藏北班公湖—怒江缝合带早白垩世双峰式火山岩的确定及其地质意义

班公湖—怒江缝合带及其两侧发育的白垩世岩浆活动记录了大洋俯冲闭合过程的信息,是反演大洋演化的关键。报道了去申拉组火山岩测年结果,并对达查沟地区新发现的去申拉组玄武岩端元进行了详细的地球化学研究。2件火山岩样品的锆石U-Pb年龄分别为104.14Ma和104.95Ma,表明去申拉组火山岩形成于早白垩世晚期。玄武岩地球化学分析结果显示出Nb、Ta、Ti亏损,以及高的Zr含量、Zr/Y和(Th/Nb)N值的特征。研究认为,玄武岩是软流圈地幔部分熔融混染地壳物质的产物。结合已报道的去申拉组酸性岩数据,研究区具有双峰式火山岩特征的去申拉组是班公湖—怒江洋壳南向俯冲过程中,板片断离导致区域伸展背景下岩浆活动的产物。

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.