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The coexistence of chloritoid and biotite in medium-pressure Barrovian terranes is quite uncommon, and the parameters controlling their equilibrium relations are still controversial. Various studies have already investigated the influence of pressure (P), temperature (T), bulk rock (X bulk) and fluid (X fluid) compositions on the stability of this assemblage. Here we apply forward thermodynamic modelling on amphibolite-facies metapelites from the upper portion of the Lesser Himalayan Sequence (eastern Nepal Himalaya) to test which parameters mostly influence the stability of the chloritoid + biotite assemblage. P–T isochemical phase diagrams calculated in the MnNKCFMASHTO system fail in reproducing the coexistence of chloritoid and biotite, predicting biotite appearance at higher temperatures than chloritoid breakdown. Neither the fluid composition (i.e. reduced H2O activity due to the presence of CO2) nor a more oxidated state of the system favours their coexistence, while slightly H2O-undersaturated conditions expand the biotite stability field toward lower temperatures, allowing the development of the chloritoid + biotite assemblage. Kinetic factors could have further contributed to the stability of this assemblage: thermal overstepping of the chloritoid-consuming and staurolite-producing reaction, induced by the difficulty in the staurolite nucleation and/or by the sluggishness of chloritoid dissolution, could have enhanced the metastable persistence of chloritoid at temperatures compatible with the presence of biotite. Being the kinetics efficiency intrinsically linked to the degree of fluid availability, the two factors (i.e. H2O-undersaturated conditions and kinetics of the chloritoid-consuming reaction) were likely complementary rather than mutually exclusive.

Post-collisional mountain-root collapse and subsequent massive partial melting occurred in the high-temperature (HT) ultrahigh-pressure (UHP) metamorphic terrane of the North Dabie complex zone (NDZ), central China. The NDZ was deeply subducted in the Triassic, producing widespread migmatites and various magmatic intrusions in the Cretaceous. Post-collisional metadiorites with distinctive large K-feldspar augen porphyroblasts, locally reported but rarely exposed in the NDZ, underwent a complex evolutional history. In this contribution, integrated studies including field investigation, petrographic observation and mineral analysis, zircon U-Pb geochronological and Hf isotopic analyses, and whole-rock elemental and Sr-Nd-Pb isotopic analyses of the metadiorites were carried out. Our results provide new constraints on the mountain-root collapse in the Dabie orogen. The metadiorites are enriched in large ion lithophile elements and light rare earth elements, whereas they are depleted in high field strength elements and heavy rare earth elements with significant Ba positive anomalies, a composition consistent with the lower continental crust. All the studied samples have moderately enriched initial 87Sr/86Sr ratios (0.707582–0.708099), low εNd(t) values (−15.3 to −20.4), and low initial Pb isotopic ratios (16.0978–16.8452, 15.3167–15.4544, and 37.1778–37.8397 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively). However, they have highly negative εHf(t) values and Paleoproterozoic two-stage Hf model ages, which are only partially consistent with data from the associated UHP metamorphic rocks. Such features suggest the metadiorites resulted from a magma produced by mixing of Triassic UHP mafic lithologies and minor amounts of mantle-derived materials. Zircon morphological analysis and U-Pb sensitive high-resolution ion microprobe dating combined with conventional thermobarometry indicate that these upwelling melts crystallized at pressure-temperature (P-T) conditions of 5.4–5.7 kbar and 750–768 °C at ca. 130 Ma and subsequently suffered HT metamorphism at ca. 125 Ma. We conclude that the metadiorites’ precursors were derived from partial melting of the Triassic subducted Neoproterozoic mafic lower-crustal rocks, with addition of minor amounts of mantle-derived materials in the Early Cretaceous, in response to mountain-root collapse of the orogen. Based on petrographic textures and mineral compositions, it is moreover inferred that formation of the distinctive K-feldspar porphyroblasts is likely related to a two-stage process, i.e., crystallization derived from biotite breakdown after the formation of the metadiorite at T = 640–703 °C and P < 4.5 kbar and coarsening related to shear deformation.

The Banchetta-Rognosa tectonic unit (BRU), covering an area of 10 km2 in the upper Chisone valley, consists of two successions referred to a continental margin (Monte Banchetta succession) and a proximal oceanic domain (Punta Rognosa succession) respectively. In both successions, Mesozoic meta-sedimentary covers discordantly lie on their basement. This paper presents new data on the lithostratigraphy and the metamorphic evolution of the continental basement of the Monte Banchetta succession. It comprises two meta-sedimentary sequences with minor meta-intrusive bodies preserving their original lithostratigraphic configuration, despite the intense Alpine deformation and metamorphic re-equilibration. Phase equilibrium modeling points to a metamorphic eclogitic peak (D1 event) of 20–23 kbar and 440–500 °C, consistent among three different samples, analyzed from suitable lithologies. The exhumation P–T path is characterized by a first decompression of at least 10 kbar, leading to the development of the main regional foliation (i.e. tectono-metamorphic event D2). The subsequent exhumation stage (D3 event) is marked by a further decompression of almost 7–8 kbar associated with a significant temperature decrease (cooling down to 350–400 °C), implying a geothermal gradient compatible with a continental collision regime. These data infer for this unit higher peak P–T conditions than previously estimated with conventional thermobarometry. The comparison of our results with the peak P–T conditions registered by other neighboring tectonic units allows to interpret the BRU as one of the westernmost eclogite-facies unit in the Alps.

