A snapshot of subduction initiation within a back-arc basin: Insights from Shiquanhe ophiolite, western Tibet

Back-arc basins are key sites for oceanic lithosphere formation and consumption at convergent plate boundaries, and their formation and subduction processes can be highly variable. The tectonic setting and evolution of the Meso-Tethys Shiquanhe-Jiali ophiolite sub-belt (SJO sub-belt) within Bangong-Nujiang Suture Zone (BNSZ), central Tibet, are disputed for the complex rock composition and ages. In this paper, we present geochronology, geochemistry and field observations on the Shiquanhe ophiolite, providing a representative ophiolite example in the western end of SJO. Based on investigation of the petrogenesis and tectonic setting of different rock types, combined with the U-Pb dating, we propose a two-stage subduction model for explaining the tectonic evolution of SJO as well as the wither away of a back-arc basin. Geochemical and geochronological data indicate that the ca. 183 Ma LAN (north of Lameila) gabbros formed in the forearc setting and represent the early-stage subduction of the Bangong Meso-Tethys. This subduction induced the back-arc spreading recorded in the ca. 170 Ma gabbros and lower pillow basalts of PL-SDN (Pagelizanong-Shiquanhe Dam Nan) ophiolitic fragments in the Shiquanhe ophiolite. The basaltic lavas overlying the lower basalts, represented by the ca. 168–164 Ma diabasic and boninite dikes have forearc characteristics, and they represent the back-arc basin subduction initiation at a late stage. This work thus recovered the multiple tectonic evolution of SJO sub-belt and emphasise the importance of the back-arc basin subduction in the evolution of ancient oceans.