United States

Microstructural investigations of late-Variscan trondhjemites (c. 310 Ma) and granites (c. 300 Ma) from the upper-amphibolite facies Aspromonte Unit of the Peloritani Mountains (NE Sicily; Fiannacca et al., 2019, 2020) revealed for the first time shear-zone deformation affecting  both granitoids during their cooling, from submagmatic (T &gt; 650  °C) to low temperature solid state conditions (T &lt; 450  °C) (Fazio et al., 2020). The magmatic texture of the granitoids is usually largely preserved, but evidence for non-coaxial shearing is provided by sigmoidal porphyroclasts, mica fish and rare domino-torn boudins in tiny andalusite crystals. The occurrence of widespread chessboard quartz, together with rare submagmatic fractures, points to deformation in the presence of late-Variscan melt, thus constraining the activity of the shear zone during the post-collisional stages of the Variscan orogenic cycle. Submagmatic deformation microstructures were largely overprinted by progressive lower temperature microstructures during cooling of the granitoid bodies. Solid state-high temperature deformation (T &gt; 450 °C) is documented by feldspar bulging, quartz grain boundary migration recrystallization, ribbon quartz and core-and-mantle structures. Finally, low-T subsolidus deformation microstructures consist of quartz bulging, subgrain rotation recrystallization, mica kinks and feldspar deformation twins. No systematic relationship between age or emplacement depth, and deformation mechanisms has been observed by comparing the microstructures developed in the older trondhjemites, emplaced at higher depth (c. 0.5 GPa), and in the younger granites, intruding the trondhjemites after their exhumation at relatively shallow crustal levels (c. 0.3 GPa).  The two continuous sequences of submagmatic to low-temperature subsolidus deformation, recorded in both the older and younger granitoids, suggest a duration of c. 20 Ma for shear-zone activity associated to middle crust exhumation in the South-European Variscan Belt.

SGI 2021

Twenty million years of supra- to sub-solidus deformation during exhumation of middle crust from the South-European Variscan Belt (Peloritani Mountains, NE Sicily)

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Microstructural investigations of late-Variscan trondhjemites (c. 310 Ma) and granites (c. 300 Ma) from the upper-amphibolite facies Aspromonte Unit of the Peloritani Mountains (NE Sicily; Fiannacca et al., 2019, 2020) revealed for the first time shear-zone deformation affecting  both granitoids during their cooling, from submagmatic (T > 650  °C) to low temperature solid state conditions (T < 450  °C) (Fazio et al., 2020). The magmatic texture of the granitoids is usually largely preserved, but evidence for non-coaxial shearing is provided by sigmoidal porphyroclasts, mica fish and rare domino-torn boudins in tiny andalusite crystals. The occurrence of widespread chessboard quartz, together with rare submagmatic fractures, points to deformation in the presence of late-Variscan melt, thus constraining the activity of the shear zone during the post-collisional stages of the Variscan orogenic cycle. Submagmatic deformation microstructures were largely overprinted by progressive lower temperature microstructures during cooling of the granitoid bodies. Solid state-high temperature deformation (T > 450 °C) is documented by feldspar bulging, quartz grain boundary migration recrystallization, ribbon quartz and core-and-mantle structures. Finally, low-T subsolidus deformation microstructures consist of quartz bulging, subgrain rotation recrystallization, mica kinks and feldspar deformation twins. No systematic relationship between age or emplacement depth, and deformation mechanisms has been observed by comparing the microstructures developed in the older trondhjemites, emplaced at higher depth (c. 0.5 GPa), and in the younger granites, intruding the trondhjemites after their exhumation at relatively shallow crustal levels (c. 0.3 GPa).  The two continuous sequences of submagmatic to low-temperature subsolidus deformation, recorded in both the older and younger granitoids, suggest a duration of c. 20 Ma for shear-zone activity associated to middle crust exhumation in the South-European Variscan Belt.

