RB-SR AGES OF MICAS FROM THE KATHMANDU COMPLEX, CENTRAL NEPALESE HIMALAYA - IMPLICATIONS FOR THE EVOLUTION OF THE MAIN CENTRAL THRUST

The Kathmandu complex, comprising the Bhimphedi and Phulchauki groups which range in age from Precambrian to Lower Palaeozoic, is situated i n the frontal part of the Main Central Thrust sheet. Although it is li kely that a tectonothermal event antedated the intrusion of early Ordo vician granites, Rb-Sr dating of muscovites in deformed pegmatites sug gests that the high-grade metamorphism in the Bhimphedi Group is of Te rtiary age. These pegmetites cooled through c. 500 degrees C at 18-14 Ma. Biotite Rb-Sr data indicate cooling through c. 300 degrees 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 degrees C to c. 500 degrees C in 7 Ma, and loss by erosion or tectonic denuda tion of c. 4-11 km of section.