Late Mesozoic ridge subduction and subduction initiation in the Bangong-Nujiang Tethyan Ocean (Tibet); evidence from two distinct arc magmatic systems

Understanding the temporal and spatial distribution patterns of arc magmatism is crucial for interpreting the evolution of ancient crust formation because the subduction of a mature ocean basin involves multiple stages from subduction initiation to collision, resulting in different kinds of arc magmas in the overriding plate. A comprehensive compilation and synthesis of arc magmatism, combined with systematic studies of key areas within relevant convergent margins, can effectively reveal specific processes, such as ridge subduction and subduction initiation. The Bangong-Nujiang Suture Zone (BNSZ) in the Tibetan Plateau records opening and closing of the Meso-Tethys Ocean and the subsequent accretion of the South Qiangtang Terrane to Asia. This study reports new data for Jurassic arc rocks in the Shiquanhe and Ritu areas of the western BNSZ. By integrating new geochronological and geochemical data with previous studies, we identify two distinct arc magmatic systems in the southern and northern belts of the BNSZ. North Belt arc magmatism was active from ca. 169 Ma to ca. 147 Ma, exhibiting characteristics of a continental arc, including isotopic characteristics (Cordilleran-type arc system). The South Belt transitioned from 165 Ma to 157 Ma high-Mg andesites/diorites (including sanukites and boninites) to low-Mg arc magmatism that continued until ca. 122 Ma, suggesting an intra-oceanic subduction system similar to the Izu-Bonin-Mariana arc system. We propose a model in which low-angle oblique ( approximately 14 degrees ) ridge-trench collision (169-156 Ma) in the North Belt induced subduction initiation (164-156 Ma) in the South Belt, which then evolved into self-sustained subduction after ca. 156 Ma. Our new model enhances understanding of the subduction evolution of Meso-Tethys Ocean and serves as a case study for comprehending multi-stage arc magmatic evolutionary processes and subduction initiation.