poster

International Ocean Discovery Program (IODP) Expedition 360 drilled 809 m-deep Hole U1473A into Atlantis Bank Oceanic Core Complex (57°E, Southwest Indian Ridge). A section of lower oceanic crust was recovered, and nine diabase dikes were sampled at different depths. Two diabase dikes in IODP Hole U1473A display magmatic mineral assemblage (clinopyroxene [Cpx] and plagioclase [Pl]) and textures suggesting late dikes emplacement in a relatively cool host rock. The other seven dikes, sampled between 292 and 469 mbsf, overall display granoblastic textures (named “granoblastic dikes” by the IODP Exp. 360 Scientific Party) and mostly consist of Pl and brown amphibole [Amph]. They also include minor Fe-Ti oxide phases and sulfides; minor proportions of Cpx and Opx locally occur in small discrete pockets. In this study, we investigate textures and mineral chemistry of the granoblastic dikes to comprehend their formation and to unravel the origin of their relatively high water contents, as documented by the amphibole modal amounts (~50 vol%). Cores of the granoblastic dikes typically display either intergranular textures with preserved prismatic Pl crystals, or fluidal textures defining a magmatic foliation. The dike rims are mainly characterized by granoblastic textures that often grade to foliated textures approaching the dike/host gabbro contact. Felsic material of dioritic to gabbronoritic composition occurs along most dike/gabbro boundaries. The felsic material generally shows sharp contacts with the host gabbro, whereas the dike/felsic material contact is irregular and characterized by a gradual decrease in crystals grain size and Pl/Amph modal ratio from the felsic material to the dike. The close association between felsic material and dikes suggests that the felsic material genesis is closely related to the granoblastic dikes. Brown Amph displays low Cl contents (<0.03 wt%) and lack of substantial chemical variability within single dikes. Overall, brown Amph has 57-67 Mg#, 7.4-11.4 wt% Al2O3, 1.9-3.1 wt% TiO2 and 1.6-2.8 wt% Na2O. Anorthite contents (An) in Pl are higher and K2O lower in the deepest dike 42-2b. Also, Pl chemistry vary between dike cores with higher An to dike rims showing relatively lower An (e.g., dike 42-2b: from An61 and 0.05 wt% K2O to An43 and 0.09 wt% K2O). Pl-Amph equilibrium temperatures range between 850 and 880 °C for the deepest analyzed dike and in the interval 750-810 °C for the shallower dikes. The temperature estimates, together with the observed granoblastic textures and mineral assemblage suggest that the granoblastic dikes equilibrated under granulite to upper amphibolite facies conditions. Compared to granoblastic dikes from fast-spreading environments, the U1473A granoblastic dikes contain remarkably higher modes of brown Amph documenting the involvement of water-rich fluids in their formation. The low Cl contents indicate that such fluids were not solely of seawater origin but included a magmatic component.

Granoblastic dikes in the lower oceanic crust exposed at Atlantis Bank (Southwest Indian Ridge)

This contribution presents the preliminary results derived from the mineralogical and petrographic study of hydrothermal alteration of deep seated mantle xenoliths. In ultramafic xenoliths, phyllosilicates are major products of hydrothermal alteration of the main mineral phases such as olivine, orthopyroxenes and clinopyroxenes. Xenolith rich outcrops were found within the Miocene tuff breccias from Valle Guffari. This locality is situated in the Hyblean Plateau (south-eastern corner of  Sicily, Italy), which consists of a thick meso-cenozoic carbonate sequence and of Neogene-Quaternary open shelf clastics with interbedded volcanic basic rocks. On the basis of petrographic and compositional features, collected xenolith population may be divided into: a) ultramafic, which consist of spinel facies peridotites and pyroxenites; b): feldspar bearing suite, mostly represented by metabasite rocks and anorthosites [Atzori et alii, 1999; Punturo et alii, 2000; Punturo, 2010]. Moreover, Manuella [2011] reports mineralogical investigations on some Hyblean harzburgite xenoliths, which show serpentinization and related secondary minerals (i.e. sulphides, aragonite, saponite). In this contest, the purpose of the present work is to describe the formation of phyllosilicates [Catalano et alii, 2014] obtained from mantle xenoliths from Miocene tuff breccias exposed, in the laboratory, to various hydrothermal conditions: temperature, 200–800 °C; pressure, 0.1–1.4 kbar; duration of treatment, 6–21 days. Xenoliths were characterized in detail before being used as starting material in hydrothermal synthesis. The starting material and run products were characterized by X-ray powder diffraction (XRPD), scanning electron microscopy with an energy-dispersive spectrometer (SEM/EDS) and differential scanning calorimetry (DSC) and thermogravimetry (TG).

