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.

In this study, we investigate the geochronology, geochemistry, zircon Lu–Hf isotopes and Sr–Nd isotopes of mafic–intermediate intrusive rocks in the Naxiguole area of the East Kunlun Orogenic Belt (EKOB), northwest China. Zircon U–Pb dating yields formation ages of 254.8 ± 1.2 Ma for gabbros and 251.9 ± 1.3 Ma for gabbrodiorites, suggesting that the mafic–intermediate intrusive rocks were formed in the late Permian. The gabbrodiorites show the geochemical characteristics of arc magmatic rocks, while gabbros were similar to those of backarc basin basalts. The gabbros yielded (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7068–0.7097, low ε_Nd(t) values of –4.40 to +0.16, and a limited range of ε_Hf(t) values of –1.09 to +0.04. The gabbrodiorites display (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7090–0.7098, ε_Nd(t) values ranging between –3.99 and –3.55, and ε_Hf(t) values of –0.81 to +0.34. The Sr–Nd and Lu–Hf isotopes from both types of samples resemble an enriched mantle but distinctly different from the crust. Based on their geochemical, and some trace elemental ratios (e.g. Ba/Th, La/Sm, Th/Yb and Sm/Yb), we suggest that the parental magmas of the gabbros and gabbrodiorites were formed by 1–5% partial melting of spinel lherzolite and garnet–spinel lherzolite, respectively. The gabbros originated from a mantle source modified by subducted slab fluids, while the gabbrodiorites were derived from a mantle source altered by subducted slab melts. By synthesising all the data, we propose that the gabbros were formed in a backarc extension setting and the gabbrodiorites in a continental arc setting. This suggests a geodynamic setting transition from backarc extension to subduction during the late Permian. In combination with previous research, we propose that the subduction mode in the Paleo-Tethys Ocean in the EKOB is intermittent.

The Paleoproterozoic Jiao-Liao-Ji Orogenic Belt is a part of the Eastern Block, North China Craton. The Shuangcha rock mass has been recently recognized in Laohe group of Jiao-Liao-Ji Orogenic Belt, and it consists of mainly porphyritic monzonitic granite and porphyritic K-feldspar granite. We present detailed studies of petrography, geochronology and geochemistry of Shuangcha rock mass to constrain the petrogenesis and tectonic evolution features of the Jiao-Liao-Ji Orogenic Belt. The mineral of the Shuangcha granite consists of K-feldspar, plagioclase, quartz and a small amount of biotite (350 ppm) and 10,000Ga/Al ratios (average 2.69), which are typical features of A-type granite. The Shuangcha A-type granites can be further subdivided into A₂-type subgroup based on some geochemical features, such as high Y/Nb ratios (0.99~3.14, average 1.84). Petrographic and geochemical studies have shown that the Shuangcha granites were the product of partial melting of quartzo-feldspathic rocks under a high temperature (>899°C), low pressure and anhydrous environment. Based on geochemical studies of Shuangcha granites, spatial and temporal distributions of regional magmatism and metamorphism records, the Jiao-Liao-Ji Orogenic Belt was in a typical post-collision extension setting in late Paleoproterozoic period (1.82~1.88 Ga) probably triggered by the slab breaks off.