The discovery of Qushenla volcanic rocks in Tasepule area of Nyima Country, Tibet, and its geological significance

This paper reports new whole-rock major and trace element data and zircon U-Pb dating results of the newly discovered Qushenla volcanic rocks in Tasepule area, Nyima County, Tibet. According to the features of major elements, the authors grouped volcanic rocks into three types, i.e., magnesian andesites, dacite and highly differentiated rhyolite. The magnesian andesites produced by the interaction between the subducted slab melt and mantle wedge are rich in MgO and Mg#. Dacite and highly differentiated rhyolites are enriched in Rb, Th, U, K, Pb and depleted in Nb, Ta, Ti, and have high Rb/Sr ratios, which shows that they might be the product of partial melting of the crust which experienced different degrees of differentiation and evolution. The dating of zircons from two rhyolitic samples by LA-ICP-MS yielded ages of 103Ma±1Ma and 107Ma±1Ma, which indicates that the Qushenla volcanic rocks in the study area were formed in late Early Cretaceous. It is considered that the plate fracturing that happened during the process of southward subduction of Bangong Co-Nujiang Ocean at about 110Ma caused the upwelling of mantle source materials, which was the geodynamic background for the formation of the Qushenla volcanic rocks. The result obtained by the authors suggests that the continent-continent collision of Bangong Co-Nujiang Ocean should occur at about 107Ma in Tasepule area.