OCEAN PLATE STRATIGRAPHY RECONSTRUCTED FROM THE BANGONG-NUJIANG MESO-TETHYS SUTURE ZONE (TIBET, CHINA): INSIGHTS INTO SUBDUCTION INITIATION AND ARC-RELATED VOLCANISM

Abstract: Ocean plate stratigraphy (OPS) refers to a lithostratigraphic succession deposited on the ocean floor in the course of its traveling from a spreading center to a trench. OPS is commonly reconstructed from accretionary complex and provides crucial insights into subduction-related processes that may not be discernible in any other geological setting. In this study, based on detailed field mapping and laboratory analyses by petrography, geochemistry and geochronology, representative OPSs were reconstructed from the Mugagangri Group accretionary complexes through the Bangong-Nujiang suture zone (BNSZ) in Gaize (Tibet, China). It reveals a series of subduction-accretion processes near the southern Qiangtang margin since the initial northward subduction of the Bangong-Nujiang Meso-Tethys ocean (BNO). (1) The subduction initiation (ca. 220Ma) was characterized by two largely concurrent, north-dipping subduction zones: a southern one was intra-oceanic and led to the development of a supra-subduction zone (SSZ) lithosphere and High-Nb basalt-related nascent arc; a northern one was beneath Qiangtang and responsible for the subduction of the SSZ lithosphere and accretion of nascent arc fragments into the accretionary wedge. (2) Continued northward consumption of the SSZ lithosphere led to a nascent arc-trench collision (ca. 210Ma), which resulted in large volumes of mass-wasting deposits, dominated by the arc-derived components, into the trench area. (3) Early Jurassic andesitic pyroclastic flow deposits (ca. 192Ma) intercalated in the trench fills indicate a northward migration of arc volcanism to the southern Qiangtang margin, and well represent the earlier continental margin arc products related to the northward BNO subduction. In combination to literature data, such a double-subduction-zone configuration in late Triassic to early Jurassic was probably ubiquitous along the southern Qiangtang active margin, and feasible to explain the early Mesozoic history of BNSZ in other areas, such as the accretion and emplacement of ophiolites (e.g., Dongco), the exhumation of high-pressure metamorphic rocks (e.g., Dongco and Amdo), and the delayed occurrence of continental margin arc magmatism until the early Jurassic (e.g., Gaize, Amdo and Jiayuqiao).

Keywords: Ocean plate stratigraphy; Bangong-Nujiang suture zone; Qiangtang; Subduction initiation; Arc volcanism