Early Triassic Forearc Sedimentation in the North Qiangtang Terrane, Tibetan Plateau: Implications for Northward Subduction of the Paleo-Tethys Ocean

The polarity and process of Paleo-Tethys subduction on the Tibetan Plateau is a key scientific issue to analyze the tectonic evolution of the Tethys, however, the sedimentary response mechanism of its continental margin still lacks a systematic study. In this paper, we take the Lower Triassic Yingshuiquan Formation at the southern margin of the North Qiangtang terrane as the research object, and comprehensively carry out petrological and sedimentary phase analyses, as well as combining the U-Pb chronology of detrital zircon with trace element geochemistry research. The study shows that the maximum depositional age of the Lower Triassic Yingshuiquan Formation is Early Triassic, and its detrital zircon has the major age group of 354-247 Ma and the minor age groups of 378-484 Ma, 550-695 Ma, 837-925 Ma, 1712-1951 Ma, and 2389-2695 Ma. The depositional environment of the Yingshuiquan Formation is carbonate terrace deposition dominated by the oolitic shoal subphase. The source area of detrital zircon in the Lower Triassic Yingshuiquan Formation is mainly the South and North Qiangtang terranes, in which the detrital zircon components of 484-378 Ma and 354-247 Ma were jointly provided by the contemporaneous bodies of the North Qiangtang terrane and South Qiangtang terrane, whereas the sedimentary detritus of >550 Ma came from the re-circulation of South Qiangtang terrane. By synthesizing the newly acquired data in this study and the previous studies on the subduction and extinction stage of the Longmu Co-Shuanghu Paleo-Tethys Ocean (LSPTO), this paper suggests that the Early Triassic LSPTO has not yet been completely extinguished, and that there is still a remnant of the oceanic basin that developed pre-arc sedimentation between the accretionary wedge and the magmatic arc, and that the Yingshuiquan Formation is the record of the deposition of the pre-arc basin.