United States

In the ophiolitic sequences, are widespread low potassium albite-rich granite, trondhjemite, tonalite and granodiorite (e.g. Coleman &amp; 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 &amp; 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 &amp; 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.

SGI 2021

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

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L'Istituto Nazionale di Oceanografia e Geofisica Sperimentale - OGS e il Dipartimento di Matematica e Geoscienze dell'Università di Trieste sono orgogliosi di ospitare il 90° Congresso della Società Geologica Italiana a Trieste dal 14 al 16 settembre 2021. La modalità del congresso è interamente online.

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The National Institute of Oceanography and Experimental Geophysics - OGS and the Department of Mathematics and Geosciences of the University of Trieste are proud to host the 90th Congress of the Italian Geological Society in Trieste on 14-16 September 2021.  The Congress will be held entirely online.

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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.

poster

In this study 80 detrital quartz grains coming from 4 sedimentary rocks samples of the Julian Basin (JB) have been analysed. Quartz can incorporate chemical impurities as defects in the crystal lattice, with substitution of Si4+ with H+ and Al+3 or B3+, by 4H+, and with LiOH. These substitutions form specific infrared absorption bands that can be investigated and analysed (Stalder & Konzett, 2012). Fourier Transform Infrared spectroscopy of quartz was used to investigate the sample set with interest to its OH defect speciation and content. OH defects are correlated to petrogenetic conditions, and they may be used as a provenance tool (Stalder, 2014). Polarized IR spectra can identify these different substitutions and allow to separate them from molecular water contained in fluid inclusions. Defect water contents were calculated for every different substitution (if present) by integration of the absorption area in the peaks’ regions. Julian Basin is a syn-orogenic basin in South-eastern Alps across Italian-Slovenian border, developed in the Late Cretaceous-Early Eocene. Quartz from the oldest sample (JB1; Maastrichtian) show a pattern suggesting a mixed source between igneous and non-igneous rocks, with a slight predominance of metamorphic material supply. Grains from sample JB17 (Thanetian) show the greatest variability in both defects and water content amount that suggests a different and more complex source. Quartz crystals from samples JB23 and JB26 (Ypresian) show a pattern of water-poor grains suggesting a mainly metamorphic rock type source. The peculiarity of JB17 is confirmed by several different previous studies based on both heavy minerals and geochemistry which demonstrated a strong difference in the supplies in the corresponding time interval (Lenaz et al., 2000; Lenaz & Princivalle, 2002; De Min et al., 2014; Velicogna, 2020).

Diverse detrital quartz supplies in SE Alps: an FTIR study

The different environmental conditions produced by the progressive restriction at the Mediterranean-Atlantic gateways during the early Messinian are abundantly investigated in the deeper water succession, while the shallow-water record is less explored. Throughout the Mediterranean, the pre-evaporite deposits in the deep-water record display changing lithologies, from hemipelagic marls and limestone alternation to sapropelitic marly limestone and euxinic clays. The shallow water successions are generally characterised by luxurious coral reefs developed in the Mediterranean area, with some, locally restricted, exceptions. During the early Messinian a first record of the progressive decay of trophic conditions in shallow water environment has been reported in the Majella structure (Central Apennines, Italy). Here the lower Messinian is represented by the Lithothamnion Limestone carbonate ramp that constitutes the uppermost carbonate unit of the Bolognano Formation. This ramp developed in the paleo Adriatic sea and it was characterised by a wide middle ramp environment in which coralline algae dominated carbonate production. This facies was associated with seagrass meadows colonizing the inner ramp. The outer ramp was characterized by bioturbated hemipelagic marl with planktonic foraminifera, pectinids and brachiopods in the aphotic zone. In this work we present facies and geochemical data providing evidence for an early, and repeated, onset of a restrictive environmental development, characterising and the Lithothamnion Limestone carbonate ramp. A stratigraphic section of ramp succession was measured in the eastern sector of Majella structure (Taranta Peligna), fourty bulk-rock samples were analysed for carbon isotopes then correlated with Mediterranean stable isotope record. Redox-sensitive elements (ICP-MS) was performed on twenty-five bulk rock samples. On these samples the Total Organic Carbon (TOC) was also measured using the Loss-On-Ignition (LOI) techniques. The foraminiferal content was analysed under a binocular microscope and classified for bio/chronostratigraphic attribution. The general negative carbon isotope trend recorded of the investigated section is interpreted as the record of the final stage of the Late Miocene Carbon Shift. Trends in the redox-sensitive elements are indicative of an evident decrease in bottom-water oxygen content and coinciding with the drowning of Lithothamnion Limestone carbonate ramp. The drowning coincides with the occurrence of Globorotalia nicolae (MMi 13b zone), when a conspicuous terrigenous input marked by a spike of Al and TOC content was recorded in the investigated succession.

