Dehydration melting studies in a 'Kyanite terrain', Manali, NW Himalayas

The Beas valley section in Himachal Pradesh is characterized by the presence of kyanite as the alumino-silicate phase and a small extent of Neogene quartzo-feldspathic intrusives. The present paper explores the reasons for the lack of extensive granite occurrences through a detailed petrologic study on samples of a drill core (200 m, inclined 60° due ESE) close to Manali, the expected highest point in P-T space in this section. The highest assemblage encountered is quartz+muscovite+biotite+garnet+plagioclase+kyanite±apatite±zircon±tourmaline±ilmenite±rutile. The chemical analyses of mineral grains were carried out on a CAMECA SX51 EPMA. The following two reactions that constrain the P-T regime of the area have been established.St+Qtz→Alm+Ky+H2OBio+Plg+Qtz+Ksp→Gar+Ky+Liq.Our result, P=8.25±1 kbar, T=638±4 °C, falls in a region between curves representing aH2O=1.0 and aH2O=0.7 of the fluid in Thompson [Trans. Royal Soc. Edinb. 87 (1996) 1] diagram. The P-T estimate falls in the kyanite field and is very close to the mica dehydration-melting curve. The studies indicate that the core samples are a part of progressive metamorphic sequence. It implies that the absence of sillimanite and large scale melting is perhaps chance brought about by thrusting and erosion. The present section experienced single stage dehydration melting.