P-T-fluid evolution of migmatites from the Leo Pargil gneissic dome, India: Insights into partial melting and exhumation processes in North Himalayan Domes

Migmatites exposed within the North Himalayan Domes result from the reworking and partial melting of different structural levels of the Indian continental crust during a major Himalayan tectonic event. Although their petrologic investigation is potentially a powerful method to unravel the anatectic and tectono-thermal processes related to the Himalayan orogeny, petrological studies of the migmatites from North Himalayan Domes are virtually lacking. This paper presents the results of a petrological and fluid inclusion study in order to understand the partial melting processes, the P-T conditions of migmatitization, and the fluid evolution of the migmatites from one of the North Himalayan Domes, the Leo Pargil Dome, exposed in central Himalaya. Phase equilibria calculated in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-TiO2-H2O (MnNCKFMASTH) system suggest peak P-T conditions of 770-790 °C, 9.3-10.5 kbar for the amphibole + biotite migmatite and of 780-820 °C, 9.3-9.8 kbar for the sillimanite-bearing migmatite, followed by an early nearly isothermal decompression. The formation of these migmatites was related to water-fluxed melting and muscovite dehydration melting. At peak metamorphic conditions, aqueous-carbonic (H2O-CO2) fluids were present in the system. Subsequently, the H2O phase was preferentially leached out from the biphase aqueous-carbonic inclusions to form primary CO2 inclusions and directly went into the anatectic melt during migmatization. The exhumation of LPD followed an early nearly isothermal decompression path and a later isochoric cooling driven by the Kaurik-Chango normal fault.