Decoding seafloor and paleoclimate conditions from the paleo-Adriatic domain during early Messinian (Majella, Apennines, Italy)

The Cilento Group (Langhian-to-Tortonian) is a thick turbiditic succession (1200-2500 m) of the Southern Apennines foreland region that unconformably overlain the Lucanian oceanic terranes.  It has been divided into four formations from NW to SE: the Pollica, San Mauro, Torrente Bruca and Albidona formations.  Here, the post-depositional history of the upper portions of the Cilento Group, the San Mauro Formation (SMF), is discussed. The SMF consists of 1400-1600 m thick succession characterized by three main turbidite facies associations: outer-fan, middle-fan, and inner-fan. The outer-fan facies association characterizes the lower part SMF and is made by 300-500 m of thin-bedded sandstones and thick mudstones and marlstones. Numerous intrabasinal carbonate turbidite beds paired with a coarse-grained volcaniclastic layer are interbedded with the outer-fan deposits. The middle-fan facies association define the middle part of the SMF and is represented by a section of about 700 m thick sandstone-lobe turbidite beds and thin fan-fringe deposits. Two impressive intrabasinal carbonate megaturbidite beds, 65 m and 35 m in thickness, respectively, are interbedded with sandstone-lobe strata. The inner-fan facies association typify the upper part of the SMF and is characterized by 400-500 m thick succession of channelized coarse-grained sandstone and conglomerate turbidite beds. Sandstones are generally characterized by quartzolithic, volcanolithic and quartzofeldspathic composition, whereas hybrid arenites and calcarenites characterize the carbonate megabeds. Several diagenetic processes reducing porosity and permeability, such as compaction and cementation, occurred within the SMF. In particular, framework grains result deeply compacted, as testified by the common deformation of ductile grains (mainly micas) and detrital matrix. Cementation occurred through the precipitation of different minerals. The main cements are: 1) carbonate, such as calcite and less dolomite, playing an important occluding role along intergranular space, even running a quite replacement in quartz and feldspar grains ; 2) authigenic quartz occurring as little overgrowths or as minor mosaic quartz; 3) phyllosilicates, mainly developing in the upper part of SMF as pore filling chlorite cement or as small and incomplete illite coatings on skeletal grains; 4) Fe-oxides occuring as pathchy or pore-filling cements. The pore system consists of inter-intra-granular pores with a small pore radius and fractures. The relationship between the compactional porosity loss (COPL) and the cementational porosity loss (CEPL) testifies that compaction is the main reduction porosity process for SMF sandstones. Burial analysis suggests two phases of intense subsidence, interpreted as the result of syn-orogenic extension at shallow crustal levels intermitted by periods of low sedimentation rates, linked to gravitational instability of a vertically growing orogen.

Diagenetic destruction of porosity in Quartzofeldspathic sandstones, Miocene Cilento group, Southern Apennines foreland basin system

