Abstract
The area around Darjeeling consists of medium grade metamorphic rocks and provides a classic example of inverted Himalayan metamorphism. The area under investigation shows upper amphibolite facies metamorphism (sillimanite-muscovite subfacies), rocks are intimately associated with the migmatites and granites. The presence of quartzite, calc-silicate rocks, graphitic schist and abundance of aluminous minerals like kyanite or sillimanite in these rocks indicate their metasedimentary character. Granet-sillimanite bearing gneisses occupy most of the area of Darjeeling but not persistent throughout. Textural relationship suggests sequential growth of progressively higher-grade metamorphic minerals during D 1 and D 2 deformation. The relative X Mg in the minerals varies in the order: biotite >staurolite> garnet, and the X Mn decreases in the order: garnet >staurolite>biotite. The P–T evolution of these garnet-sillimanite gneiss has been constrained through the use of conventional geothermobarometry, internally consistent TWEEQU programme and Perple_X software in the KFMASH model system, the combination of these three approaches demonstrates that the Darjeeling gneisses experienced peak pressure and temperature at 7.0±0.3 kbar and 700±30∘C. The observation in this study has important bearing on the inverted metamorphism in the Himalayan metamorphic belt.
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