Geochemistry of the ~326 Ma Xinyuan mafic intrusion in the Eastern Junggar Terrane, Northwest China: implications for tectonic setting and magmatic Ni–Cu mineralization potential

Abstract

The Xinyuan intrusion of the Eastern Junggar terrane is one of the mafic–ultramafic intrusions located in the Northern Xinjiang region and is associated with the southern part of the Central Asian Orogenic Belt (CAOB). Based on the secondary ion mass spectrometry (SIMS) zircon U–Pb dating, the intrusion is 326.2 ± 1.1 Ma old. Positive zircon ɛHf(t) values (+12.4 to +15.7) and mantle-like δ¹⁸O values (5.0–5.5‰) suggest that the parental magma was derived from a depleted mantle source with a low degree of crustal contamination. Significant negative Nb–Ta anomaly and large iron lithophile element (LILE) enrichment of the Xinyuan intrusion are identical to those of the coeval basalt–andesite rocks in the study area. Temporal, spatial, and geochemical evidence suggest that the Xinyuan intrusion and coeval basalt–andesite rocks are associated with the northward subduction of the Kelameili Ocean. This, together with the ages of alkaline granites and sedimentary record in the Eastern Junggar, suggests that closure of the Kelameili Ocean was probably between 326 Ma and 310 Ma. Fractional crystallization modelling indicates that the parental magma of the Xinyuan intrusion has experienced significant fractional crystallization at depth. The Xinyuan parental magma was estimated to contain ~20 ppm Ni, which is significantly lower than the coeval basaltic magma (~200 ppm). Magma that is depleted in Ni has potentially undergone extensive fractional crystallization, during which most of the Ni has been sequestered into olivine in the early stages. The Xinyuan intrusion has Hf isotope and trace element characteristics similar to the Permian sulphide-bearing mafic–ultramafic intrusions. However, the low degree of crustal contamination apparent in δ¹⁸O data is more characteristic for the pre-Permian sulphide-poor intrusions in the area. Extensive fractional crystallization and the low degree of crustal contamination in the parental magma of Xinyuan intrusion experienced are suggestive of a low-magmatic Ni–Cu mineralization potential.