The Crotone Province (Calabria - South Italy) extends for a total surface of circa 1.736 km² in which a wide spectrum of geological, geomorphological and tectonic configurations alternate from the mountainous hinterland portions towards the coastal plain settings. Through the integrated collection of data coming from separated databases such as Geoportale della Calabria, Geoportale Nazionale, Videpi Project, ARPACAL Multirisk Functional Center, CARG project, CASMEZ, a complete geodatabase which make available a wealth of different spatial and cartographic data useful for the study and monitoring of Province of Crotone was implemented through open source technologies such as Qgis and PostgreSQL. The processed data consist in: 1) A new unified Geological map obtained through the comparison of the CASMEZ 1: 25000 and CARG the 1: 50000 maps and the reinterpretation of the stratigraphies with the adaptation of the old nomenclature with the new standards defined by the CARG project; 2) Geomorphological map obtained through the integration of  the available bibliography with new data coming from geoportale della Calabria; 3) Topographic and land cover maps obtained through the processing of both national and regional data; 4) Aspect, Slope and Curvature maps of the studied territory obtained using the Digital Elevation Models (DEM); 5) Rainfall and drainage density maps obtained through the re-elaboration of the ARPACAL databases; 6) Subsidence and land movements maps obtained though the processing of the ascending and descending orbit of RADARSAT 1 and 2 (Band C) satellites (2013-2018) using PSP-DIFSAR technique, and the ascending orbit of the TerraSarX satellite (X Band) and the descending orbit of the CosmoSkyMED satellite (X Band) (2013-2018)  using SBAS technique. Lastly, using the qgis2web plugin an interactive WebMap was created with the final aim to disclose, query and interactively view as much information as possible from a single database which also allows periodic updating without major investments and with very low maintenance costs.

Development of a unified geographic database from separated existing data through open-source softwares, Crotone Basin, Southern Italy

The Tertiary Piedmont Basin of NE Italy is an episutural basin which lies at the junction between the Alpine and Apennine chains. Its late Eocene to Pliocene sedimentary fill is thus important because it may provide a record of a source to sink system modulated by tectonics. This study focuses on sedimentology and petrography of the sedimentary succession cropping out in the Arquata Scrivia-Carrosio area, a key sector separating the Alto Monferrato area to the west from the Borbera-Curone sub-basin to the east. The litostratigraphic succession indicates initial establishment of a conglomeratic fan-delta (Conglomerati di Savignone and Molare Fms., Rupelian), which is drowned and onlapped by slope hemipelagic marlstone and channelized turbidites (Gremiasco Fm., Chattian). These are followed up section by a suite of confined turbidites, mass transport deposits, and channelized turbidites (Castagnola and Costa Montada Fms., Aquitanian-Early Burdigalian pro-parte), which record a further deepening culminating in the establishment of a basin plain environment (Costa Areasa Fm., Early Burdigalian-Langhian). Lastly, from the late Langhian onward, tectonic uplift resulted in a regional unconformity capped by shelfal marly deposits with storm layers and hyperpycnites (Marne di Cessole Fm., Langhian). Quantitative petrographic analysis of the above succession allowed us to recognise 11 petrofacies, which were compared and contrasted in a stratigraphic perspective. In agreement with previous studies, we identified lithic fragments from the Voltri Group, the Sestri-Voltaggio Zone, the Ligurian Appenine Units and the Mortara Volcanic complex, which can be therefore regarded as the main sediment sources. Results show that quartz and feldspars fractions increase up-section, whereas that of lithics decreases. In the lithic fraction, a decrease in the content of metamorphic lithics is observed in favour of volcanic and sedimentary lithotypes. The increase in quartz and feldspars content requires further investigation to be linked to candidate source terrains (Dora Maira? Cristallino di Valosio?). On the other hand, the increase of the volcanic fraction is partly coeval to the Mortara Volcanic complex (Upper Oligocene-Lower Miocene), now buried about 50 km to the north of the study area, and may thus correlate to an acme of volcanic activity and/or dismantling of its products. Lastly, the increase of the carbonate fraction can be interpreted to reflect the establishment of carbonate platforms of similar age documented at outcrop both to the N and S of the study area (P. da Cantoni Fm. of the Basso Monferrato and Visone Fm. of the Alto Monferrato, respectively) and in the subsurface (Quargnento High) to the N.