In the Valmala sector of the southern Dora Maira Massif (Western Alps), two different eclogite- and blueschist-facies units (i.e. the Rocca Solei and Dronero units, respectively), are separated by a shear zone (i.e. the Valmala Tectonic Unit), which peculiarly consists of mixed slices of ophiolitic and continental rocks. A detailed geological map at 1:10,000 scale allowed to point out that the tectonic slices within the Valmala Tectonic Unit consist of 'native' rock slices wrenched from the overlying Dronero Unit, and 'exotic' rocks likely sourced from other units of the Dora Maira and from a continental margin and an oceanic basin. On the contrary, rock slices sourced from the underlying Rocca Solei Unit are lacking. The overall tectonic stack results after an early subduction-related deformation phase (i.e. the D1), and the pervasive overprinting of two subsequent exhumation-related deformation phases (i.e. the D2 and D3). The Valmala Tectonic Unit is inferred to have played a role in decoupling the southern Dora Maira Massif during subduction, and/or in driving exhumation of the ultra-high pressure rocks occurring in the adjoining Brossasco-Isasca Unit.

Calcite-bearing sediments (calcareous pelites, marls, impure limestones) are among the most investigated sources of carbon in collisional settings (e.g. Groppo et al., 2017, 2021, 2022; Rapa et al., 2017). Dolomite- and magnesite-bearing sediments, however, can also be important constituents of evaporitic sequences deposited along passive margins and involved in collisional orogenic processes. So far, decarbonation reactions in dolomite- and magnesite-bearing rocks have been rarely investigated, and their contribution to the orogenic carbon cycle substantially neglected. As a contribution to the understanding of the influence of dolomite- and magnesite-bearing lithologies on the global Earth's carbon cycle, a petrologic study was focused on the Lesser Himalayan Sequence (LHS) in central Nepal. The LHS is a thick Proterozoic sedimentary sequence originally deposited on the northern margin of the Indian plate, metamorphosed during the Himalayan orogeny. Abundant dolomite- and magnesite–bearing lithologies occur in the Upper-LHS, whose protoliths can be grouped in: (1) a dolomitic series (dolostones, dolomitic marls, dolomitic pelites), and (2) a magnesitic series (sparry magnesite ores, magnesitic pelites). The magnesite deposits associated to dolomitic lithologies are interpreted as the evidence of evaporitic environments during the Proterozoic.The schists derived from dolomitic pelites show mineral assemblages similar to those of normal metapelites, but with significant amounts of Ca-rich minerals (e.g. plagioclase) and with biotite anomalously enriched in Mg. The schists derived from magnesitic pelites are, instead, characterized by uncommon assemblages such as orthoamphibole + kyanite + garnet + phlogopite. Thermodynamic forward modelling (P/T-X(CO2) pseudosections) applied to these schists allowed to: (1) understand the nature of the main decarbonation reactions; (2) constrain the P-T conditions at which these reactions occurred, and (3) estimate the amounts of dolomite/magnesite consumed during prograde metamorphism, and the correspondent amounts of released CO2. The main results are:the observed assemblages formed during a heating decompression stage, at P-T conditions of 620 ± 20°C, 8.5 ± 0.2 kbar, consistent with those registered by the associated metapelites; the observed peak assemblages are predicted to be stable in equilibrium with a CO2-bearing fluid, even in those samples where carbonates are no more preserved; the overall results point to an internally buffered P/T-X(CO2) evolution. The amount of carbonates consumed during prograde metamorphism varies in the range 7-20 vol%, corresponding to 3-10 wt% of CO2 These CO2 amounts are nearly double the CO2 released by calcareous pelites (Groppo et al., 2021). The main consequence of this study is that the CO2 productivity of dolomitic and magnesitic pelites is significant and that these lithologies could be relevant sources of CO2, possibly contributing to the diffuse Himalayan CO2 degassing (e.g. Girault et al., 2014, 2018). ReferencesGirault et al. (2014). Geoph. Res. Lett. 41, 6358–6366Girault et al. (2018). Nat. Comm. 9, 2956Groppo et al. (2017). J. Petrol. 58, 53-83.Groppo et al. (2021). J. metam. Geol. 39, 181-207.Groppo et al. (2022). Comm. Earth Environ, doi: 10.1038/s43247-022-00340-wRapa et al. (2017). Lithos, 292–293, 364–378.

