Rb-Sr Geochronology of the Rocks of the Himalayas, Eastern Nepal : Part I. The Metamorphic Age of the Himalayan Gneiss

Abstract

Rb-Sr isotopic measurements were carried out for whole rock and small sliced rock from the Himalayan gneiss which constitues the metamorphosed basement of the Tethyan sediments. The results of the measurements on whole rock from the Barun migmatite, the Barun gneiss and the Irkhua gneiss indicate that complete Sr isotopic redistribution occurred about 520 m.y. ago. This age is interpreted as the time of the regional metamorphism. The analytical results of the small sliced slabs of the Himalayan gneiss indicate that Sr isotopic redistribution occurred among the sliced slabs 33.3±13.2 m.y. ago. This age is interpreted as the time of the metamorphism in the sillimanite-amphibolite facies. The high initial 87Sr/86Sr ratio of 0.7372±0.0031 from the Barun migmatite of the Himalayan gneiss suggests the Precambrian origin of the source rocks. The analytical results on the sliced slab of the Barun migmatite indicate that the original age of the rocks is about 800 m.y. The rock is interpreted as the remobilized Precambrian crustal rock. The high initial 87Sr/86Sr ratio of the Barun gneiss and the Irkhua gneiss of the Himalayan gneiss (0.7234±0.0013) indicates also the Precambrian origin of the source rocks.