Geochronology and Geodynamic Settings of Zenong Group Tuff from Shiquanhe of Middle Lhasa Terrane, Tibet

The formation age of the Zenong Group along the Shiquanhe-Yongzhu-Namucuo-Jiani mélange in the middle Lhasa terrane remains controversial between the Late Jurassic and the Early Cretaceous, and there is a lack of comparative study along the east-west direction. In this study, it focuses on the Zenong Group tuff from the Shiquanhe area in the western segment, and systematic analyses of zircon U-Pb age, trace element and Hf isotope for zircon were done for the tuff samples. Zircons from the tuff samples are elongated grains, showing obvious oscillatory zones, and the trace elements also verify the magmatic origin of zircons. Two tuff samples show LA-ICP-MS zircon U-Pb ages as 157.5±0.9 Ma and 159.2±1.3 Ma. Combined with previous research results, the Zenong Group was formed in the Late Jurassic to Early Cretaceous. Zircon trace elements and Hf isotopes of the tuff samples indicate that the Zenong Group were formed in the continental arc setting, and the hybrid of ancient continent crustal basement material and minor mantel material might be the magma source. Combined with the previous studies, the initial time of the subduction might vary from the earlier one in the western part to the later one in the eastern part.