The Tibetan Plateau, largely derived from the accretion of several Gondwana microplates to the southern margin of Asia since the late Palaeozoic, is the highest and largest topographic relief on Earth. Although the first order geodynamic processes responsible for its pre-Cenozoic evolution are quite well-known, many issues are still debated, among which is the timing of collision of each terrane with the southern margin of Asia. Even more uncertain is the pre-Palaeozoic history of these terranes, due to the lack of basement exposures. As a contribution to understanding the pre-Cenozoic evolution of the Tibetan Plateau, this paper focuses on the Aghil Range, a remote and poorly investigated area close to the Karakorum Fault between Kunlun and Karakorum (Xinjiang, China) in western Tibet. The tectono-metamorphic and magmatic evolution of the Aghil Range is investigated using a multidisciplinary approach that combines field mapping, petrology and geochronology (U Pb on titanite, zircon, monazite and xenotime using SHRIMP-RG). We demonstrate that the Aghil Range preserves a coherent slice of Neoproterozoic crystalline basement with a late Palaeozoic sedimentary cover deposited on a passive continental margin during the Gondwana break-up. This represents the westernmost exposure of Precambrian crystalline basement known so far in the Tibetan Plateau. Furthermore, petrological and geochronological results allow reconstructing the Mesozoic poly-metamorphic evolution of this sector of the Tibetan Plateau, which records the evidence of Middle Jurassic (ca. 170 Ma) and Late Cretaceous (66 Ma) collisional events, as well as of the Late Jurassic (ca. 150 Ma) early subduction of an accretionary complex developed on its southern margin. Evidence of Late Cretaceous subduction-related magmatism preceding the last collisional event is also recorded. These results allow tentative correlation of the different terranes of Central Tibet with those of the Pamir-Karakorum Range on both sides of the Karakorum fault.

Peculiar coesite + chloritoid + garnet ± glaucophane talcschists have been used to make at least four quern-stones, unearthed in the ruins of a villa rustica belonging to the Roman imperial period and located at Costigliole Saluzzo, Western Alps. The site of the villa rustica and the presence of coesite relics could suggest a possible provenance of these rocks from one of the already known ultra-high pressure ( UHP ) units of the Western Alps, i.e ., either the Brossasco-Isasca Unit (BIU) of the southern Dora-Maira Massif or the Lago di Cignana Unit (LCU) of the Piemonte Zermatt-Saas Zone. However, similar talcschists have never been reported from these units. Two samples of coesite-bearing, chloritoid + garnet ± glaucophane talcschist collected from two different specimens of quern-stones have been petrologically investigated with the aim of defining their peak pressure–temperature ( P-T ) conditions. The stability field of the coesite + garnet + talc + chloritoid + glaucophane assemblage has been constrained using isochemical phase diagrams modelled in the MnNCFMASHO system; prograde P-T conditions have been additionally constrained using the yttrium-in-garnet (YAG) geothermometer. Thermodynamic modelling tightly constrains peak P-T conditions at 480–510°C, 27–31 kbar. The unusual Mg-rich composition of the talcschists suggests that they originated by Mg-metasomatism of either a continental crustal protolith (granodiorite or Fe-rich metapelite) or an oceanic crustal protolith (altered oceanic crust). A mechanism similar to that proposed for the well-known pyrope-bearing whiteschists of the UHP BIU, i.e . influx of antigorite-derived fluids along shear zones during subduction, can be envisaged. Although the field occurrence of these coesite + chloritoid + garnet ± glaucophane talcschists is still unknown, the obtained results clearly show that these rocks cannot belong to the UHP BIU, whose peak P-T conditions are at significantly higher T and P (730°C, 40–43 kbar). Therefore, this finding opens the challenging hypothesis of the existence of a further, still unmapped, UHP Unit in the Western Alps that also experienced UHP metamorphism and fluid influx from underlying serpentinites during subduction. A detailed mapping and petrologic investigation of the tectono-metamorphic continental ( i.e ., southern Dora-Maira Massif) and oceanic ( i.e ., Monviso meta-ophiolite Complex) units in the proximities of Costigliole Saluzzo is required in order to further constrain the location and the dimension of this new UHP unit.

This contribution presents a geological mapping of the Quaternary succession of the Bule and low Alpetto tributary valleys (Po Valley, Piedmont), together with preliminary data on its petrography. The investigated area, covering an altitude range between 3015 and 1485 m a.s.l., is located in the Monviso Massif Ophiolite Complex, at the contact with marble and calcschist of the Dora-Maira Massif cover. The Quaternary sequence mainly consists of glacial, outwash, debris and landslide deposits, with local torrential/avalanche and colluvial bodies. The investigation on the petrographic composition of these sediments has allowed to recognize the occasional occurrence of jadeitite boulders, likely exploited during the Neolithic period for axe head production. Four possible jadeitite blocks have been found; one of them is described here for the first time, sizing up to 1 m3. In detail, one of the finds is a Neolithic site of jade polishing in the high Bule Valley that was referred to the middle of V millennium BC (Allisio, La Preistoria e l’alta Valle Po, Lar Editore, Perosa Argentina, 2012). The jadeitite blocks have been found in the debris (site A), landslide (site B), glacial (site C) and outwash (site d) sediments. The petrographic characterization of the new jadeitite samples might potentially shed light on the still controversial genesis of these peculiar rocks.

In this preliminary note, we describe our strategy for collecting, storing and representing new and old geological data acquired in several sectors of the central-eastern Himalaya where, since the 70's, researchers of the CNR- IGG Torino and the Department of Earth Sciences of the University of Torino addressed a significant part of their activity. All the geologic features (including rock samples, petrographic, petrologic and structural observations and measurements) are stored and harmonized in a suitable geodatabase using ArcGIS. Standard criteria are used to describe their typology and quality, facilitating future homogeneous data implementation and a correct data traceability. This geodatabase represents the fundamental starting point for the editing of a digital geological map of the central-eastern Nepal Himalaya.

