Origin of Qushenla Formation Volcanic Rocks in the Nawucuo Area, Northern Tibet, and Constraints on the Subduction Polarity of the Bangong-Nujiang Tethys Ocean

The Qushenla Formation volcanic rocks are widely exposed in the northern margin of the Bangong-Nujiang suture zone (BNSZ). Research on these rocks is of great significance for understanding the tectonic evolution of the Bangong-Nujiang Tethys Ocean (BNTO). In this study, a systematic geological survey was conducted on the Qushenla Formation volcanic rocks that are widely exposed in the Nawucuo area, in the northern margin of the western segment along the BNSZ. The whole-rock geochemistry, zircon U-Pb dating, and in situ zircon Lu-Hf isotopes were carried out in this study, aiming to constrain the formation age, rock genesis, magma source and tectonic setting of the volcanic rocks. The zircon U-Pb dating shows that the Qushenla Formation volcanic rocks in the western BNSZ erupted during the period of 120-108 Ma, i.e., Early Cretaceous. The Qushenla Formation volcanic rocks are a suite of intermediate-basic volcanic and pyroclastic rocks belonging to the medium-K calc-alkaline series. They are relatively enriched in light rare earth elements (LREEs) and incompatible elements such as Rb, K, La, Th, Sm, and Hf, whereas depleted in heavy REEs (HREEs) and high field strength elements (HFSEs) such as Nb, P, Zr, and Ti. The in situ zircon epsilon(Hf)(t) values of the volcanic rocks range from 8.95 to 12.57, with an average of 10.40. The Mg-#, Th/La and Th/Ce values are between those of the mantle-derived magma and the continental crust. The formation of the Qushenla Formation volcanic rocks can be explained through the following process: (1) As the nospheric materials that upwelled during the rollback of the subducting Tethys Ocean slab induced the large-scale partial melting of the mantle wedge and the formation of the initial basaltic magma. (2) These mantle-derived magmas ascended and induced the partial melting of the lower crust to generate peraluminous melts. (3) The mixing of the peraluminous melts and mantle-derived melts generated the initial magma with homogeneous Sr-Nd isotope compositions. (4) Last, the eruption of the magma produced the widespread Qushenla Formation volcanic rocks at the surface. When combining this information with the regional geological background, it is believed that the Qushenla Formation volcanic rocks, the Meiriqicuo Formation volcanic rocks and the Late Jurassic-Early Cretaceous intrusive rocks together constitute the tectonic magmatic arc of the active continental margin on the southern margin of Qiangtang, which was formed in the tectonic setting of the northward subduction of the Bangong-Nujiang oceanic crust beneath the Qiangtang Block.