Early Paleozoic Magmatism in Himalayan Orogen: The Geochronological Study on Augen Gneisses from Gyirongand Nyalam Areas,Southern Tibet

In the Gyirong and Nyalam areas, a massive amount of augen gneisses are extensively exposed in the middle Himalayan orogen. They consist of quartz, K-feldspar, plagioclase, biotite and minor muscovite. Zircons from augen gneisses have magmatic rims indicated by concentric oscillatory zoning. LA-ICP-MS zircon U-Pb dating gave weighted mean ages of (488.5±1.1) Ma (MSWD=0.6)、(475.1±0.7) Ma (MSWD=1.5) and (468.1±2.5) Ma (MSWD=4.2), hinting early Paleozoic magmatism in the Greater Himalayan Crystalline complex (GHC). The data in this study and other published geochronological results of Cambrian-Ordovician magmatites demonstrated that early Paleozoic orogenesis existed in the Himalayas. Early Paleozoic tectonic events preserved in Himalayas are well compared with the contemporaneous ones in the Lhasa terrane, Qiangtang terrane, Baoshan terrane and Tengchong terrane located in the south and southeast of Tibet Plateau. Integrating previous studies, we suggested an Andean-type orogeny corresponding to dynamic adjusting of the plates by subduction of the Proto-Tethys Ocean lithosphere along the northern margin of Gondwana, instead of Pan-African orogeny that was characterized by the continent-continent collisions during Gondwana assembly.