Collection and sharing of geoheritage information are nowadays enhanced by using digital tools and IT (Information Technology) applications that allow homogeneous storing of data, building of sharable databases, and transfer of knowledge between experts and communities. In the frame of the PROGEO-Piemonte project (PROactive management of GEOlogical heritage in the PIEMONTE region), the geological and geomorphological trail in the Monviso Massif (Western Alps) is an example of how IT-applications may support management of geoheritage information. The trail particularly allows walking across the meta-ophiolite succession of an ancient ocean and on glacial landforms. Its realization was supported by different tools that allow (i) capturing of data via GPS handheld mobile device, (ii) managing of field data by means of GIS (Geographic Information Systems) applications, and (iii) virtual visualization of data by means of GIS-based 3D viewer. The different (virtual) views of the geology along the trail can be implemented by adding photos and bookmarks and easily shared through web mapping tools that ensure wide accessibility of geoheritage information.

International audience; The interplay between the processes controlling the mobility of H2O and C-bearing species during subduction zone metamorphism exerts a critical control on plate tectonics and global volatile recycling. Here we present the first study on fresh, carbonate-bearing, lawsonite eclogite-facies metabasalts from Alpine Corsica, France, which reached the critical depths at which important devolatilization reactions occur in subducting slabs. The studied samples indicate that the evolution of oceanic crustal sequences subducted under present-day thermal regimes is dominated by localized fluid-rock interactions that are strongly controlled by the nature and extent of inherited (sub)seafloor hydrothermal processes, and by the possibility of deep fluids to be channelized along inherited or newly-formed discontinuities. Fluid channelization along inherited discontinuities controlled local rehydration and dehydration/decarbonation reactions and the stability of carbonate and silicate minerals at the blueschist-eclogite transition. Fluid-mediated decarbonation was driven by upward, up-temperature fluid flow in the inverted geothermal gradient of a subducting oceanic slab, a process that has not been documented in natural samples to date. We estimate that the observed fluid-rock reactions released 20–60 kg CO2 per m3 of rock (i.e. ~0.7–2.1 wt% CO2), which is in line with the values predicted from decarbonation of metabasalts in open systems at these depths. Conversely, the estimated time-integrated fluid fluxes (20–50 t/m2) indicate that the amount of carbon transported by channelized fluid flow within the volcanic part of subducting oceanic plates is potentially much higher than previous numerical estimates, testifying to the percolation of C-bearing fluids resulting from devolatilization/dissolution processes operative in large reservoirs.

A detailed structural, lithological and petrological study of different transects in the Rasuwa district of central Nepal Himalaya allows the characterization of the tectonostratigraphic architecture of the area. It also facilitates constraining the P-T evolution of the different units within the Lesser (LHS) and Greater (GHS) Himalayan Sequences. Peak P–T conditions obtained for the studied metapelite samples using the pseudosection approach and the Average PT method highlight the existence of four different T/P ratio populations in different tectonometamorphic units: 80 ± 11 °C/kbar (LHS), 66 ± 7 °C/kbar (RTS), 73 ± 1 °C/kbar (Lower-GHS) and 101 ± 12 °C/kbar (Upper-GHS). Integration of structural and petrological data emphasizes the existence of three tectonometamorphic discontinuities bounding these units, characterized by top-to-the-south sense of shear: the Ramgarh Thrust, which separates the LHS (peak metamorphism at ∼600 °C, 7.5 kbar) from the overlying RTS (peak metamorphism at ∼635 °C, 10 kbar); the Main Central Thrust, which separates the RTS from the Lower-GHS (peak at 700–740 °C, 9.5–10.5 kbar with a prograde increase in both P and T in the kyanite stability field), and the Langtang Thrust, which juxtaposes the Upper-GHS (peak at 780–800 °C, 7.5–8.0 kbar with a nearly isobaric heating in the sillimanite stability field) onto the Lower-GHS. An increase in the intensity of deformation, with development of pervasive mylonitic fabrics and/or shear zones, is generally observed approaching the discontinuities from either side. Overall, data and results presented in this paper demonstrate that petrological and structural analysis combined together, are reliable methods adequate to identify tectonometamorphic discontinuities in both the LHS and GHS. Geochronological data from the literature allow the interpretation of these discontinuities as in-sequence thrusts.

The determination of the oxidation state of transition metals at high spatial resolution is a crucial issue for many fields of science, including solid state physics, earth sciences, biology, bio-chemistry and catalysis. Among the other available analytical methods, micro-XANES allows to probe in situ the oxidation state with high lateral resolution, enabling an unprecedented level of description in heterogeneous samples. In geological samples the determination of the Fe 3+ /ΣFe ratio is of particular interest since it can be used as an indicator of the oxygen fugacity ( f O 2 ) at which a mineral formed. With this respect, we performed a micro-XANES experiment aiming to investigate the Fe-redox state variation across single-crystals of both garnet and omphacite exploiting the X-ray microprobe available at the ESRF ID22 beamline to reach a spot size of 1.7 μm × 5.3 μm. For garnet, the absolute Fe 3+ content was determined in a space-resolved way. In the case of omphacite, the analysis of the XANES data is not straightforward owing to the presence of a significant dichroism effect and to the random orientation of the different grains in the mineral assemblage. The investigated samples are highly complex materials which represent a challenge for the micro-XANES technique. These zoned micro-crystals are therefore ideal systems to develop analytical procedures which can be subsequently generalized to other relevant fields of science such as the Fe speciation in a single cell or a single grain for life science and catalysis applications, respectively.

