Two contrasting eclogite types in the Himalayas: implications for the Himalayan orogeny

The metamorphic evolution of granulitized eclogites recently discovered in the Eastern Himalaya compared to that of the eclogites of the Northwestern Himalaya (upper Kaghan Nappe and Tso Morari Dome) suggests the possibility of a Himalaya-wide eclogitic metamorphism of Early Tertiary age. Eclogites from the Northwestern Himalaya record peak metamorphic temperatures of 580- 600 degrees C at metamorphic pressures in excess of 23-24 kbar. They have g laucophane as a retrograde phase and followed a nearly isothermal decompres sion path into the field of epidote amphibolite facies. In contrast, the Ea stern Himalaya eclogites have a strong granulite-facies overprint at metamo rphic temperatures of about 750 degrees C and pressures of 7-20 kbars, and followed a clockwise decompression path strongly convex towards high metamo rphic temperatures. The main difference between the crystalline nappes of t he Northwestern Himalaya and those of the East Himalaya appears to lie in t he different PT path they followed during exhumation. In particular the Nor thwestern Himalaya crystalline nappes lack the Miocene high temperature and low pressure overprinting which is characteristic of the Eastern Himalaya, where thermal relaxation of the thickened continental crust erased almost completely the mineralogical record of the early stages of continental coll ision. (C) 2000 Elsevier Science Ltd. All rights reserved.