Abstract

The Kathmandu complex, comprising the Bhimphedi and Phulchauki groups which range in age from Precambrian to Lower Palaeozoic, is situated in the frontal part of the Main Central Thrust sheet. Although it is likely that a tectonothermal event antedated the intrusion of early Ordovician granites, Rb-Sr dating of muscovites in deformed pegmatites suggests that the high-grade metamorphism in the Bhimphedi Group is of Tertiary age. These pegmatites cooled through c. 500°C at 18–14 Ma. Biotite Rb-Sr data indicate cooling through c. 300°C at 7.5 Ma. Thermobarometric and radiometric data from central Nepal suggest an evolutionary model for the emplacement history of the Main Central Thrust sheet involving cooling from a peak temperature c. 660°C to c. 500°C in 7 Ma, and loss by erosion or tectonic denudation of c. 4–11 km of section.

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