In Piemonte, stone has always been the most widely used raw material for buildings, characterizing the architectural identity of the city of Turin. All kinds of rocks, metamorphic, igneous, and sedimentary, are represented, including gneisses, marbles, granitoids, and, less commonly, limestones. The great variety of ornamental stones is clearly due to the highly composite geological nature of the Piemonte region related to the presence of the orogenic Alpine chain and the sedimentary Tertiary Piemonte Basin. This paper provides a representative list of the most historic ornamental stones of Piemonte, which have been used over the centuries in buildings and architecture. The main stones occurring in Turin have been identified and described from a petrographic and mineralogical point of view in order to find out the corresponding geological units and quarry sites, from which they were exploited. This allows the associated cultural and scientific interest of stones to be emphasized in the architecture of a town which lies between a mountain chain and a hilly region.

A variety of deep-seated xenoliths occur within the Mesozoic Jiagou dioritic porphyry in the southeastern margin of the North China Craton (NCC). In this study we present a combined petrologic, geochronological, Hf isotope and geochemical study on the different types of xenoliths and use these data to better constrain the composition and age of the deep crust beneath the area. Most of the xenoliths are mafic meta-igneous rocks, among which garnet-bearing lithologies are common. The xenoliths can be classified into three broad petrographic groups: spinel-bearing garnet clinopyroxenite/phlogopite clinopyroxenite/spinel pyroxenite (Group 1), garnet amphibolite or hornblendite/garnet granulite/mafic gneiss lacking pyroxene (Group 2), and garnet-bearing felsic (intermediate-acid) gneiss (Group 3). Among these, the mafic–ultramafic rocks constitute the dominant category. The protoliths of the studied xenoliths range from basalt through andesite to dacite. Geochemical and Hf-isotope data indicate that most xenoliths belonging to Groups 2 and 3 resemble magmatic rocks formed at convergent continental margin arc setting. A few of them (mostly belonging to Group 1) represent mantle-derived products. Multiple metasomatic imprints, with contribution from subduction-related or mantle-derived fluids or melts have been recognized from the multistage mineral assemblages and ages. SHRIMP zircon U–Pb dating, Hf isotope and geochemical data offer evidence for subduction-related adakite-like and arc-related rocks in the southeastern margin of the NCC at ca. 2.5 Ga and 2.1 Ga, and confirm the occurrence of high-pressure granulite-facies metamorphism at ca. 1.8 Ga. These data suggest an episodic growth of Precambrian lower crust beneath this region in response to two stages of subduction–accretion and one vertical accretion of mantle-derived basaltic magma at the base of the lower crust. Additionally, a previously unknown late mantle-derived basaltic magmatism at 393 ± 7 Ma has also been recognized. The data presented in this paper demonstrate that the deep crust beneath the southeastern margin of the NCC is composed of hybrid protoliths derived from Paleozoic, Paleoproterozoic and late Neoarchean sources.

Impure calcite marbles from the Precambrian metamorphic basement of the Wuhe Complex, southeastern margin of the North China Craton, provide an exceptional opportunity to understand the depositional processes during the Late Archean and the subsequent Palaeoproterozoic metamorphic evolution of one of the oldest cratons in the world. The studied marbles are characterized by the assemblage calcite + clinopyroxene + plagioclase + K-feldspar + quartz + rutile ± biotite ± white mica. Based on petrography and geochemistry, the marbles can be broadly divided into two main types. The first type (type 1) is rich in REE with a negative Eu anomaly, whereas the second type (type 2) is relatively poor in REE with a positive Eu anomaly. Notably, all marbles exhibit remarkably uniform REE patterns with moderate LREE/HREE fractionation, suggesting a close genetic relationship. Cathodoluminescence imaging, trace elements and mineral inclusions reveal that most zircons from two dated samples display distinct core-rim structures. Zircon cores show typical igneous features with oscillatory growth zoning and high Th/U ratios (mostly in the range 0.3-0.7) and give ages of 2.53 - 2.48 Ga, thus dating the maximum age of deposition of the protolith. Zircon rims overgrew during granulite-facies metamorphism, as evidenced by calcite + clinopyroxene + rutile + plagioclase + quartz inclusions, by Ti-in-zircon temperatures in the range 660-743 °C and by the low Th/U (mostly < 0.1) and Lu/Hf (< 0.001) ratios. Zircon rims from two dated samples yield ages of 1839 ± 7 Ma and 1848 ± 23 Ma, respectively, suggesting a Palaeoproterozoic age for the granulite-facies metamorphic event. These ages are consistent with those found in other Precambrian basement rocks and lower-crustal xenoliths in the region, and are critical for the understanding of the tectonic history of the Wuhe Complex. Positive Eu anomalies and high Sr and Ba contents in type 2 marbles are ascribed to syn-depositional felsic hydrothermal activity which occurred at 2.53 - 2.48 Ga. Our results, together with other published data and the inferred tectonic setting, suggest that the marbles' protolith is an impure limestone, rich in detrital silicates of igneous origin, deposited in a back-arc basin within an active continental margin during the late Archean and affected by synchronous high-T hydrothermalism at the southeastern margin of the North China Craton.

