A new ophiolitic mélange containing boninitic blocks in Alxa region: Implications for Permian subduction events in southern CAOB

The Alxa region, located in the southernmost part of Central Asian Orogenic Belt, is a key region for understanding the tectonic processes associated with the closure of the Paleo-Asian Ocean. Issues of late Paleozoic tectonic settings and tectonic unit divisions of the Alxa region still remain controversial. In this study, we report a new ophiolitic mélange named the Tepai ophiolitic mélange in the northern Alxa region, northwest of Alxa Youqi. The tectonic blocks in the Tepai ophiolitic mélange are mainly composed of serpentinized peridotites, serpentinites, mylonitized gabbros, gabbros, basalts, and quartzites, with a matrix comprising highly deformed clastic rocks. A gabbro exhibits a zircon LA-ICP-MS U–Pb age of 278.4 ± 3.3 Ma. Gabbros exhibit high MgO and compatible element contents, but extremely low TiO2, totally rare earth element and high field strength element contents. These rocks exhibit light rare earth element depleted patterns, and display enriched in large-ion lithophile elements and depleted in high field strength elements. Boninite-like geochemical data show that they were formed in a subduction-related environment, and derived from an extremely depleted mantle source infiltrated by subduction-derived fluids and/or melts. The Tepai ophiolitic mélange exhibits similar zircon U–Pb–O isotopic compositions and whole-rock geochemical characteristics to those of the Quagan Qulu ophiolite. Therefore, we propose that the Tepai ophiolitic mélange may have been the western continuation of the Quagan Qulu ophiolite. Our new finding proves the final closure of the Paleo-Asian Ocean might have taken place later than the early Permian.