Geodynamic setting and metallogenic potential of Permian large-sized mafic-ultramafic intrusions in Beishan area, Xinjiang, China

Abstract: Permian large-sized mafic-ultramafic complexes such as Pobei, Luodong, Bijiashan, Hongshishan and Xuanwoling, are developed as a cluster in Beishan area, Xinjiang. They are mainly composed of peridotite, pyroxenite and gabbro. They have significant cumulate rhythmic layers of magmatic minerals, and are commonly considered to be layered mafic-ultramafic intrusions hosting Ni-Cu sulfide deposits. Their intrusive ages (260.7-289 Ma) are close to the age of the Tarim large igneous province. Beishan layered mafic-ultramafic intrusions show positive correlation between Fo values and Ni content of olivine minerals. Their trace elements are characterized by negative anomalies of high field strength elements (Zr, Hf, Nb, Ta and Ti), and richness of Rb, Sr and Pb in comparison with the primitive mantle (PM). They have LREE-depleted-flat rare earth element (REE) patterns. The rocks display low PGE content and IPGE-depleted PGE primitive mantle normalized distribution patterns. The Sr, Nd isotopes are located between depleted mantle (DM) and OIB ranges, whereas Re-Os isotopic compositions overlap the ranges of mantle plume magmatic Noril’sk Cu-Ni sulfide deposits and Emeishan basalts. Simulated calculations show that the parental magmas of Beishan complexes had high MgO content and high magmatic initial liquidus temperature and crystallization temperature, and underwent fractional crystallization and different degrees of crustal contamination in their evolution process. Parental magma experienced sulfide segregation in the mantle source or during the emplacement, and resulted in depletion of PGE. It is inferred that large-sized mafic-ultramafic complexes in Beishan area were formed by high degree of partial melting of lithospheric mantle modified by subduction fluid at high temperature due to lithosphere delamination and asthenosphere upwelling triggered by a superposed mantle plume in a post-collisional extension tectonic setting, and thus show large metallogenic potentials for the formation of magmatic deposits.