The Mortisa landslide is located in the urban area of Cortina d'Ampezzo, north-east Italy. The landslide unit consists of three mudslides; the affected area is 3500 m long, stretching from 1750 to 1300 m a.s.l. The landslide has been investigated since 1998 as its track crosses a national route and some buildings have been severely damaged by the movements.Through radiocarbon dating it was possible to reconstruct the dynamic of the events that led to the formation of the present-day Mortisa slope. The landslide body is composed by interdigitated layers of gravel in a silty clay matrix originated from subsequent earth and debris flows events since the Lateglacial.To assess the stability conditions of the Mortisa landslide via numerical modelling it is crucial to consider the mechanical influence of the gravel lenses in the dynamic of the whole landslide body. However, their position is almost unknown inside the clay matrix with just some certain positions, where stratigraphic data are available. Therefore, in order to calibrate the model on the monitoring data through back analysis, a new procedure has been implemented. A new code permits to generate stochastically several soil configurations which are automatically elaborated by the commercial software FLAC. The results are organized and filtered based on the monitoring data, hence it is possible to select the model that fits the data best. In this way it it possible to obtain a sound back-analysis model of the Mortisa landslide, to be used in order to design countermeasure works and mitigate the risk associated with the phenomenon.

Cordierite-bearing anatectic rocks inform our understanding of low-pressure anatectic processes in the continental crust. This article focuses on cordierite-bearing lithologies occurring at the upper structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite-bearing gneisses from different geological transects (from Mt Everest to Kangchenjunga) have been studied, in which cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of the microstructures related to melt production and/or melt consumption allows the distinction to be made between peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (from two-mica metapelite/metagreywacke to biotite-metapelite) has been investigated at low-pressure conditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineral assemblages and microstructures. The results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low-pressure conditions, the two-mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval. Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along P-T paths with a peculiar dP/dT slope of about 15–18 bar °C−1. Overall, our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low-P regional anatectic terrane. The cordierite-bearing migmatites may well represent the source rocks for the Miocene andalusite-bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low-P isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism.

In the southern Dora-Maira Massif, Western Alps, slivers of continental crust with similar lithologies, but recrystallized during the Alpine orogeny at different peak-P conditions, are exposed. They include the Brossasco-Isasca Unit (BIU) where coesite was first discovered in continental crust. A new 1:20,000-scale geologic map and related cross-sections of the whole BIU and adjoining units is presented, in which the most significant features useful to infer the pre-Alpine history and the Alpine tectonic and metamorphic evolution, are summarized. Thanks to detailed petrography and petrology, the geologic map shows the precise location of ultra-high pressure (UHP) minerals (such as coesite), and the locations of the most significant mineral assemblages (such as kyanite + jadeite). This innovative approach is used to distinguish the BIU from the adjacent units. Relict pre-Alpine structures (such as igneous intrusive contacts with basement xenoliths and metagranitoids) are summarized in a sketch illustrating...

The metamorphic evolution of a granulitized eclogite from Punta de li Tulchi, NE Sardinia, Italy, reconstructed utilizing a combined microstructural (symplectitic, coronitic and kelyphytic features) and thermodynamic approach, involved a complex metamorphic history with equilibrium attained only at a domainal scale. Microstructural analysis and mineral zoning allow recognition of reactants and products involved in successive balanced mineral reactions. The P–T conditions at which each microstructure was formed are constrained by calculating isochemical phase diagrams (pseudosections) for the composition of effectively reacting domains. A pre-symplectite stage developed during prograde metamorphism under conditions ranging from 660–680 °C, 1.6–1.8 GPa to 660–700 °C at 1.7–2.1 GPa. Pseudosections calculated for subsequent clinopyroxene + plagioclase and orthopyroxene + plagioclase symplectitic coronae using the composition of effectively reacting microdomains suggest temperature in excess of 800 °C and pressures of 1.0–1.3 GPa. Modelling the development of later plagioclase + amphibole coronae around garnet during decompression yields conditions of 730–830 °C and 0.8–1.1 GPa. H2O (wt%) isomodes indicate that the granulitized eclogites were H2O-undersaturated at peak-P conditions and during most of the subsequent heating and decompression. This allowed the preservation of prograde garnet zoning in spite of the strong granulite facies overprint. The P–T evolution of Punta de li Tulchi granulitized eclogite is very similar in shape to that registered by other NE Sardinia retrogressed eclogites thus suggesting a common tectonic scenario for their evolution.

