Permian oceanic slab subduction in the southmost of Central Asian Orogenic Belt: Evidence from adakite and high-Mg diorite in the southern Beishan

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Abundant Permian magmatism is exposed in the southern Beishan Orogenic Collage, and records key details on the closure of the Paleo-Asian Ocean. The Xiaoxigong granites, exposed in the southern Beishan orogen, exhibit middle Permian zircon UPb ages (269–267 Ma). They are sodium-rich and exhibit adakite-like geochemical characteristics, such as high Sr content and low Yb and Y contents, coupled with high Sr/Y (99–120) and (La/Yb)N (41–66) values. The Xiaoxigong granites are depleted in whole-rock Nd isotopes [εNd(t) = +4.2 to +4.5] and zircon Hf isotopes [εHf(t) = +4.3 to +11.4]. Geochemical compositions show that the Xiaoxigong granites formed by partial melting of the subducted Liuyuan oceanic slab, and subducted sediments also played important roles. Two types of dikes were identified in the early Permian host granite (~280 Ma) in the Yinwaxia area. The Group I dikes (267 Ma) are calc-alkaline gabbros, exhibit some subduction-derived geochemical imprints, and are interpreted as derived from an E-MORB-like source that was metasomatized by subduction-related fluids; the Group II dikes are sodium-rich, calc-alkaline diorites, and exhibit high MgO (5.45–6.73 wt%) and compatible element contents, and depleted isotopic compositions. They are sanukitic high-Mg diorites derived from interactions between melts of subducted sediments and overlying mantle peridotites. The association of contemporaneous high-Mg dioritic dikes, arc-like gabbroic dikes, and adakitic granites indicate middle Permian oceanic slab subduction in the southern Beishan. Furthermore, we propose that a subducting oceanic slab window can account for Permian magmatism in the Shibanshan unit, and that the final termination of the Paleo-Asian Ocean in the southern Beishan occurred after the middle Permian.