Hydrothermal alteration of ultramafic xenoliths from Miocene tuff breccias from Valle Guffari (southern Sicily, Italy): results from laboratory tests

Shear zones localise most of deformation in narrow areas and provide efficient ways to mobilize significant volumes of fluids and melts up and down the crust. Deformation may localise if a specific domain focuses higher differential stress or strain rate with respect the host rocks. This concentration is commonly associated with variations in rock composition/texture, temperature/pressure conditions or presence of fluid/melts, which are even likely the dominant factor for weakening. In this contribution, we investigate exhumed shear zone developed in the middle crust with the aim to decipher which heterogeneities promoted the strain localisation in a fully ductile regime. We present microstructural and petrological data to characterise the Pressure-Temperature- Deformation (P-T-D) path across a major extensional shear zone of the Ivrea-Verbano Zone (IVZ, Western Alps) exposed along the Ossola Valley, the Anzola shear zone (ASZ). The ASZ developed at the transition between amphibolite and granulite facies conditions and consists of metabasites rock showing mylonitic fabric alternating layers made mainly of clinopyroxene, amphibole and plagioclase porphyroclasts and in recrystallized grains. The SZ boundaries consist in undeformed gabbroic rocks to one side and felsic and mafic granulite to the other side. A calcsilicate layer occurs at the contact among the gabbroic rocks and SZ. Our preliminary results suggest that deformation in the ASZ took place under granulite/amphibolite facies conditions at approximately between 650-800°C and confining pressures of around 8 kbar. Microstructural and textural data indicate that dissolution-precipitation creep is the dominant deformation mechanism. Evidence of synchronous deformation and mineral reactions of clinopyroxene suggests that metabasite become mechanically weak during the general transformation weakening process. This process was driven by melt flux deformation demonstrated by the occurrence of large amphibole porphyroclasts and interstitial ilmenite. Petrographic and textural observations suggest that the SZ was characterized by melt-rock interaction with the adjacent gabbroic and metamorphic wall rocks. In fact, our results show that the ASZ is extremely heterogeneous consisting by alternation of amphibolitic and calcsilicate layers, supported by the geochemical observations (e.g., occurrence of two different composition cpx: diopside and hedenbergite). The possible pre-existing heterogeneity (i.e., layering and grain-size variations) was probably the loci of concentrated shear deformation (Altenberger, 1997) and the magmatic fluids could be the source of clinopyroxene weakness and of the abundant amphibole. In this last case, geological interpretations already given (Brodie, 1981; Stünitz, 1998) require reappraisal.

