Discovery and tectonic significance of the Early Permian high-Mg diorite in the East Tianshan

The East Tianshan is characterized by the Early Permian metaluminous-peraluminous granitoids and mafic-ultramafic intrusions associated with Ni-Cu sulfide deposits. However, the absence of coeval diorite in this region hinders a better understanding of the tectonic-magmatic evolution of the East Tianshan in the Early Permian. The newly discovered Longdong diorite, located in the central part of the East Tianshan, provided an excellent rock record for the detailed study of the Early Permian tectonic-magmatic evolution of this area. Zircon U-Pb dating results demonstrate that the Longdong diorite was emplaced at ca.275.8±1.6Ma. This diorite is characterized by low SiO2(53.30%~54.87%) and K2O (0.82%~0.94%) contents, high MgO (5.83%~10.50%) and Na2O (2.60%~3.84%) contents and high Mg# (0.61~0.85) values, suggesting it belonging to a sodic calc-alkaline series of high-Mg diorite. It also has low Sr and Sr/Y, high Y, enrichment of Rb, Th and U, and depletion of Nb, Ta and Ti geochemical features, which is consistent with those of typical sanukitoids. Furthermore, low Ba, La/Yb, Th/Yb, and 87Sr/86Sr(t)(0.703367~0.703716) values, coupled with high εNd(t)=(+6.53~+7.81) and εHf(t)=(+12.40~+15.74) values, indicate that its parental magma was derived from partial melting of a mantle wedge metasomatized by fluids from a subducted oceanic slab. The existence of Early Permian high-Mg diorite confirms that oceanic subduction persisted in the East Tianshan during the Early Permian. Integrating the diversity of contemporary magmatism (e.g., MORB-type mafic rocks, and I-, S- and A-type igneous rocks, and high-Mg diorite), large-scale magmatic Ni-Cu mineralization and the dextral strike-slip movement, it is suggested that ridge subduction best explains the Early Permian tectonic-magmatic framework and associated magmatic Ni-Cu metallogeny in the East Tianshan.