Temporal variations in the mantle source beneath the Eastern Tianshan nickel belt and implications for Ni–Cu mineralization potential

Abstract

Many small mafic–ultramafic intrusions are found in the Eastern Tianshan Ni belt along the southern margin of the Central Asian Orogenic Belt (CAOB). The Ni–Cu mineralized Hulu intrusion is located in the eastern part of the Jueluotage area of the Eastern Tianshan Ni belt. This mafic–ultramafic intrusion comprises a shallow layered sequence and deeper dike-like unit. Precise zircon U–Pb dating yielded ages of 381.0 ± 1.3 and 377.0 ± 3.0 Ma for sulfide-bearing samples from the layered unit and 388.6 ± 2.7 Ma for a mineralized gabbro from the dike-like unit. 176Hf/177Hf values of zircons from the layered unit range from 0.282846 to 0.283165 and correspond to εHf(t) values of +10.6 to +20.4. δ18O values for these zircons range from +4.8‰ to +5.5‰. Similarly, zircons from the sulfide-bearing gabbro of the dike-like unit have εHf(t) and δ18O values from +13.0 to +18.2 and +5.0‰ to +5.6‰, respectively. Normalized SiO2, Al2O3, TiO2, and Na2O + K2O contents increase with decreasing MgO content, whereas the FeOtotal content is positively correlated with MgO, which indicates that the different lithologies resulted from the fractional crystallization of a common parental magma. Furthermore, the mafic–ultramafic intrusions and their calculated parental magmas in the Jueluotage area have similar rare earth element (REE) patterns with slight enrichment of the light REE relative to heavy REE. All of the rocks and estimated parental magmas exhibit depletions in high-field-strength elements (HFSE; e.g., Nb, Ta, and Zr) and enrichments in large-ion lithophile elements (LILE; e.g., U). The pronounced light REE and LILE enrichments and HFSE depletions of the mafic–ultramafic intrusions and calculated parental magmas are indicative of a subduction-modified mantle source. Clinopyroxenes in the Hulu intrusion have variable Al2O3 (4.39–7.32 wt%) and low TiO2 (0.33–0.67 wt%) contents, which are comparable to typical Alaskan-type complexes that were emplaced in subduction environments. In a TiO2 versus Alz (AlIV × 100/2) diagram, data for the Devonian mafic–ultramafic intrusions in the Jueluotage area fall within the fields of Alaskan-type complexes, whereas data for most of the Permian intrusions fall outside of these fields. Systematic in situ zircon O and Hf isotope analyses from the Devonian mafic–ultramafic intrusions in the Jueluotage area exhibit higher δ18O and lower εHf(t) values than those of the Permian intrusions, which may be explained by a change from a Devonian fluid- to Permian fluid- and melt-modified mantle source. The mafic–ultramafic intrusions that were emplaced in both subduction and post-collisional extensional environments in the Eastern Tianshan Ni belt were favorable hosts to magmatic Ni–Cu deposits, despite their different mantle sources.