Rb-Sr Age and Initial Sr Isotopic Ratio of Some Himalayan Rocks

Abstract

Granitic and gneissic rocks of the Kangchenjunga range, Eastern Nepal, were analysed for Rb-Sr age and initial ⁸⁷Sr/⁸⁶Sr ratio. Biotite and muscovite ages are ranging from 11m. y. to 16m. y. and from 10m. y. to 17m. y. respectively, which seem to show the final geologic event of the Himalayan granite. The short range of these age data seems to reflect the rapid cooling rate of the Himalayan rocks. The age of tourmalin granite was not obtained from its whole rock isochron, but from the internal isochron 12m. y. for the Notite-muscovite-whole rock and 29m. y. for the potassium feldspar-whole rock, which may correspond to the young mineral ages of the Kangchenj unga gneisses. The Kangchenj unga gneiss and its augen gneiss defined the same whole rock isochron of 455m. y. and gave initial⁸⁷Sr/⁸⁶Sr ratio of 0.724. However, potassium feldspar mineral ages of these gneisses are young and scattered widely from 98m. y. to 270m. y., which suggest partial rejuvenation by the later geologic events.