Microstructural and petrological characterization of a major extensional shear zone in the middle crust (Val d’Ossola, Ivrea-Verbano Zone, Western Alps)

The Soana valley is located on the border between Piedmont and Aosta Valley regions; it offers countless opportunities for studying the lithological and structural features of the Alpine collisional chain. In particular, the Upper Soana valley displays, from west to east and from lower to higher structural levels, a natural, sectional profile across the Gran Paradiso Massif, the ophiolitic Piedmont Zone, the Santanel nappe and the Sesia-Lanzo Zone. These units represent transposed fragments of the paleo-European continental crust, the Ligurian-Piedmont ocean basin, and the paleo-African continental crust, respectively. The study area is located at Colle della Borra (2.578 m), within the Piedmont Zone, where new detailed observations on the geological context and on lithologies associated to massive sulphide mineralization, were made. This area pertains to the Colle della Rosa-Dondena Unit (CRD), which has previously been described as a greenschist-facies tectonic unit interposed between two eclogitic-facies ophiolitic units. The ophiolitic sequence exposed at Colle della Borra, consisting of dominant calcschists with subordinate metabasites layers, quartzites and marbles, has therefore been considered to be of Combin type. In this work, garnet-chloritoid-bearing leaden micaschists, equivalent to those described in previous works (Battiston, 1984) in the adjacent Rosa dei Banchi klippe (Monte Nero eclogitic unit) were found, suggesting the presence of a possible eclogitic-facies metamorphic imprint inside the CRD. The long lasting high-pressure metamorphic history of these micaschists is evidenced by the presence of chloritoid associated either with a main schistosity together with garnet, or with a primary schistosity marked by relict isoclinal folds. The lithological contact between the micaschists and the overlying calcschists is gradational, suggesting its stratigraphic nature. Furthermore, the finding in calcschists of numerous lawsonite relics completely replaced by epidote, white mica and quartz is clear evidence of the high-pressure metamorphic signature of the Colle della Borra sequence. This suggests that the entire CRD, in which the calcschist is the dominant lithotype, may have experienced, a metamorphic peak at a pressure at least exceeding the greenschist-facies. The certain presence of previously unreported high-pressure mineral assemblages in the metasediments of the Colle della Borra ophiolitic sequence calls for reconsidering the tectono-metamorphic evolution of the entire CRD and marks the starting point for further studies in this area.

New evidence of high-pressure metamorphism in an ophiolite sequence in the Upper Soana valley, Western Alps

In the geological setting of the Italian Western Alps, the Rocca Canavese Thrust Sheet Unit (RCT) is a small unit (only a few km2) marking the innermost sector of the Sesia-Lanzo Zone of the Austroalpine Domain, between the Eclogitic Micaschists Complex to the west and the Canavese Zone of the Southern Alpine Domain to the east. This unit has been described for the first time by Pognante (1989) as a series of thin thrust sheets made by pre-Alpine continental crust and upper mantle, separated by blueschists facies mylonites. Recently, the RCT has been referred to as a subducted-related mélange (Roda et al., 2018, 2019). This contribution presents new data on the structural and metamorphic evolution of the RCT. This unit is mainly made of ultramafic, mafic and metasedimentary rocks. Serpentinite crops out in the easternmost part of the unit in contact with orthoglaucophanite and metasediment, and embeds plurimetric bodies of metagabbro. Orthoglaucophanite consists of massive to gneissic rocks in contact with metasediment and serpentinite. Metasediments have significant compositional variations from pelitic/felsic (micaschist and gneiss) to mafic (Wm-rich glaucophanite). Moving to the boundary with the Canavese Zone, laterally discontinuous masses of mylonitic orthogneiss occur within serpentinite. Five tectono-metamorphic events have been recognized. The first event is recorded by a relict foliation preserved in microlithons or as introfolial folds, and whose mineralogical association (Wm+Lws+Gln in metasediment and orthoglaucophanite) is typical of Lws-blueschists facies conditions. The second event marks the eclogitic peak of the RCT because it is defined by the assemblage Jd/Omp+Grt (metasediment, orthoglaucophanite and metagabbro) and by millimetric Omp-veins (orthoglaucophanite). The third event is responsible for the development of a very pervasive transpositive foliation defined by Wm+Gln+Ep s.s. assemblages (metasediment and orthoglaucophanite), recording Ep-blueschists facies conditions. This event led to the formation of millimetric Omp+Chl+Ep s.s. veins in metagabbro. A general late retrogression under greenschists-facies conditions is recorded by the last two stages defined in this study: the former develops crenulation cleavage in serpentinite and open to close folds in the other lithologies; the latter is characterized by Pl+Chl millimetric to pluricentimetric veins in metasediment and orthoglaucophanite.

