Geology of the Western Arunachal Himalaya in Parts of Tawang and West Kameng Districts, Arunachal Pradesh
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The tourmaline bearing pegmatitic High Himalayan leucogranite crops out in the higher altitudes of the High Himalayan Sequence. The primary mineral assemblages recorded are tourmaline + muscovite + biotite + garnet apart from quartz and K-feldspar. This intrudes the migmatite of Sela Group parallel as well as oblique to earlier gneissosity as lens shaped bodies. The Sela's are thrust over the metasedimentary rocks of Dirang Formation. Earlier workers marked this thrust as the Main Central Thrust (MCT). During the present studies the whole Dirang Formation was found to be a high strained zone with many shear planes, almost all the lithounits are sheared with prolific graphite/carbon emplacement along the permeable shear planes.
The Dirang Formation comprises quartzite, kyanite ± staurolite + garnet ± biotite schist, crystalline limestone and carbon phyllite. Basic and ultrabasic rocks intrude these lithounits. The Bomdila Gneiss, which forms a part of the Lesser Himalayan Sequence, is a large body of granite orthogneiss. The primary mineral assemblage includes quartz, K-feldspar, biotite and garnet. Though the gneisses have an intrusive relationship with the Dirang metasediments the contact is marked by a thrust.
Metamorphic grade increases from the base of Dirang to the top of the Sela Group. The base of the Dirang Formation is characterized by biotite + garnet assemblages followed by kyanite + staurolite at the top the Dirang Formation. The base of the Sela Group is marked by kyanite ± garnet ± biotite followed by sillimanite ± biotite at the top of Sela Group. Two phases of deformations are recorded from Sela and Dirang rocks with type III interference pattern folds.
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