced an important tectonic event during or soon after their crystallization. Melting of crustal country rocks continued after the deformation event and produced the undeformed leucocratic charnockites. The study area exemplifies some of the possible effects of the intrusion of a large volume of basic magma into hot crust.

1990 - Geochemical variability in the greenstone belts of Goiàs (Brazil): the Hidrolina and Crixàs sequences [Articolo su rivista]
Rivalenti, Giorgio; Mazzucchelli, Maurizio; Girardi, V. A. V.; Fornoni Candia, M. A.; Coltorti, M.; Correla, C. T.; Siena, F.; Finatti, M. C.
abstract

The greenstone belts of Crixás and Hidrolina of Goiás, Brazil, are dismembered components of the same body or of originally adjacent belts. Their tectono-metamorphic history and stratigraphy is similar, as well as their components (meta-komatiites, meta-basalts and felsic meta-volcanics). Chemically, they differ for the CaO and Al2O3 versus MgO trends and for the behavior of many incompatible and compatible trace elements (Cr, Ni, Ti, Sc, Y, Zr, P). The difference in the Ca and Al behavior, which is anomalous at Crixás, is tentatively attributed to an unconstrained igneous crystallization process. This process should imply the growth of clinopyroxene into an olivine-dominated crystal mush, and has acted at Crixás and not at Hidrolina. The difference in the trace element behavior has been mainly attributed to a compositional and modal heterogeneity of the mantle source, which was probably spinel-richer at Crixás than at Hidrolina.

1990 - Isotopic evidence from the Ivrea Zone for a hybrid lower crust formed by magmatic underplating [Articolo su rivista]
H., Voshage; Aw, Hofmann; Mazzucchelli, Maurizio; Rivalenti, Giorgio; S., Sinigoi; I., Raczek; G., Demarchi
abstract

The large mafic complex overlying mantle peridotites in the Ivrea Zone of northern ltaly originated by underplating of mantle-derived magma onto the base of continental crust, and may be a unique outcrop of typical mafic lower crust, sampled elsewhere only by mafic xenoliths. Neodyrnium and strontium isotope data show that the magmas assimilated large and roughly uniform proportions of crustal material. The resulting hybrid magmas have the same isotopic signature as, and may represent the lower-crustal 'roots' of, upper-crustal Hercynian granitoid intrusions.

1989 - Geochemical Models for the Petrogenesis of Komatiites from the Hidrolina Greenstone Belt, CentraI Goias, Brazil [Articolo su rivista]
Rivalenti, Giorgio; Girardi, V. A. V.; Coltorti, M.; Correia, C. T.; Mazzucchelli, Maurizio
abstract

The Archaean greenstone belt of Hidrolina (Centrai Goiás, Brazil) is a severely tectonized and metamorphosed volcano-sedimentary sequence. It consists of a lower sequence (LS) with ultramafic and mafic flows (compositionally corresponding to komatiites) and an upper scquence (US) with intercalated mafic and felsic flows (compositionally corresponding to tholeiites, dacites, and rhyolites). Practically no primary structure or texture is preserved. Geochemical major and trace element modelling allows distinction between liquid compositions and cumulates. Crustal contamination does not seem to have been effective in controlling the geochemical variations. Batch partial melting models and CMAS plot indicate that: (a) the komatiites probably resulted from moderate (28-30%) melting of a spinel peridotite rather than a garnet one; (b) the komatiites were formed by the mixing of a liquid of basaltic composition with peridotite source material; (c) the mantle source was heterogeneous, the flows higher in the stratigraphy coming from a more depleted source than the lower ones; and (d) US tholeiites resulted from lower degree melting of the same depleted source that produced the upper LS flows.

1989 - Sub-solidus Reactions and Corona Structures in the Niquelandia Layered Complex (Central Goias, Brazil) [Articolo su rivista]
Fornoni Candia, M. A.; Mazzucchelli, Maurizio; Siena, F.
abstract

Corona structures occur in the upper units (UGAZ and UA) or the Niquelandia mafic-ultramafic complex (Central Goias, Brazil), where olivine gabbros,mgabbros, anorthosites and "amphibolites" are interlayered. In olivine gabbros, where the corona structures are most complex, primary igneous minerals are olivine, plagioclase ± clinopyroxene and ilmenite. The instability between olivine and plagioclase results in three types of coronas:1) ol + pl → opx + (cpx + sp)sympl2) ol + pl → opx + (amph + spsympl3) ol + pl → opx + amph + garIn the second corona type the amphibole is pargasite; in the third type the amphibole approaches Mg-hornblende in composition. The occurrence or secondary amphibole indicates the presence of a fluid phase during re-equilibration. Reaction calculations, obtained by means of a "MIXING" program, show that the mineralogical changes in the three corona types may reasonably have occurred in a closed system (with the exception of the fluid phase). The chemical composition of the reacting igneous phases (especial1y the Mg/Fe2+ ratio or the mafic phases) constrains the composition or the products, but not the type of secondary assemblage. The nature of these products depends mainly on the variations of aH20, in relation to temperature decrease. In gabbros, anorthosites and "amphibolites", where orthopyroxene and/or amphibole may be intercumulus phases, the following reactions occur between pyroxenes and plagioclase:4) opx + cpx + pl_→ hbl5) opx + cpx + pl → hbl + gar ± qz6) opx + pl → cpx + gar + qzIn these rocks, garnet formation as a product in reaction (5) depends on the partial pressure of a vapour phase. Reaction (6) develops only when the orthopyroxene is very iron-rich. Estimates of the re-equilibration pressure are of 5-8 kb; re-quilibration temperatures vary from about 800 °C (anhydrous corona in olivine gabbros) to 560 °C (plag + hbl + gar + qz secondary assemblages). Temperatures obtained from the hbl – gar geothermometer are directly correlated with the pargasitic component in secondary amphiboles. The absence of deformation suggests that the coronas formed during slow cooling.

