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

Abstract

Mineral thermobarometric and fluid inclusion studies were carried out on the 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 metamorphism from chlorite to kyanite towards the north. The pressure calculated from garnet–biotite–muscovite–plagioclase phase equilibria, increases from 1.9 to 8 kbar and the temperature obtained through garnet–biotite and chlorite geothermometer varies between 360 and 562°C. The mineral assemblages and these thermobarometric estimates reveal that the grade of metamorphism in the MCT zone increases from south to north. Considering the inclusion 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 retrogression. This observation is also substantiated by the systematic increase in CO 2 density from 0.75 g/cm 3 in CO 2–H 2O inclusions near MCT-III to 1.01 g/cm 3 in pure CO 2 inclusions near MCT-I. The pressure and temperature estimated using fluid isochores varies from 1.9 to 4.8 kbar and 360 to 562°C. When compared with mineral phase thermobarometry, these P–T conditions suggest a post peak-metamorphic nature of the fluids. A smooth trend of mineral and fluid phase P–T conditions is observed in the MCT zone. Based on combined mineral P–T data and fluid isochores, a decompressional uplift path is suggested. The decrease in P–T and the 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.