Geochronology and petrogenesis of the Late Silurian Shitoukengde mafic–ultramafic intrusion, NW China: Implications for the tectonic setting and magmatic Ni-Cu mineralization in the East Kunlun Orogenic Belt

Abstract

ABSTRACT The Shitoukengde mafic–ultramafic intrusion, located in the East Kunlun Orogenic Belt, is spatially and temporally associated with the Xiarihamu Ni-Cu sulphide deposit, the second-largest Ni deposit in China. This intrusion comprises mainly lherzolite, harzburgite, olivine websterite, and gabbroic rocks. The Shitoukengde mafic–ultramafic rocks are enriched in light rare earth elements (REE) relative to heavy REE and show pronounced negative Nb–Ta anomalies. Magmatic zircon crystals of the studied samples have high δ18O values (6.5 to 8.0‰) with a weighted mean value of 7.3‰ that is higher than the typical mantle value (~5.3‰). They are characterized by high εHf(t) values (–2.0 to 9.2) and variable εNd(t) values (–4.46 to 2.83). Olivine grains from the ultramafic rocks have high forsterite (Fo) values of 79.3–86.8 and variable Ni contents of 746–3347 ppm. Our calculations indicate that parental magma of the Shitoukengde intrusion had a high-Mg basaltic composition with 10.9 wt% MgO and 9.8 wt% FeOT. Fractional crystallization and significant crustal contamination triggered S saturation and sulphide enrichment in the magma. Zircon LA-ICPMS and SIMS U–Pb analyses reveal the consistent Silurian ages of 420–424 Ma for the gabbronorite and Ol websterite, which is coeval with the Xiarihamu Ni-Cu deposit. Based on these data and the regional tectonic setting, we suggest that the Shitoukengde mafic-ultramafic rocks formed in the post-collisional, extensional environment. The cessation of subduction may have resulted in break-off of the dense subducted slab, and triggered ascent of the hot convective asthenospheric mantle. The mantle wedge was modified by the subduction-related fluids and these enriched mantle components were incorporated into the ascending asthenospheric melts at relatively shallow depths, generating the high-Mg basaltic magmas that formed the compositionally variable mafic-ultramafic rocks (e.g., Shitoukengde and Xiarihamu intrusions) in the Eastern Kunlun Orogenic belt.