The Fate of Pyroxenes in Mafic Xenoliths from the Kinnaur Kailash Granite, Sutlej Valley, NW Himalaya: Effect of Retrograde Hydration and Insights on the Rare Occurrence of High-Grade Metamorphic Rocks in the Himalayan Orogen

Abstract

Abstract Pseudosection modelling of post-peak mineral assemblages in metamorphosed mafic xenoliths from the Kinnaur Kailash Granite, Northwest Himalaya, has provided new insights into pre-Himalayan metamorphism and constrained the P–T conditions of hydration. P–T and P–MH2O pseudosections constructed in the NCFMASH system show that retrograde hydration occurred during destabilisation of orthopyroxene and clinopyroxene in mafic granulites after decompression to 0.3 GPa pressure, and as a result, water-containing phases such as cummingtonite and hornblende formed. Results from two of the studied xenoliths indicate that the secondary mineral assemblage is controlled not only by P and T, but also by the amount of water available for hydration. In contrast, another example indicates the development of cummingtonite from hornblende via dehydration reaction after decompression to conditions appropriate for cummingtonite stability. The study shows that the Higher Himalayan Crystalline Sequence (HHCS), from which the xenoliths have been derived, must have been subjected to pre-Himalayan high-grade metamorphism, although it is not evident because of pervasive overprinting by Himalayan metamorphism. The present-day metamorphic pattern of the HHCS is expectedly the result of both pre-Himalayan and Himalayan metamorphism, and careful petrological investigation is required to distinguish their signatures so that a better understanding of the tectonothermal evolution of the Himalayas can come into picture. Isotopic dating of the xenoliths is required to establish the timings of the pre-Himalayan metamorphism and decompression.