The fate of calcareous pelites in collisional orogens

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