Sedimentological and petrographic evolution of the Oligo-Miocene succession in the Carrosio and Arquata Scrivia area (eastern Tertiary Piedmont Basin, NW Italy) 

This work illustrates a combined modelling approach using digital photogrammetry and geological modelling to create a high detailed 3D facies model of the inner environment of the Monte Carrubba carbonate ramp outcropping in the South Sicily. The Monte Carrubba Formation is the youngest marine Miocene carbonate deposit of the Hyblean region, prior to the Messinian crisis of the Mediterranean. In particular, Faro Santa Croce outcrop shows the most proximal sector of this ramp.The Faro Santa Croce outcrop offers the opportunity to investigate and reproduce a highly-detailed facies heterogeneity 3D model for a very narrow and limited area of few squared kilometers, developed in a tectonically stable area. Within this small areal, five facies have been recognized and modelled revealing a high level of facies heterogeneity. In this area marine ooidal shoals (ooidal grainstone to packstone) interfingered with shallow water seagrass environment (green-algal-floatstone facies and bioclastic grainstone-to-packstone) with abundant mollusc fauna. The distal part of this vegetated environment (fine mollusc-packstone facies) pass basinward into coral mounds (coral boundstone). The combination of digital photogrammetry and 3D geological modelling software has allowed to obtain a very high-resolution model of facies heterogeneity, evidencing the complexity of facies associations and, in particular, the development of a facies mosaic that can be underestimated by a classical 1D or 2D field analysis, especially in limitedly-exposed outcrops.

Facies modelling of proximal environment in the upper Tortonian-lower Messinian carbonate ramp of Hyblean domain (Faro Santa Croce, Sicily, Central Mediterranean)

During Permian, the Pangea supercontinent experienced the most critical and dramatic restocking of its configuration. Meanwhile, in the Mediterranean region, the occurrence of late Paleozoic siliciclastic strata experienced peculiar tectonic and climatic environment. Here, we detail the Permian sedimentary succession types displayed in some portions of the circum-Mediterranean region, providing new insights about the composition of Permian siliciclastic strata of the basal Apulian Unit, in Puglia 1 well log, and the Sosio Valley succession in Sicily. Permian sandstones, and related mudrocks, record a synchronously phase of post-Variscan collision and subsequent final closure of Paleotethyan ocean. Sedimentological and stratigraphic framework, also associated to the distribution of mineralogical composition of sandstones and mudrocks, are pivotal factors to reconstruct the sedimentary evolution of the Permian paleogeography. The Apulia Unit played a critical role because it stood relatively still in the Mediterranean paleogeographical contest and received a great Permian sedimentary load. Nowadays these strata were intersected in Puglia 1 well between 7070 m to 6110 m in depth, for 960 m in thickness, with sandstone, mudstone and breccia mostly. Meanwhile a Sicily Permian succession (the San Calogero Formation) is well preserved, outcropping around the Sosio Valley (northwestern Sicily), by few tens of meters of marine sandstones and clays strata. Sandstones of the Puglia 1 Well are quartzolithic, with abundance of quartz and  metasedimentary lithic fragments (mainly phyllite and schist). Feldspars are minor. Sandstones of the San Calogero Fm. in Sicily, are more variable in composition, from quartz-rich sandstone, to hybrid arenites and calclithite, and include abundant extrabasinal carbonate fragments and metasedimentary lithic fragments, as such as important contributions of intrabasinal carbonate grains. Interbedded volcanic and volcaniclastic layers are also present. These siliciclastic strata experienced a diverse burial history. The Puglia 1 succession presents moderate carbonate cements, that are dominant, and consist of crystals of dolomite, ankerite and calcite (related to dedolomitization/calcitization processes of the first carbonates. Mudrocks display K-enrichments, intense paleoweathering under a hot, episodically humid, climate with a prolonged dry season, and sediment recycling processes. Meanwhile Sicily succession is pervaded by carbonatic poikilotopic cements mostly, siliciclastic matrix and less iron cements. Quartz overgrowths are locally present. In terms of provenance, sandstone detrital modes are typical of quartzolithic sandstones, hybrid arenite and calclithite: that should be related to the erosion of upper crustal levels of the Variscan orogenic terranes , with most of metasedimentary and sedimentary rocks occurring in Calabria-Peloritani, southern Alps and in internal domains of the Circum-Mediterranean orogens.

