The discovery of Early Cretaceous bimodal volcanic rocks in the Dachagou area of Tibet and its significance

As a result of the subduction, the Cretaceous magmatic rocks must record the information of the process and therefore provide the opportunity for understanding the evolution of the Bangong Co-Nujiang River Ocean basins and boundary conditions for models of the origin and evolution of the Tibetan Plateau. This paper reports two zircon U-Pb ages of Qushenla Formation volcanic rocks and also new whole-rock major and trace element data of the newly discovered basalt unit. LA-ICP-MS dating of zircons from two rhyolitic samples yielded ages of 104.14Ma and 104.95Ma, which indicate that the Qushenla volcanic rocks in the study area were formed in late Early Cretaceous. The basalts are depleted in Nb, Ta, and Ti, and characterized by high Zr abundances and high Zr/Y and (Th/Nb)N ratios, which suggests that they were generated by partial melting of hot asthenospere with contributions from the crust. The new data obtained by the authors, together with recently published data of rhyolites, have led the authors to believe that the generation of the bimodal volcanic suite in an extensional setting resulted from the slab break-off of the southward subduction of the Bangong Co-Nujiang River Ocean lithosphere.