Geochemistry, Geochronology, and Hf-S-Pb Isotopes of the Akechukesai IV Mafic-Ultramafic Complex, Western China

The Akechukesai IV mafic-ultramafic complex, located in the western segment of the eastern Kunlun orogenic belt (EKOB), represents a newly-discovered complex, containing Ni ores at grades of up to 0.98% Ni. It is dominated by olivine pyroxenite, pyroxenite, and gabbro units. The gabbros are enriched in lithophile elements (e.g., Rb, U, and K) and light rare-earth elements (LREE), with negative anomalies in high field-strength elements, except Zr, Ta. Nb/Ta(similar to 5) and Zr/Hf (similar to 10) ratios lower than the primitive mantle and chondrites, respectively, indicate the influence of the mantle metasomatic process or fractionation of accessory mineral phases. Zircon U-Pb dating of the gabbro yielded an age of 423.9 +/- 2.6 Ma, indicating that the complex formed contemporaneously with the Xiarihamu Ni deposit (423 +/- 1 Ma). The gabbro has negative epsilon Hf(t) values (-11.3 to -1.2) with corresponding T-DM1 ages of 1535-1092 Ma. The vein-like and disseminated mineralization (i.e., pyrite and pyrrhotite) have delta S-34 values of 13.1 parts per thousand-13.4 parts per thousand and 5.0 parts per thousand-8.5 parts per thousand, respectively, suggesting that the magmas that formed the complex assimilated crustal sulfur. They yield Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 values of 17.323-18.472, 15.422-15.626, and 37.610-38.327, respectively, indicating Pb derived from multiple sources (i.e., mantle crustal sources). Geochemical and Hf-S-Pb isotopic characteristics suggest that the complex formed from a primitive magma derived by partial melting of a spinel- and garnet-bearing lherzolite mantle at variable degree of 5%-10%. This source region was geochemically enriched by previous interaction with slab-related fluids. Tectonic reconstruction suggests that the Akechukesai IV complex was generated in a post-collisional extensional environment.