Fluid events and exhumation history of the main central thrust zone Garhwal Himalaya (India)

Mineral thermobarometric and fluid inclusion studies were carried out on th e low to medium grade metamorphics which occur at different tectonic levels of the main central thrust (MCT) zone in Garhwal Himalaya (MCT-I in north, MCT-II in the centre and the southern most MCT-III). The augen gneiss and pelitic schists present in this zone show an increase in the grade of metam orphism from chlorite to kyanite towards the north. The pressure calculated from gamet-biotite-muscovite-plagioclase phase equilibria, increases from 1.9 to 8 kbar and the temperature obtained through garnet-biotite and chlor ite geothermometer varies between 360 and 562 degreesC. The mineral assembl ages and these thermobarometric estimates reveal that the grade of metamorp hism in the MCT zone increases from south to north. Considering the inclusi on types, it is apparent that the carbonic fluid increases towards the MCT- I, which is related to increase in P-T conditions whereas the aqueous phase is more pronounced near the MCT-III reflecting fluid assisted retrogressio n. This observation is also substantiated by the systematic increase in CO2 density from 0.75 g/cm(3) in CO2-H2O inclusions near MCT-III to 1.01 g/cm( 3) in pure CO2 inclusions near MCT-I. The pressure and temperature estimated using fluid isochores varies from 1. 9 to 4.8 kbar and 360 to 562 degreesC. When compared with mineral phase the rmobarometry, these P-T conditions suggest a post peak-metamorphic nature o f the fluids. A smooth trend of mineral and fluid phase P-T conditions is o bserved in the MCT zone. Based on combined mineral P-T data and fluid isoch ores, a decompressional uplift path is suggested. The decrease in P-T and t he evidence of movement along well distributed shear fabric across the MCT zone indicate that exhumation from north to south occurred under decreasing P-T conditions. (C) 2001 Elsevier Science Ltd. All rights reserved.