Partial melting of deep continental crust may occur during either prograde heating or decompression. Although the effect of temperature on crustal melting has been widely investigated, few experimental studies have addressed the question of the influence of pressure on crustal anatexis.Tounderstand the influenceofdecreasingpressure onpartialmelting processes, the thermodynamic approach of isochemical phase diagrams has been applied to garnet^K-feldspar^kyanite^sillimanite anatectic gneisses (Barun Gneiss) from the Higher Himalayan Crystallines (HHC)ofeasternNepal.Themainmelt-producing reactions, the amount of melt produced during heating vs decompression, and the effects ofmelt loss on themineral assemblagesand compositions have been investigated along four idealP^T trajectories, dominated by either heating or decompression. Based on these results, the observed microstructures and mineral compositions of the Barun Gneiss have beeninterpreted intermsofmelt-producingvsmelt-consumingreactions (e.g. growth ofperitecticgarnetwith preserved ‘nanogranite’inclusions vs microstructures related to back-reactions between solids and melt), and used to derive the metamorphic evolution of the studied samples. The P^T pseudosection modelling predicts that at least 15^20 vol.% ofmeltwasproducedat peakP^Tconditions through dehydrationmelting of both muscovite and biotite, and that melt production was mainly triggered by heating, with or without the combined effect ofdecompression.The preserved granulitic peak metamorphic assemblage, however, is consistent with a significant loss ofmost of this melt.TheP^Tevolution inferred forsamplesfrom different, strategically located, structural levelsoftheBarunGneiss isconsistentwiththeexpectationsofa‘channel flow’model, including: (1) the clockwise shape of the P^T paths; (2) the estimatedPat peakT(newdata:10^8 kbarat 8008C;model:13^7 kbar at 8008C); (3) the decreasing P structurally upward, which defines a ‘normal’ metamorphic sequence, in contrast to the inverted metamorphic sequence occurring in the lowermost Main Central Thrust Zone; (4) the nearly isothermal exhumation of the structurally lowest sample, reflecting the progressive exhumation of rocks that have been entrained in the deep, high-T region of the channel, versus the nearly isobaric heating of the structurally uppermost sample, reflecting the evolution of those rocks that flowed outwards with the underlying channel.

The occurrence of crystallized and glassy melt inclusions (MI) in high-grade, partially melted metapelites and metagraywackes has opened up new possibilities to investigate anatectic processes. The present study focuses on three case studies: khondalites from the Kerala Khondalite Belt (India), the Ronda migmatites (Spain), and the Barun Gneiss (Nepal Himalaya). The results of a detailed microstructural investigation are reported, along with some new microchemical data on the bulk composition of MI. These inclusions were trapped within peritectic garnet and ilmenite during crystal growth and are therefore primary inclusions. They are generally isometric and very small in size, mostly ≤15 μm, and only rarely reaching 30 μm; they occur in clusters. In most cases inclusions are crystallized (‘nanogranites’) and contain a granitic phase assemblage with quartz, feldspar and one or two mica depending on the particular case study, commonly with accessory phases (mainly zircon, apatite, rutile). In many cases the polycrystalline aggregates that make up the nanogranites show igneous microstructures, e.g. granophyric intergrowths, micrographic quartz in K-feldspar and cuneiform rods of quartz in plagioclase. Further evidence for the former presence of melt within the investigated inclusions consists of melt pseudomorphs, similar to those recognized at larger scale in the host migmatites. Moreover, partially crystallized inclusions are locally abundant and together with very small (≤8 μm) glassy inclusions may occur in the same clusters. Both crystallized and partially crystallized inclusions often display a diffuse nanoporosity, which may contain fluids, depending on the case study. After entrapment, inclusions underwent limited microstructural modifications, such as shape maturation, local necking down processes, and decrepitation (mainly in the Barun Gneiss), which did not influence their bulk composition. Re-homogenized nanogranites and glassy inclusions show a leucogranitic and peraluminous composition, consistent with the results of partial melting experiments on metapelites and metagraywackes. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes.

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In Alpine Corsica (France), deeply subducted metabasalts are well preserved as lawsonite-bearing eclogite (Law-Ecl), occurrence of which is restricted to similar to 10 localities worldwide. The Corsican Law-Ecl, consisting of omphacite + lawsonite + garnet + phengite + titanite, occurs as both single undeformed metabasaltic pillows surrounded by lawsonite blueschist (Law-Bs), and carbonate-bearing eclogitic veins. Law-Bs are found as variably deformed metabasaltic pillows locally cross-cut by eclogitic veins and consist of glaucophane + actinolite + lawsonite + garnet + phengite + titanite. Field evidence and microstructures reveal that both Law-Ecl and Law-Bs are stable at the metamorphic peak in the lawsonite-eclogite stability field. Isochemical phase diagrams (pseudosections) calculated for representative Law-Ecl and Law-Bs samples indicate that both lithologies equilibrated at the same conditions of similar to 520 +/- 20 degrees C and 2.3 +/- 0.1 GPa. Therefore, the coexistence at the same peak metamorphic conditions of Law-Ecl and Law-Bs implies that different portions of deeply subducted oceanic crust may store significantly different H2O contents, depending on bulk-rock chemical composition. In addition, thermodynamic modelling of phase equilibria indicates that the occurring progressive dehydration reactions, which are significantly depending on bulk-rock chemical composition, strongly influence rock densification and eclogite formation in subducting slabs.