Structural and metamorphic evolution of the Rocca Canavese Thrust Sheet Unit (Sesia-Lanzo Zone, Western Alps, Italy)

In the ophiolitic sequences, are widespread low potassium albite-rich granite, trondhjemite, tonalite and granodiorite (e.g. Coleman & Peterman, 1975; Koepke et al., 2007). Concerning the Tethyan realm, plagiogranites have been recognized as dikes and beds in the ophiolithic rocks of the western Alps, in Corsica, and in the northern Apennines (Borsi et al., 1996; Castelli et al., 2002; Montanini et al., 2006). Also if the oceanic plagiogranites are volumetrically minor in the ophiolitic sequences, their origin has been largely debated due to importance of these rocks to understand the Earth’s continental lithosphere evolution of (Marien et al., 2019). Several authors (Beccaluva et al., 1999; Floyd et al., 2000; Niu et al., 2002; Rao et al., 2004; Berndt et al., 2005; Dylek & Thy, 2006) proposed that the plagiogranites are the products of fractional crystallization of mid-ocean ridge basalt (MORB) magma at low pressures conditions including, for instance, those exposed in the Oman, Cyprus, Serbia and Turkey ophiolitic sequences (Dylek and Furnes, 2011; Milovanovic et al., 2012; Kocsal et al., 2017; Marien et al., 2019). Others Authors (Koepke et al., 2004; 2007) have suggested an origin by partial melting (i.e. anatexis) of  cumulate gabbro (Koepke et al., 2004; Brophy, 2009; Brophy & Pu, 2012). A further model suggests that plagiogranites formed through the interaction of immiscible liquids with mafic solutions (Floyd et al., 2000; Kiliç, 2009). Geochemical studies (Koepke et al., 2007; Brophy 2009) stated that the distribution of some elements, including SiO2, TiO2, and REE, can be used in order to distinguish among these different processes. The aim of this research is to characterize for the first time, with a petrological and geochemical study, the plagiogranites from the ophiolitic rocks of the North Calabria Unit of Pollino Massif and to discuss their origin and geodynamic significance. We also compare the geochemical data of these plagiogranites with those of Western Alps (Castelli and Lombardo, 2007) and Northern Apennines (Borsi et al., 1996; Montanini et al., 2006) and with further Mediterranean plagiogranites such as Turkey (Koksal et al., 2017), Cyprus (Marien et al., 2019), Serbia (Milovanovic et al., 2012) and Oman (Dylek and Furnes, 2011). The results of this work will provide new contributions and constaints about the geodynamic evolution of the Tethyan realm.

Geochemistry and petrology of plagiogranites in the ophiolites from the Pollino Massif (Southern Italy): Origin and tectonic significance

