LA-ICP-MS zircon U-Pb ages of adakitic rocks in Dongco area, Tibet, and their tectonic implications

The Ban'gong Co-Nujiang suture zone is one of the main suture zones in the Tibetan Plateau. Its closure time has been controversial, which causes some restrictions to the study of the evolution of the Tibetan Plateau. This paper deals with the discovered trachyandesite which occurs in Bangong Co-Nujiang suture zone. LA-ICP-MS zircon U-Pb analyses suggest that the trachyandesite was formed at 102±1.9Ma, and that the volcanic rocks were formed in late Early Cretaceous. They are characterized by ada-kite-like rock:high Na content, SiO₂, Al₂O₃, Mg^# and high Sr/Y values(25.2~42.2), and low Nb(11.1×10^-6~16.6×10^-6), Y (11.7×10^-6~18.3×10^-6)and Yb(1.06×10^-6~1.77×10^-6) values. The differentiation between HREE and LREE is obvious. The authors hold that high Mg^# adakite-like rock(Mg^#=(50.0~54.1) resulted from the reaction between the rising melt of remnant oceanic crust and the mantle wedge. Therefore, the origin of Dongcuo adakitic-like rocks may be related to the collision between Lasha and Qiangtang blocks after the closure of the Bangong Co-Nujiang Ocean. Based on these data, the authors consider that Bangong Co-Nujiang Ocean had been closed before 102Ma.