1988 - lsotopic constraints on the origin of ultramafic and mafie dikes in the Balmuccia peridotite (Ivrea Zone) [Articolo su rivista]
Voshage, H.; Sinigoi, S.; Mazzucchelli, Maurizio; Demarchi, G.; Rivalenti, Giorgio; Hofmann, A. W.
abstract

The Balmuccia peridotite massif in the central Ivrea Zone constitutes an upper mantle slice which has been tectonically emplaced into the crust. It represents the residue from partial melting of undepleted mantle material and varies in composition from lherzolite to harzburgite and subordinate dunite. Dikes of websterite and gabbroic pods within the peridotite can be subdivided into an older Cr-diopside suite and a younger Al-augite suite. Nd isotopic data on whole rocks of these lithotypes in combination with independent observations suggest that the dikes formed during a Hercynian event about 270 Ma ago. The rocks of the Cr-diopside dikes, in particular, display isotopic signatures similar to those of the lherzolite and represent fractionates from partial melts derived from the lherzolite wall rock. The Sm - Nd data of the pyroxenites and gabbros of the Al-augite suite, in contrast, scatter widely and suggest that partial melting of lherzolite was triggered or at least accompanied by introduction of fluids and/or liquid phases. These fluids or liquids carried exotic isotopic components from elsewhere in the crust-mantle complex, and deposited them within the rocks by metasomatic reactions. Two distinct types of metasomatism must have operated not only within the Balmuccia body, but also in the complex of Finero: The first type of metasomatism introduced mantle-derived volatiles and is responsible for formation of amphibole. The other type has a crustal source and led to formation of phlogopite, which occurs mainly within mantle rock of Finero, but occasionally, within the Balmuccia body also.

1986 - Geotectonic Significance of the Metabasites of the Kinzigitic Series, Ivrea-Verbano Zone (Western Italian Alps) [Articolo su rivista]
Mazzucchelli, Maurizio; Siena, F.
abstract

Geochemical investigations have been carried out on the metabasites of the Ivrea-Verbano Zone (NW-Italy). The Ivrea-Verbano Zone is commonly considered to be a section through the lower continental crust. It is constituted by a steeply dipping sequence of metamorphic rocks (Kinzigitic Series), intruded by a mafic-ultramafic complex. The metabasites of the Kinzigitic Series have a meta-igneous origin, Their protoliths show tholeiilic affinity in the NW and central part of the zone and alkali affinity in the SE. This fact together with the rock associations suggests that a variation of the paleogeographic environment from NW to SE occurs in the Ivrea-Verbano Zone.

1983 - Textural features and olivine fabrics as petrogenetic indicators in the Niqualandia Complex of Central Goiás (Brasil) [Articolo su rivista]
Mazzucchelli, Maurizio; Siena, F.; Girardi, V. A. V.
abstract

The Niquelàndia Complex belongs to an alignment of mafic-ultramafic massifs situated in Central Goiás (Brasil). The Complex displays the following stratigraphy: a) a basal gabbroic zone (BGZ); b) a basal peridotite zone (BPZ); c) a layered ultramafic zone (LUZ); d) a layered gabbroic zone (LGZ). LUZ is a sequence formed by alternating peridotite and pyroxenite layers. The Complex has been interpreted in various ways. The most recent interpretations are those of Danni et al. (1982), who consider the Complex as a protoophiolitic suite, with presence of mantle peridotites (BPZ), and that of Rivalenti et al. (1982), who consider the Complex as a layered sequence formed by fractionation of a basic magma, without presence of mantle peridotites. Nevertheless, in both cases LUZ is considered as formed by fractionation and cumulus. The textural features do not constitute an unequivocal discrimination tool for the peridotites of the Complex, as foliated and cumulus textures occur both in BPZ and LUZ. The lack of discriminant textural features is stressed by fabric analysis. α-olivine fabric is usually weak and appears mainly related to the textural type rather than to the stratigraphic unit. The data presented in this paper suggest the idea that BPZ and LUZ rocks are kindred and formed by the same process.

1983 - The upper zone of the Ivrea-Verbano layered complex (Italian Western Alps) [Articolo su rivista]
Mazzucchelli, Maurizio
abstract

The Ivrea-Verbano Complex is generally considered as a deep seated multiple intrusion. A layered series occurs at the base of the Complcx, which has been subdivided in a basal, an intermediate and an upper zone (BZ. IZ andUZ). Thc three zones, possibly, derive by fractionation from a unique magma.The upper zone is mainly constituted by a gabbroic layered series, dominated in the lower part by melanocratic terms and in the upper part by leucocratic ones, up to anorthosite. Olivine gabbros occur al various levels in this series. Texture is mainly cumulitic and it becomes coronitic by subsolidus reactions.The fractionation pattern presents plagioclase, olivine and pyroxenes as phases of the liquidus and is similar to that of the pure system MgO-Al2O3-CaO-SiO2 at low pressure. Formation of coronitic textures is determined by instability of mafic minerals in presence of plagioclase. This instability is controlled by the composition of the reacting phase and is attributed to re-equilibration by slow cooling and not to a metamorphic event.