Resolving the subduction initiation of the Bangong–Nujiang Tethyan Ocean: New insights from the andesites in the southern margin of southern Qiangtang

The formation and evolution of the Bangong-Nujiang Tethyan Ocean played a key role in the evolution of the Tibetan plateau before the India-Asia collision. However, the timing of the Bangong-Nujiang Ocean’s subduction initiation and the resulting magmatism remain subjects of ongoing debate. In this study, we focus on new identified andesites from the Amdo area, Southern Qiangtang Terrane. Using zircon U-Pb isotopes, bulk rock geochemical data, and whole-rock Sr-Nd isotopic data, we attempt to temporally and petrogenetically constrain the magmatism associated with the subduction initiation of the Bangong-Nujiang Tethyan Ocean. LA-ICP-MS zircon U-Pb ages demonstrate that the Zhaquxiang andesites were generated during the Late Triassic (ca. 211.5 Ma). They exhibit geochemical features resembling those of arc magmatic rocks, characterized by enrichment in large ion lithophile elements and depletion in high field strength elements. The calculated ε_Nd(t) values and initial ⁸⁷Sr/⁸⁶Sr ratios for the andesites are from 2.6 to 4.4 and from 0.7048 to 0.7050, respectively. The andesites have low Mg^# (31-40), Cr (30-38 ppm) and Ni (21-24 ppm) contents. These geochemical characteristics suggest that the Zhaquxiang andesites were most probably produced by partial melting of mafic lower crust, with sediment-derived melt incorporated into their source. Combining our new data and the field investigation we conclude that the formation of these andesites was triggered by the northward-subducting Bangong-Nujiang Tethyan oceanic lithospheric during the Late Triassic. Consequently, the subduction initiation of the Bangong-Nuijiang Ocean would be the Late Triassic (ca. 211.5Ma).