Detrital modes of Permian sandstones in southern Italy

The geological record of deep seismic activity in subduction zones is generally limited due to common rock overprinting during exhumation and only a few regions allow studying well-preserved exhumed deep structures. The Northern Apennines (Italy) are one such area, granting access to continental units (Tuscan Metamorphic Units) that were subducted to high-pressure conditions, were affected by brittle-ductile deformation while accommodating deep tremor and slip and then exhumed back to surface, with only minor retrogression. Our approach is based on detailed fieldwork, microstructural and petrological investigations. Field observations reveal a metamorphosed broken formation composed of boudinaged metaconglomerate levels enveloped by metapelite displaying a pervasive mylonitic foliation. Dilational shear veins occur in both lithologies, but are more common and laterally continuous in the metapelite. They are mostly parallel to the foliation and composed of iso-oriented stretched quartz and Mg-carpholite (XMg>0.5) fibres, which are single-grains up to several centimetres long. These fibres define a stretching direction coherent with that observed in the metaconglomerate and metapelite, which is marked by K-white mica and quartz. Thermodynamic modeling constrains the formation of the high-pressure veins and the mylonitic foliation to ~ 1 GPa and 350°C, corresponding to c. 30-40 km depth in the subduction channel. Dilational shear veins developed in subducted (meta)sediments are a key indicator of episodic tremors and slip events (e.g. 1). We propose that these structures reflect the repeated alternation of localised brittle failure, with dilational shear veins development, and more diffuse viscous deformation. These cycles were probably related to the fluctuation of pore pressure that repeatedly reached lithostatic values. Concluding, these structures can be considered the geological record of episodic tremors and slip events occurring at >30 km of depth in the Apenninic subduction channel.

Brittle-ductile deformation in high-pressure continental units and deep episodic tremors and slip events

We present a detailed geochemical and petrographic study on samples from the Lar alkaline igneous complex, which is located in the Sistan belt (SE, Iran), a poorly studied area of the Alpine-Himalayan orogenic belt. The complex is composed of intrusive and hypabyssal rocks with shoshonitic to ultrapotassic affinity with variable silica saturation degree. The most alkaline rocks include lamprophyres, nepheline-syenites and felsic dykes of phonolitic composition, whereas silica-saturated and over-saturated rocks are represented by syenites/trachytic dykes and monzonites, respectively. Lamprophyres and nepheline-syenites show petrographic textures suggesting subsolidus unmixing processes between nepheline and K-feldspar recalling the occurrence of pseudoleucite and represent the most alkaline potassic rocks of the Oligocene magmatism in the Sistan-Lut collisional belt. Incompatible trace element patterns show geochemical features compatible with a subduction-related setting (i.e., LILE enrichments and HFSE depletions), with the alkaline potassic and silica-undersaturated rocks characterized by higher LREE/HREE fractionation (LaN/YbN from 12 to 22) with respect to the silica-saturated and -oversaturated rocks (LaN/YbN from 10 to 12). Preliminary investigation on the mineral chemistry highlights that lamprophyres (mg# 0.70-0.79) are in fact cumulates of olivine (Fo up to 75) and clinopyroxene (mg# up to 0.83), thus attesting the absence of near primary melts in the complex. Ne-syenites (mg# 0.50-0.60) show comparable clinopyroxene compositions (mg# up to 0.82) and equilibrium temperature around 950°C, in agreement with the results of phlogopite-melt geothermometer. The nepheline compositions in the pseudoleucite textures indicate stabilization temperatures under 700°C. The Sr-Nd-Pb isotopic compositions are rather homogeneous and suggest a possible common mantle source for the Lar igneous rocks and that the crustal contamination/assimilation processes affect only the most differentiated products. A geochemical and isotopic-based modelling suggests that the metasomatic agents, which modified the mantle beneath the study area, are mainly constituted by melts derived from carbonate-rich sediments, whereas melts from carbonate-poor sediments are subordinate. In the general framework of the Alpine-Himalayan orogen, magmas of the Lar igneous complex show interesting geochemical similarities with those of the post-leucititic phase of the Neapolitan district in the Roman Magmatic Province, suggesting analogies in the magma genesis among different sectors of the Tethyan realm.

The Lar alkaline igneous complex: genesis and geochemical features of a subduction-related magmatic event in the Sistan belt (SE Iran)

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.

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Diverse detrital quartz supplies in SE Alps: an FTIR study

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