The record of metamorphic conditions may be highly heterogeneous in spatially close rocks with different rheology and composition. The Cima di Gagnone area in the Central Alps represents such an example, where 1– to 100 s-meter scale ultrahigh–pressure and high–temperature ultramafic lenses are enveloped within amphibolite–facies metasediments. Structural investigation demonstrate that ultramafics and metapelites experienced a common Alpine deformation history, excluding their coupling within a tectonic mélange (Maino et al., 2021). In this contribution, we present new field observations, microstructural and petrological analyses, and thermodynamic modelling results focused on the metasediments, confirming that these rocks generally experienced medium pressure and medium temperature conditions of 1.0–1.2 GPa and 640–700 °C, followed by a retrograde stage around 0.6–0.8 GPa and 600–675 °C. However, a few samples from the immediate proximity of the ultramafic lenses record significantly higher P–T conditions of 1.3–1.7 GPa and 750–850 °C, approaching the high pressure and high temperature conditions of the ultramafic bodies (1.5–3.1 GPa, 650–850 °C). Mineral/bulk chemistry changes during growth of new mineral phases indicate to local melt/fluid interaction (i.e., metasomatism) between metasediments and ultramafics during the high temperature deformation. Preliminary U-Pb LA–ICP–MS dating suggests that zircon grains from the metasomatic reaction zone have been fully re–equilibrated during the early stage of Alpine exhumation (~36 Ma), while the large part of the metasediments records only pre–Alpine ages. Our results documented that, even though metasediments share the same structural evolution with UM, large differences in the local metamorphic conditions (Δ P up to 2 GPa; ΔT up to 160 °C) are recorded and are not randomly distributed but they are in relation to: 1) deformation (micro)structures, and 2) distance from the UM lenses. Moreover, fluid–assisted metasomatism is furthermore documented as being strongly localized at the interface between ultramafic lenses and the metapelitic host throughout all part of the metamorphic evolution, including the HP–HT stage. Therefore, in the Cima di Gagnone type–locality, the interplay between metapelites and ultramafic exerts a crucial first–order control to allow assemblage equilibrium during HT metamorphism and amphibolite–facies retrogression. Furthermore, the new findings exclude that the different metamorphic record may be attributed only to differential preservation during the retrograde path. We finally discuss these new data into the regional P–T–t–D paths and discuss the possible processes that controlled the exhumation of HP rocks.

Local variations of metamorphic record from compositionally heterogeneous rocks: Inferences on exhumation processes of HP rocks from the Adula-Cima Lunga unit

The Acatlan Complex (Puebla state) is the largest outcrop of metamorphic Paleozoic rocks in Mexico. This Complex is built by metaigneous and metasedimentary units, including polydeformed metagabbros, metabasites, metapelites, serpentinites and granitoids, which have registered, at least, three independent orogenic events that occurred between the Ordovician-Silurian and the early Permian. The most pervasive of these events is the Acatecan Orogen of Early Mississippian age that is characterized for having reached metamorphic peak conditions in the eclogite facies, at up to 22 kbar and 760 °C (Hernández-Uribe et al., 2018). The resulting lithologic and structural diversity has been interpreted as evidence for tectonic processes that involved a complete Wilson cycle, as well as accretionary processes and the subduction of Paleozoic global oceans (e.g. Nance et al. 2006; Ortega-Gutiérrez et al., 2018). For this reason, the Acatlán Complex represents a superb natural laboratory for the complete study of deep-seated inhumation processes and those involved in the eventual exhumation of the rocks that occur in orogenic convergent margins.  In this work, we present a preliminary study based on the metamorphic retrogression of eclogitic rocks from the Acatlán Complex. Field and detailed petrographic observations of these eclogites and associated rocks reveal that their retrogression is preserved in different kinds of microstructures, from plagioclase-epidote and amphibole-plagioclase symplectites, to chlorite-epidote coronae on garnet and titanite rims around rutile. These rocks also record retrograde compositional zoning in garnet, tourmaline, epidote, amphibole and phengite, and different generations of clinopyroxene, garnet, epidote, zoisite, amphibole, plagioclase, biotite, chlorite and calcite. Our observations suggest that the retrogression and subsequent exhumation of this eclogitic rocks took place in at least three stages; thus, our data represents the basis for the reconstruction of critical retrograde segments for the P-T trajectories followed by the Acatlán Complex high pressure lithologies.  This approach provides new elements that eventually could help to solve how HP-rocks generated under more than 75 km deep where exhumed and brought back to the surface. 

