Experimental study of granitic rocks of Darjeeling (West Bengal, India) and its application to the origin of himalayan granites

Abstract

The area around Darjeeling consists of low-to-high grade metamorphic rocks with the development of typical index minerals, viz. chlorite, biotite, almandine, kyanite and sillimanite. At the highest stage of regional metamorphism (sillimanite—orthoclase subfacies), rocks are intimately associated with the migmatites and granites. The high-grade gneisses contain inclusions of isolated, round, oval or thin lenticular masses of quartz + calcic plagioclase + garnet + clinopyroxene ± hornblende ± biotite ± sphene +- graphite +- zoisite rocks typical of hornblende/pyroxene granulite facies. Low-grade rocks are devoid of these inclusions.Phase relationships of a granite and a gneiss from Darjeeling have been determined at 4 and 7 kbar pressure in the presence of excess water. The beginning of melting and the disappearance of the phases, viz. sodic-plagioclase, K-feldspar, muscovite, biotite, and quartz have been noted. Plagioclase is the first phase melted out in the granite. In the gneissic rock, plagioclase melts out at a slightly higher temperature than muscovite. Both quartz and biotite are the silicate liquidus phases in the granite and gneiss. Complete fusion of both the rocks was achieved at temperatures below 800°C.The new data demonstrate that crystal—liquid equilibria played a dominant role in the formation of magmas of granodiorite → granite → pegmatite, the sequence of which is also established in the field. The anatectic magma left behind a more dessicated refractory rock now occurring as hornblende or pyroxene granulites.The granites of the Lower Himalaya are formed relatively at a lower temperature and pressure than the younger leucocratic tourmaline granites of the Higher Himalaya.