Retrograde corona texture in pre-Himalayan metamorphic mafic xenoliths, Sutlej valley, NW Himalaya: Implication on rare occurrence of high-grade rocks in the Himalaya

Abstract

The present study documents extensive retrogression in mafic xenoliths embedded in the Kinnaur Kailash Granite (∼500Ma), Sutlej valley, NW Himalaya. Most of the mafic xenoliths are hornblende-rich and are characterized by numerous retrograde corona textures such as garnet coronae around clinopyroxene and plagioclase, titanite coronae around ilmenite, and hornblende coronae around clinopyroxene. This implies that the mafic xenoliths had undergone granulite-facies metamorphism in the early stage of metamorphic evolution, and have been extensively retrograded at a later stage to the present day amphibole-rich mafic xenoliths. The retrogression path traced by these mafic xenoliths can be constrained through P–T estimates using hbl–grt and hbl–pl geothermometry, hbl–grt–pl geobarometry, and Thermocalc3.21 calculations. The estimated results suggest temperatures in the range of 536–662°C and pressures in the range of 4.5–6.7kbar for the formation of the corona textures. P–T calculation of garnet forming reaction rim around clinopyroxene further shows that retrogression had started at ∼650°C and ∼7.3kbar. These observations suggest that retrogression played a major role in the obliteration of most of the high-grade pre-Himalayan xenoliths from the Himalaya.