Rb-Sr MINERAL AGES OF HIGHER HIMALAYAN CRYSTALLINES, BHAGIRATHI VALLEY, GARHWAL HIMALAYA

Abstract

The Himalaya is a unique and classic example of continental collision tectonics in an orogenic belt where leading continental margin of the Indian plate has collided with the Eurasian plate. The later phase of collision tectonics of continental crust of the'two plates during the Cenozoic has been mainly responsible for remobilization of the Proterozoic basement and it's splitting by many thrusts. The present study encompasses the determination of the whole rock ages of Bhaironghati Granite and minerals ages along Bhagirathi Valley, Garhwal Himalaya by using Rb-Sr technique. For this 18 rocks samples have been collected from the Bhagirathi Valley and muscovite and biotite minerals have been used for dating and constraining the ages within the closure temperature of 550 ± 50 and 300 f 50°C respectively. The muscovite cooling ages from Jhangla and Bhaironghati are showing exceptionally high ages about 350-450 Ma, indicating a partial resetting of the minerals which has not reached the 550°C geotherm and hence indicates that the temperature of metamorphism has gone below 550 ± 50°C while a sample around Bhatwari gives a low age (about 60 Ma) for muscovite which might be due to the reactivation of the MCT causing disturbance in Rb-Sr systematics. On the contrary, the bioite cooling ages are quite low and good about 5-17 Ma indicating complete resetting of the minerals during the Himalayan metamorphism. We can say that there has been a local resetting of the minerals which have resulted in such low ages of the biotites (i.e. 5-17 Ma) and partial resetting of the muscovite giving higher ages (about 350-450 Ma).