A petrological study of the Punta Orvili metabasite in NE Sardinia has been carried out, integrating quantitative pseudosection modelling with reaction balancing, with the aim of reconstructing the metamorphic evolution and P–T path. The Punta Orvili metabasite preserves microstructural evidence of: (i) a pre-symplectite polyphase and prograde stage (M1) indicated by the occurrence of amphibole inclusions in garnet, by Na-rich diopside and by compositional zoning in garnet, clinopyroxene and amphibole; (ii) a symplectite stage (M2) represented by the occurrence of Cpx2 + Pl1 symplectite; (iii) a corona stage (M3) documented by the formation of micrometre-thick Pl2 ± Am3 ± Ilm coronas around garnet; and (iv) a late stage (M4), documented by the growth of epidote and albite and by the replacement of biotite and clinopyroxene by chlorite. The M1 pre-symplectite stage has been modelled by P–T pseudosections calculated in the NCKFMASTH model system at a(H2O) = 1 (for a(H2O) = 0.5 temperature values are ~ 50 °C lower). Mg and Ca zoning in garnet and Na zoning in clinopyroxene testify to a progressive increase in temperature and pressure during garnet and clinopyroxene growth from 610 The corona stage (M3) was modelled in the NCFMASTH model system using the bulk composition of the effectively reacting microdomain, calculated from mineral compositions and stoichiometric coefficients of the corona-forming reaction. T = 610–670 °C and P ~ 0.7 GPa have been determined for this stage. Presumed P–T conditions of the latest re-equilibration stage (M4 stage) are around 300–400 °C and 0.2–0.3 GPa. The prograde evolution of the Punta Orvili metabasite took place under a geothermal gradient of ~ 10 °C/km, compatible with a relatively hot subduction of a small, young marginal basin. Subsequent increase of the geothermal gradient up to 20–30 °C/km suggests that subduction was followed by the Variscan continental collision. The P–T path of the Punta Orvili metabasite has significant analogies with that of the retrogressed eclogite of Golfo Aranci, NE Sardinia, and with other eclogites from the Migmatite Complex of NE Sardinia.

Lawsonite eclogites represent fossil records of processes occurring during subduction of cold- and fast-subducting slabs, and provide an opportunity to investigate the thermal and metamorphic evolution of palaeosubduction zones. Occurrences of lawsonite eclogites are rare because lawsonite is often replaced during exhumation. We report here, for the first time, the occurrence of a lawsonite eclogite from the Monviso meta-ophiolite (Western Alps), representing the product of the Alpine metamorphism of a FeTi-oxide gabbro. The prograde metamorphic evolution of this lawsonite eclogite has been investigated using the petrological approach of pseudosections. The reconstruction of its prograde P-T evolution allowed us to monitor changes in chemical and physical properties (mineral assemblages and compositions, density, H2O content, oxygen fugacity) during subduction. The pseudosection modelling suggests peak metamorphic conditions of T epsilon 550 degrees C, P = 25-26 kbar, and a decrease in the thermal gradient during subduction from about 9 degrees C km(-1) to less than 7 degrees C km(-1), which may be interpreted as related to an increase in the subduction rate. During its prograde evolution, at a depth of about 65-70 km, the eclogite-facies metagabbro experienced a significant dehydration passing from the Grt(1) + Omp(1) + Lws + Chl + Qtz + Rt assemblage (stage I) to the Grt(2) + Omp(2) + Tlc + Qtz + Rt assemblage (stage II). The breakdown of lawsonite and chlorite caused the release of up to 3 wt % of H2O and also of oxygen, as modelled by the redox equilibrium Lws + Qtz + Chl + Omp(1) = Grt + Omp(2) + H2O + O-2. This redox-equilibrium represents the boundary between an earlier, more oxidized assemblage (stage I), stable at lower T, and a later, more reduced assemblage (stage II), stable at higher T. These results have possible implications for the understanding of the complex interactions between crust and mantle in subduction zones, especially in clarifying the processes locally involved in the oxidation of the mantle wedge overlying the subducting slab.

DOI = 10.3126/hjs.v5i7.1257 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.55

The Lago di Cignana ultra-high-pressure unit (LCU), which consists of coesite–eclogite facies metabasics and metasediments, preserves the most deeply subducted oceanic rocks worldwide. New constraints on the prograde and early retrograde evolution of this ultra-high pressure unit and adjoining units provide important insights into the evolution of the Piemontese–Ligurian palaeo-subduction zone, active in Paleocene–Eocene times. In the LCU, a first prograde metamorphic assemblage, consisting of omphacite + Ca-amphibole + epidote + rare biotite + ilmenite, formed during burial at estimated P 3.2 GPa. The prograde path was characterized by a gradual decrease in the thermal gradient from ∼9–10 to ∼5–6 °C km−1. This variation is interpreted as the evidence of an increase in the rate of subduction of the Piemonte–Ligurian oceanic slab in the Eocene. Accretion of the Piemontese oceanic rocks to the Alpine orogen and thermal relaxation were probably related to the arrival of more buoyant continental crust at the subduction zone. Subsequent deformation of the orogenic wedge is responsible for the present position of the LCU, sandwiched between two tectonic slices of meta-ophiolites, named the Lower and Upper Units, which experienced peak pressures of 2.7–2.8 and

This paper describes the metamorphic evolution and the tectonic significance of the Aghil Range, a poorly known terrane located between Kunlun and Karakorum north of K2 in the framework of western Tibet. The Aghil Range consists of different units separated by syn- to late-metamorphic thrusts and post-metamorphic faults of similar attitude; among other units, the Surukwat Complex is a composite sequence of thrust sheets trending WNW–ESE and steeply dipping SSW showing a general increase of metamorphic grade from lower to higher structural levels. P–T pseudosections and conventional thermobarometry of metapelites at the top of the Surukwat Complex tightly constrain the P–T path of this highest-grade metamorphic portion of the Aghil Range. The prograde path is characterized by an early increase in both P and T and by a later, nearly isothermal, increase in P, from 500 As concerning the tectonic setting of this terrane, a number of geologic and petrologic similarities link the Aghil Range and the central Qiangtang metamorphic belt, suggesting that the Aghil Range is the possible NW extension of the Qiangtang microplate separating Kunlun from Karakorum.