The fate of calcareous pelites in collisional orogens

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.