P-T EVOLUTION, GEOCHRONOLOGY OF MIGMATITE FROM LEO PARGIL DOME, KINNAUR DISTRICT, HIMACHAL PRADESH, INDIA

Abstract

Migmatites exposed within the North Himalayan Domes are the result of reworking and partial melting of different 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, petrologic studies of migmatites from North Himalayan Domes are virtually lacking. The results of a petrological and fluid inclusion study aimed at understanding 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. The thermodynamic approach of isochemical phase diagrams has been applied. Phase equilibria calculated in the MnO–Na2O CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–H2O (MnNCKFMASTH) system suggest peak P–T conditions of 810–825°C, 9.2–11.0 kbar for the amphibole+biotite migmatite and of 700–745°C, 7.2–7.9 kbar for the sillimanite+muscovite migmatite, followed by an early nearly isothermal decompression. The formation of migmatites in the study area was related to both water-fluxed melting and muscovite dehydration melting. The Leo Pargil shear zone flushed the excess water in the system leading to water–flux melting and provided the environment suitable for the formation of migmatites and leucogranites completed around 15 Ma. Anatexis was limited to the shear zone, where melting of a sedimentary protolith (e.g. calcareous pelite and/or marl) generated leucosomes with peritectic hornblende and the melting of neighbouring metasedimentary rocks lack anhydrous peritectic phases. Primary CO2 fluid inclusions entrapped in quartz at 5.5–6.8 kbar: 695–790°C; whereas the secondary carbonic fluid was entrapped at 2.2–4.2 kbar, 320–550°C due to density reversal during exhumation. This study suggests that the primary carbonic-aqueous (VCO2 – LH2O) fluid inclusions were initially trapped in the system, later H2O content was lost during exhumation, leading to the formation of pure CO2 fluid inclusions. The carbonic fluids were may be sourced locally from decarbonation reactions of the associated carbonate rocks during migmatization or from a deep-seated reservoir through Kaurik-Chango normal fault. U–Pb geochronology, Hf isotopes, and trace element chemistry of zircon grains from migmatite of the upper Sutlej valley (Leo Pargil), Northwest Himalaya, reveal a protracted geologic evolution and constrain anatexis and tectono-thermal processeses in response to Himalayan orogenesis. U-Pb geochronology and εHf record separate clusters of ages on the concordia plots in the migmatite (1050-950, 850-790, and 650-500 Ma). The εHf data from the 950-1050 Ma i zircon (-6.9 to +13.6) imply derivation from mixed depleted and enriched crustal sources. This event is analogous to the Rodinia-age zircon population observed in the Eastern Ghats orogeny of India, the Albany Fraser belt of Australia and Antarctica, and the Rayner Complex of Antarctica. The presence of minor amounts of Paleoproterozoic grains were likely derived from the Indian craton. The potential source rock of 930-800 Ma detrital zircons may be granitoid present in Greater Himalayan rocks themselves and Aravalli Range which has 870-800 Ma granitic rocks. The arc type basement within the Himalayan-Tibet orogen recorded (900–600 Ma) igneous activity, which may depict a north easterly extension of juvenile terranes in the Arabian–Nubian Shield. The granitoid of 800 Ma may be a potential source for 790 Ma detrital zircon due to scatter in 206/238 dates. The other prominent U-Pb age peak of detrital zircon (650 498 Ma) with εHf(t) value (-16.8 to +3.6) advocates their derivation from the East African Orogen and Ross-Delamerian Orogen of Gondwana. The Cambrian-Ordovician magmatism during the Bhimphedian Orogeny, and observed late Neoproterozoic to Ordovician detrital zircon have been derived to some extent from the Greater Himalayan magmatic sources. Our isotope data also reveal that the studied area represents a part of the Greater Himalayan Sequence distinct from the Tethyan Himalaya Sequence. One sample yielded a discordia lower intercept age of 15.6 ± 2.2 Ma, the age of melt crystallization.