Late Paleozoic subduction system in the northern margin of the Alxa block,Altaids: Geochronological and geochemical evidences from ophiolites

The northern margin of the Alxa block (NMAB), located in the southernmost part of the Altaids, is important for understanding the tectonic processes associated with the closure of the Paleo-Asian ocean. In this study, we report results from our studies on two ophiolitic belts (the Enger Us and Quagan Qulu ophiolitic belts) to constrain the tectonic evolution of the Altaids. The tectonic blocks in the Enger Us ophiolite are mainly composed of ultramafic and mafic rocks, with a matrix comprising highly deformed Carboniferous clastic rocks and tuffs. Zircons from a pillow lava sample yielded SHRIMP zircon U–Pb age of 302 ± 14 Ma. Massive and pillow basalts in the Enger Us ophiolite exhibit N-MORB geochemical affinities, displaying high TiO₂ and low K₂O contents with tholeiitic signatures. They are characterized by depletion of light rare earth elements (LREEs) without fractionation of high field strength elements (HFSEs) and negative Nb–Ta anomalies. It is inferred that the magmas of these rocks were derived from a depleted mantle source in a mid-ocean ridge setting. The Quagan Qulu ophiolite is composed of tectonic blocks, including ultramafic, gabbros and siliceous rocks, and matrix, including deformed clastic rocks and limestones. Zircons in a gabbro sample from the Quagan Qulu ophiolite yielded SHRIMP zircon U–Pb age of 275 ± 3 Ma. The gabbros show high MgO contents, compatible elements (Ni, Co, Sc, and V), and Al₂O₃/TiO₂ ratios, but low TiO₂ and SiO₂ contents. They are enriched in large-ion lithophile elements (LILEs) and depleted in LREEs and HFSEs, indicating that they were derived from an extremely depleted mantle source which was infiltrated by a subduction-derived fluid or melt. Our geochemical data suggest that gabbros in the Quagan Qulu ophiolite were formed in a back-arc basin setting. A synthesis of evidence from geochemistry, regional geology, and paleobiogeography support the notion that the Enger Us ophiolitic belt represents the major suture of the Paleo-Asian Ocean in the NMAB and the Quagan Qulu ophiolitic belt represents a back-arc basin. These two ophiolitic belts, together with the Zongnaishan–Shalazhashan arc have been suggested to be a late Paleozoic ocean-arc–back-arc basin system in the southernmost part of the Altaids. The geochronological data suggest that the subduction process occurred even in the early Permian, indicating that the final closure of the Paleo-Asian Ocean might have taken place later than the early Permian.