Metamorphic P–T conditions and CO2 influx history of medium-grade metapelites from Karakorum, Trans-Himalaya, India

Abstract

The medium grade metapelites of Pangong-Tso area in the trans-Himalayan region underwent sillimanite-grade metamorphism initiated during the Cretaceous, associated with the collision of the Kohistan arc and the Indian plate with Asia. This paper present results from a petrological and fluid inclusion study to understand the metamorphic P–T conditions and fluid history of these rocks. The calculated phase equilibria in the Na2O–CaO–K2O–FeO–MgO–MnO–Al2O3–SiO2–H2O–TiO2 (NCKFMMnASHT) system suggest P–T conditions of 8kbar and 650°C for the peak metamorphic event. Primary fluid inclusions occur in staurolite and garnet, whereas quartz carries mostly secondary fluid inclusions. The trapped fluids in primary inclusions show initial melting temperatures in the range of −56.9 to −56.6°C, suggesting nearly pure CO2 composition. The secondary fluids are of mixed carbonic-aqueous nature. The re-equilibrated inclusions show annular morphology as well as necking phenomena. The CO2 isochores for the primary inclusions indicate pressures of 6.1–6.7kbar, suggesting that the CO2-rich fluids were trapped during post-peak exhumation of the rocks, or that synmetamorphic carbonic fluids underwent density reversal during isothermal decompression. The secondary CO2–H2O fluids must have been trapped during the late exhumation stage, as their isochores define further lower pressures of 4.8kbar. The morphology of re-equilibrated fluid inclusions and the rapid decrease in pressure are consistent with a near-isothermal decompression trajectory following the peak metamorphism. The carbonic fluids were probably derived locally from decarbonation reactions of the associated carbonate rocks during metamorphism or from a deep-seated reservoir through Karakorum fault.