Exhumation of the HP eclogites from the Acatlán Complex (south Mexico): new insights from retrogressive microstructures

Metamorphic rocks often show evidence of reacting phases, whose chemical reactions have been “frozen” due to rapid rock exhumation rates. Symplectitic textures are an example of such microstructures that depict ongoing reactions events of thermodynamic instability. They generally occur as vermicular intimate intergrowths of two or more reacting minerals. When preserved in exhumed rocks, they can be analysed to extract considerable amounts of petrological information, since they make possible to identify both products and reactants of the metamorphic reaction in progress. In the present work we applied image analysis techniques to unravel the P-T conditions associated with the development of symplectitic microstructures, occurring as Pl-Hbl-Cpx coronitic intergrowths around relict eo-Variscan garnets in a late Variscan amphibolite sample from the Aspromonte Unit (NE Sicily). With the software Perple_X (Connoly, 1990), we calculated PT-pseudosections for the Eo-Variscan prograde evolution and for the subsequent retrograde stage in amphibolite facies conditions. This was opportunely performed using the XRF bulk rock chemistry and the progression of the Effective Bulk Chemistry (EBC – Zuluaga et al., 2005; Ortolano et al., 2014), respectively. In particular the EBC was calculated by means of the software Quantitative X-Ray Maps Analyser (Q-XRMA – Ortolano et al., 2014, 2018), analysing the calibrated X-ray maps of three different micro-domains from the same thin section sample. Such a procedure was functional to isolate the chemical contribution of the reactant phase volumes, that effectively took part in the symplectite-forming reaction. We managed to constrain a) an Eo-Variscan metamorphic peak condition of 12.0-12.6 Kbar and 745-780 °C and b) a retrograde P-T range of 4.8-5.4 Kbar and 540-560 °C, the latter associated with the development of the symplectitic coronae. These results quantitatively constrain for the first time previous petrographic evidences of relatively HP relics preserved within the Aspromonte Unit, depicting also the late-Variscan exhumation stage trajectory.   

X-ray quantitative image analyses of progressive symplectite formation during granulite/amphibolite facies retrogression

The Ivrea-Verbano Zone (IVZ) is a well-exposed middle-lower continental crustal section located in the southern Alps of northwest Italy. This crustal section consists mainly of middle-high grade metapelites and metabasites (Kinzigite Formation) with numerous marble lenses. In particular, the Kinzigite Formation is best exposed in the Valle Strona di Omegna, showing progressively higher T conditions, from amphibolite to granulite facies, with increasing crustal depth. Several previous studies focused on the P-T conditions of the metapelites and metabasites (e.g., Kunz and White 2019 and references therein). We present new petrological and geochronological data for metacarbonate rocks occurring at the different crustal levels. We focus on titanite (CaTiSiO5), which is a common and almost unique accessory mineral in the metacarbonates for the lack of other petro-chronological tools (e.g. zircon, monazite and rutile). Titanite may contain numerous important geochemical elements, such as REE, Pb, U, Zr and Nb that are useful for petrological and geochronological reconstruction (Kohn, 2017). Major and trace elements were determined for titanite from metacarbonates with different mineral assemblages along the crustal profile. The chemistry of titanite shows interesting correlations with lithologies, mineral assemblages and titanite modal abundance. Zr-in titanite thermometer (Hayden et al 2008) provided temperature estimates ranging from 700 to 900 °C, coherently with the high temperature conditions recorded by surrounding metapelites/metabasites at Carboniferous-Permian interval (Kunz and White, 2019; Redler et al., 2012). The titanite U concentration ranges from about 20 to 300 ppm; U–Pb dating was only performed for titanite grains from granulite facies metacarbonate, containing more than 100 ppm of U. Preliminary U–Pb dates range from Middle Triassic to Lower Jurassic, suggesting a decoupling between Zr-in titanite geothermometer and U–Pb isotopic results.

Titanite from metacarbonates: a (unique) petrological and geochronological tool. An example from the Valle Strona di Omegna (Ivrea-Verbano Zone, Western Alps, Italy)

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Granoblastic dikes in the lower oceanic crust exposed at Atlantis Bank (Southwest Indian Ridge)

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