 |
TOMMASO GIOVANARDI
Ricercatore t.d. art. 24 c. 3 lett. B
Dipartimento di Scienze Chimiche e Geologiche - Sede Dipartimento di Scienze Chimiche e Geologiche
|
Home |
Curriculum(pdf) |
Didattica |
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
- Early exploitation of Neapolitan pozzolan (pulvis puteolana) in the Roman theatre of Aquileia, Northern Italy
[Articolo su rivista]
Dilaria, Simone; Secco, Michele; Ghiotto, Andrea R.; Furlan, Guido; Giovanardi, Tommaso; Zorzi, Federico; Bonetto, Jacopo
abstract
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
- Slip localization by cataclasis and fluid-rock interaction in seismogenic crustal faults (Gole Larghe Fault, Italy)
[Abstract in Atti di Convegno]
Mittempergher, Silvia; Di Toro, Giulio; Aretusini, Stefano; Gratier, Jean-Pierre; Bistacchi, Andrea; Giovanardi, Tommaso
abstract
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.
2022
- Multiple crustal and mantle inputs in post-collisional magmatism: Evidence from late-Variscan S`arrabus pluton (SE Sardinia, Italy).
[Articolo su rivista]
Secchi, Francesco; Giovanardi, Tommaso; Naitza, Stefano; Casalini, Martina; Kohút, Milan; Conte Aida, Maria; Oggiano, Giacomo
abstract
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.
2021
- Occurrence and characterization of tremolite asbestos from the Mid Atlantic Ridge
[Articolo su rivista]
Di Giuseppe, D.; Perchiazzi, N.; Brunelli, D.; Giovanardi, T.; Nodari, L.; Della Ventura, G.; Malferrari, D.; Maia, M.; Gualtieri, A. F.
abstract
Tremolite is one of the most common amphibole species and, in the fibrous form (i.e., characterized by crystals/particles consisting of fibres with length > 5 µm, width < 3 µm and aspect ratio > 3), one of the six asbestos minerals. Until now the attention of crystallographers has focused only on samples from continental environment. Here we report the first chemical and structural data of a tremolite asbestos found along the Mid Atlantic Ridge (MAR) at the eastern intersection of the Romanche Transform Fault (Equatorial MAR). Tremolite is associated with chlorite and lizardite and was formed through the green shale facies lower than zeolite in a predominantly fluid system. MAR tremolite asbestos shows very slight deviations from the ideal crystal structure of tremolite. Differences in cation site partitioning were found with respect to tremolite asbestos from ophiolitic complexes, attributed to the different chemical–physical conditions during the mineral formation. In particular, oceanic tremolite asbestos is enriched in Al and Na, forming a trend clearly distinct from the continental tremolites.
2020
- Early life of Neanderthals
[Articolo su rivista]
Nava, A.; Lugli, F.; Romandini, M.; Badino, F.; Evans, D.; Helbling, A. H.; Oxilia, G.; Arrighi, S.; Bortolini, E.; Delpiano, D.; Duches, R.; Figus, C.; Livraghi, A.; Marciani, G.; Silvestrini, S.; Cipriani, A.; Giovanardi, T.; Pini, R.; Tuniz, C.; Bernardini, F.; Dori, I.; Coppa, A.; Cristiani, E.; Dean, C.; Bondioli, L.; Peresani, M.; Muller, W.; Benazzi, S.
abstract
The early onset of weaning in modern humans has been linked to the high nutritional demand of brain development that is intimately connected with infant physiology and growth rate. In Neanderthals, ontogenetic patterns in early life are still debated, with some studies suggesting an accelerated development and others indicating only subtle differences vs. modern humans. Here we report the onset of weaning and rates of enamel growth using an unprecedented sample set of three late (∼70 to 50 ka) Neanderthals and one Upper Paleolithic modern human from northeastern Italy via spatially resolved chemical/isotopic analyses and histomorphometry of deciduous teeth. Our results reveal that the modern human nursing strategy, with onset of weaning at 5 to 6 mo, was present among these Neanderthals. This evidence, combined with dental development akin to modern humans, highlights their similar metabolic constraints during early life and excludes late weaning as a factor contributing to Neanderthals’ demise.
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
- Fast offline data reduction of laser ablation MC-ICP-MS Sr isotope measurements: Via an interactive Excel-based spreadsheet 'SrDR'
[Articolo su rivista]
Lugli, F.; Weber, M.; Giovanardi, T.; Arrighi, S.; Bortolini, E.; Figus, C.; Marciani, G.; Oxilia, G.; Romandini, M.; Silvestrini, S.; Jochum, K. P.; Benazzi, S.; Cipriani, A.
abstract
Strontium isotopes are applied to a wide range of scientific fields and to different types of materials, providing valuable information foremost about provenance and age, but also on diagenetic processes and mixing relationships between different Sr reservoirs. The development of in situ analytical techniques, such as laser ablation ICP-MS, has improved our understanding of Sr isotope variability in several fields of application, because of the possibility to discriminate small-scale changes and their spatial distribution. However, large outputs of Sr isotope data are produced by laser ablation MC-ICP-MS systems, which necessitate multiple offline steps to correct and assess the data. This requires the availability of simple and user-friendly tools, easily manageable by non-specialists too. With this in mind, we developed SrDR, an Excel-based interactive data reduction spreadsheet ('SrDR', Sr-Data-Reduction) for the processing of Sr isotopes measured by LA-MC-ICP-MS. The SrDR spreadsheet is easily customizable (a) to meet user-specific analytical protocols, (b) for different instruments (i.e. Nu plasma vs. Neptune), and (c) for diverse target materials (e.g. rare earth element enriched or depleted samples). We also include several examples relevant to low and high temperature geochemistry fields-a fossil tooth, a modern seashell, a speleothem sample and plagioclase crystals-to show how different sample materials are corrected for different interfering masses.
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
- 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.
2019
- The 2.0–1.88 Ga Paleoproterozoic evolution of the southern Amazonian Craton (Brazil): An interpretation inferred by lithofaciological, geochemical and geochronological data
[Articolo su rivista]
Roverato, M.; Giordano, D.; Giovanardi, T.; Juliani, C; Polo, L
abstract
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.
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
- 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 Hf-INATOR: A free data reduction spreadsheet for Lu/Hf isotope analysis
[Articolo su rivista]
Giovanardi, T.; Lugli, F.
abstract
During the last decade, the in situ Lu-Hf isotope methodology has been widely used in Earth sciences and the number of laboratories performing Lu-Hf analysis has increased together with dedicated scientific software. However, free and user-friendly software for the Lu-Hf data reduction is currently missing. We present the ‘Hf-INATOR’, a new interactive Excel spreadsheet, simple to use and completely free (compatible with Libreoffice and Openoffice) that performs data reduction of the Lu-Hf analytical outputs and calculates the main parameters of the Lu-Hf systematic, such as the 2σ errors, the εHf(0), the εHf(t), the TDM, the TDMC and the fLu/Hf.
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
- 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
- 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
- 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
- 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
- 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.
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
- 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 & 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
- 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
- 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
- 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
- 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.
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
- 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
- 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
- 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.