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

• Main Authors: Jiaming Yan, Fengyue Sun, Ye Qian, Liang Li, Yongsheng Zhang, Zhengping Yan
• Format: Article
• Language: English
• Published: MDPI AG 2019-05-01
• Series: Minerals
• Subjects: Akechukesai IV mafic–ultramafic complex; Hf–S–Pb isotopes; spinel and garnet lherzolite mantle; enriched primitive magma; post-collisional tectonic environment; eastern Kunlun orogenic belt (EKOB)
• Online Access: https://www.mdpi.com/2075-163X/9/5/275

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(∼5) and Zr/Hf (∼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 εHf(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 δ³⁴S values of 13.1‰−13.4‰ and 5.0‰−8.5‰, respectively, suggesting that the magmas that formed the complex assimilated crustal sulfur. They yield ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb 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.