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The Albergian unit (AU), belonging to the Liguria-Piemonte oceanic domain of the Western Alps, comprises a thick sequence of calcschists and minor meta-ophiolitic bodies, from metric to kilometric in size. The occurrence of fresh lawsonite in its metasedimentary cover is known since the end of the XIX century. This paper reports the occurrence of fresh lawsonite also in the metamafic rocks and presents petrological data to constrain the tectono-metamorphic evolution of one of these metamafic bodies exposed in the upper Chisone Valley. In the Monte Albergian - Gran Mioul area, the AU consists of a kilometric body of basalt and/or diabase metabreccias (with minor metagabbros and meta-plagiogranite clasts and blocks), covered by discontinuous layers of quartzitic metasandstones, black micaschists, and eventually a thick sequence of calcschists. This paper focuses on the petrographic and mineral chemistry features of these lithologies, with emphasis on the preserved magmatic textures in the metamafic clasts, and on fresh lawsonite occurrence and its replacement. The second part of the paper deals with phase equilibrium modeling of two representative samples from the area: a Mg-Al metagabbro and a metabreccia with plagiogranite clasts. Thermodynamic modeling of isochemical phase diagrams (i.e., P-T pseudosections), performed for the first time in this unit, allowed constraining the metamorphic peak in the lawsonite blueschist-facies conditions, at about 18-21 kbar and 380-430°C, followed by a partial re-equilibration in the epidote-stability field. [ABSTRACT FROM AUTHOR]

New petrologic data from meta‐ophiolites exposed in a relatively poorly known sector of the Western Alps (i.e. Susa Valley, Internal Piedmont Zone) are reported. Garnet+chloritoid‐bearing phengitic micaschist and Fe–Ti metagabbro preserving eclogite facies assemblages have been investigated in detail to constrain the HP tectono‐metamorphic evolution of meta‐ophiolites in the study area. Microstructural investigations allowed to identify different assemblages related to different tectono‐metamorphic stages, whose pressure–temperature (P–T) conditions of the HP stages were constrained using the pseudosection modelling approach combined with multi‐equilibrium thermobarometry. Four main metamorphic stages have been recognized: (a) an Alpine peak‐P stage (M1a), which reached the HP–UHP eclogite facies boundary (P = 25–29 kbar, T = 460–510°C); (b) a slightly prograde decompression stage (M1b), still under eclogite facies conditions (P = 21–25 kbar, T = 500–530°C); (c) a greenschist facies re‐equilibration stage (M2); (d) a nearly isobaric late heating stage (M3), developed at the boundary between greenschist‐ and amphibolite facies conditions. The new P–T path inferred for the HP phases recorded by the studied meta‐ophiolites is compatible with that reported from other meta‐ophiolite units in the Western Alps, that is, the Zermatt–Saas ophiolite to the north and the Monviso ophiolite to the south, confirming that subduction of the Tethys oceanic lithosphere occurred at similar P–T regimes all along its length. The comparison with the tectonic evolution of the adjacent Dora Maira continental crustal unit suggest that the early exhumation of eclogitized oceanic crust likely occurred along the plate interface in the subduction channel, without the buoyancy contribution of continental crust. [ABSTRACT FROM AUTHOR]

Although calcareous pelites are important constituents of sediments involved in orogenic processes, their prograde metamorphism is significantly less studied than that of pure pelites with negligible amounts of modal calcite. This paper presents mineral equilibria modelling in the system MnO–Na2O–K2O–CaO–FeO–MgO–Al2O3–SiO2–TiO2–H2O–CO2, with the aim of constraining the prograde evolution of calcareous pelites in collisional orogenic settings. A suite of model bulk‐rock compositions is used to investigate the influence of different proportions of calcite in the protolith on (a) the equilibrium assemblages at different pressure, temperature and fluid composition (P–T–X(CO2)) conditions; (b) the melt fertility and (c) the fluid evolution and the main decarbonation reactions occurring during prograde metamorphism of calcareous pelites. In spite of being purely theoretical, the reliability of the modelling is tested by comparing the predicted assemblages with those observed in a wide set of natural samples from the Himalayan metamorphic core. Comparison between the predicted and the modelled assemblages demonstrates that even a small amount of calcite in the calcareous pelitic protoliths has a strong influence on the final mineral assemblages and compositions, with potential effects on their melt productivity. Specifically, it appears that up to ~800°C, the melt productivity of calcic metapelites remains low, and melt production occurs gradationally because it is mostly controlled by continuous biotite dehydration melting reactions, rather than by muscovite breakdown. Moreover, the study demonstrates that calcareous pelites could be non‐negligible CO2‐source rocks in orogenic settings, and that in such contexts, an internal buffered behaviour is likely for most of them. [ABSTRACT FROM AUTHOR]