Mafic and pelitic xenoliths in the Kinnaur Kailash Granite, Baspa river valley, NW Himalaya: Evidence of pre-Himalayan granulite metamorphism followed by cooling event

Abstract

Mafic and pelitic xenoliths occurring within the early Palaeozoic Kinnaur Kailash Granite (KKG) in the Baspa river valley, NW Himalaya record pre-Himalayan regional metamorphism at a range of pressure (P)–temperature (T) conditions that span amphibolite to granulite facies. The key evidence of granulite metamorphism is a xenolith of two-pyroxene mafic granulite in which orthopyroxene occurs as both discrete grains and microscopic needles exsolved parallel to prismatic cleavage of the clinopyroxene host. The rock records an average peak metamorphic temperature of 840°C. Garnetiferous mafic xenoliths display coronae of garnet around plagioclase and clinopyroxene, and of sphene around ilmenite. These coronae were developed by near-isobaric cooling after peak metamorphism at 730°C and 8kbar. Pelitic xenoliths have the assemblage biotite–plagioclase–quartz±garnet±K-feldspar±muscovite, and record P–T ranges of 7.0–9.0kbar and 500–700°C which indicate lower to middle amphibolite facies metamorphism of these rocks. Quartz, feldspar and mica in the pelitic xenoliths commonly show optical evidences of crystalloplastic deformation which indicate that the rocks were sheared before being engulfed as xenoliths in the KKG. The findings of this study imply that the present day metamorphic assemblages and shear fabrics in HHCS rocks need not be attributed solely to the Himalayan orogeny.