Zircon LA-ICP-MS U-Pb ages and geochemical characteristics of rhyolite in Nagqu area, Tibet

This paper reports LA-ICP-MS zircon U-Pb ages of rhyolite from Middle-Late Jurassic Lagongtang Formation in Nagqu area,Tibet. The age of rhyolite is 116.6±0.81Ma,regionally consistent with the age of Early Cretaceous Duoni Formation and Zelong Group,which indicates products of the same geological event and suggests that this volcanic rock does not belong to Lagongtang Formation, and should unconformably overlie Lagongtang Formation. The average SiO2 content of rhyolite is 71.01%,with the enrichment of Rb and Th and the depletion of P, Eu and Ti. The εHf(t) values are concentrated on -0.030～2.277, 0.75 on average,with the corresponding TDM2 crustal model ages being 682～782Ma and 1015～1175Ma,larger than the formation age of the rhyolite. The rhyolite is characterized by syncollision volcanic rock instead of by traditional island-arc volcanic rock.The authors hold that there exist two kinds of possible tectonic environments: The Bangong Co-Nujiang Tethys subduction had ended and entered into the continent-continent collision stage, and the collision between Qiangtang massif and Lhasa block began; the Bangong Co-Nujiang Ocean had not been completely closed, the collision between residual ocean and Lhasa block occurred, and during the collision, the lower crust continuously thickened and experienced partial melting to form acid rhyolite.