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

摘要

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 delta O-18 values (6.5 to 8.0 parts per thousand) with a weighted mean value of 7.3 parts per thousand that is higher than the typical mantle value (similar to 5.3 parts per thousand). They are characterized by high epsilon(Hf)(t) values (-2.0 to 9.2) and variable epsilon(Nd)(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.