Geochronology, geochemistry, and Sr–Nd–Hf isotopic compositions of mafic–ultramafic intrusions in the Niubiziliang Ni–(Cu) sulfide deposit, North Qaidam Orogenic Belt, NW China: Implications for magmatic source, geodynamic setting, and petrogenesis

Abstract

Whole-rock geochemical and SrNd isotopic data, and zircon U–Pb–Hf isotopic data are reported for mafic–ultramafic intrusions in the Niubiziliang Ni–(Cu) sulfide deposit, located in the westernmost segment of the North Qaidam Orogenic Belt (NQOB), NW China. NiCu sulfide ores of economic interest are found within these intrusions, with the Niubiziliang III and IV intrusions hosting most of the Ni–(Cu) mineralization. The Niubiziiang mafic–ultramafic intrusions intrude biotite–plagioclase gneiss of the Paleoproterozoic Dakendaban Group. Zircons separated from the gabbro, plagioclase-bearing clinopyroxenite, and olivine websterite yield UPb ages of 389 ± 2, 373 ± 2, and 380 ± 2 Ma, respectively, indicating a genetic linkage with Middle–Late Devonian regional magmatism (390–370 Ma). Gabbro zircons have εHf(t) values of 4.2–10.9 with older TDM1 ages (0.86–0.59 Ga). The Niubiziliang intrusions contain many xenoliths of Paleoproterozoic biotite–plagioclase gneiss, and the intrusions have similar trace element patterns with strong enrichment in LILEs (e.g., K, Rb, and Th) and moderate depletion in HFSEs (e.g., Nb, Ta, P, and Ti). The major and trace element characteristics, and Sr–Nd–Hf isotopic compositions indicate the parental magmas originated from a metasomatized, asthenospheric mantle source that had previously been modified by subduction-related fluids. The magmas experienced significant crustal contamination both in the magma chamber and during ascent, triggering sulfur over-saturation that resulted in the deposition and enrichment of sulfides. Considering the tectonic evolution of this region, we infer that the Niubiziliang mafic–ultramafic intrusions formed in a post-collisional extensional setting related to delamination of the subcontinental lithosphere.