Geochronology and Precambrian tectonic evolution of the Namche Barwa complex from the Eastern Himalayan syntaxis, Tibet

Abstract

The Namche Barwa complex located in the eastern syntaxis of the Himalayan orogenic belt and south of the India-Yarlung Tsangpo suture zone has been subjected to the high-pressure regional metamorphism and intensely migmatization. The felsic gneisses distributing widely in this area were chosen to study its lithology and isotopic geochronology. Most of the gneisses have the metamorphic mineral assemblages of amphibolite facies except minor pelitic gneisses with a high-pressure granulite facies mineral assemblage of garnet + kyanite + ternary feldspar + quartz. Most of their protoliths are diorite and granodiorite characterized by the chemical composition of igneous rocks formed in a magmatic arc environment. The zircons in gneisses commonly have a core-rim texture. SHRIMP and LA-ICP-MS U-Pb zircon in-situ analyses indicate that the zircon rims give multi-stage metamorphic and magmatic event ages ranging from 500Ma to 10Ma, while the zircon cores yield at least four populations of Precambrian ages: ∼2500Ma, ∼1800Ma, ∼1600Ma and ∼1000Ma. The zircon grains analyzed show obvious magmatic crystallization zoning and have relatively high Th/U ratios, typical of magmatic origin, and thus these four populations of ages are interpreted as ages of multiphase Precambrian tectonic-magmatic events. Moreover, these ages are comparable to those of Precambrian tectonic-magmatic events of the Greater Himalayan Crystalline complex and the Indian craton. Therefore, we conclude that the protoliths of the Namche Barwa complex and the Greater Himalayan Crystalline rock may have been derived from the intrusive rocks formed from the late Archean to the late Proterozoic, recorded the formation and breakup of the Columbia, Rodinia and Gondwana supercontinent, and undergone the regional metamorphism and magmatism during the Himalayan orogeny.