Paleozoic tectonic evolution of the Dananhu-Tousuquan island arc belt, Eastern Tianshan: Constraints from the magmatism of the Yuhai porphyry Cu deposit, Xinjiang, NW China

Abstract

The Yuhai intrusions (quartz diorite, granite and pyroxene diorite) are located in the eastern part of the Dananhu-Tousuquan island arc belt of the Eastern Tianshan, and associated with the early Paleozoic porphyry Cu mineralization. LA-ICP-MS zircon U-Pb dating yielded emplacement ages of 443.5±4.1Ma for the quartz diorite, 325.4±2.5Ma for the granite, and 291±3.0Ma for the pyroxene diorite. These rocks are tholeiitic to calc-alkaline and metaluminous, with A/CNK values ranging from 0.66 to 1.10. The Silurian ore-bearing Yuhai quartz diorite is rich in LREEs and LILEs (e.g., K, Ba, Pb and Sr), and depleted in HREEs and HFSEs (e.g., Nb, Ta and Ti). These rocks are MgO-rich (1.90–3.80wt.%; Mg#=37–72), with high Sr/Y, La/Yb and Ba/Th ratios, positive εNd(t) (6.31–6.84) and εHf(t) (13.26–16.40), low (87Sr/86Sr)i (0.7037–0.7039), and low Nb/U and Ta/U ratios. The data suggest that the quartz diorite was generated by the partial melting of subducted juvenile oceanic slab. The oxygen fugacity (ƒO2) of the quartz diorite, calculated by zircon Ce4+/Ce3+ ratios, is higher than that of the granite and pyroxene diorite, implying that the quartz diorite was more favorable to porphyry Cu mineralization. The Carboniferous Yuhai granite reveals similar geochemical features with the quartz diorite, except for the lower Mg# (27–33), and the more elevated Th/U and Th/La ratios. Furthermore, these rocks also show high εNd(t) (5.2–5.8) and εHf(t) (11.03–14.85) values, and low (87Sr/86Sr)i (0.7036–0.7037). These features indicate that the parental magma of the granite was probably derived from a juvenile lower crust with no significant mantle component involvement. Different from the Yuhai quartz diorite and granite, the early Permian Yuhai pyroxene diorite contains low SiO2 (50.76–55.74 wt.%) and high MgO (3.96–4.33 wt.%; Mg#=40–44). The εNd(t), εHf(t) and (87Sr/86Sr)i values of the pyroxene diorite are 5.77–6.42, 7.99–12.10 and 0.7035–0.7040, respectively. The pyroxene diorite shows slight enrichments in LREEs ((La/Yb)N=2.04–2.55), Ba, U, K and Pb, weak depletions in Nb, Ta and Ti, elevated Th/U ratios, and low Ni contents and Ce/Pb ratios. Integrating with the regional tectonic evolution, we suggest that the pyroxene diorite was likely originated from the partial melting of depleted mantle metasomatized by subducted slab-derived fluids. In addition, crustal contamination likely occurred when the pyroxene diorite magma traversed the continental crust.Integrating our new results with published works on the early Paleozoic Dananhu-Tousuquan and Bogeda-Haerlike island arc belts, we propose that the Yuhai quartz diorite may have formed in a subduction setting related to the N-dipping subduction of the North Tianshan oceanic plate. The younger Yuhai granite was likely generated by the bipolar subduction of the North Tianshan oceanic plate, which formed both of the Dananhu-Tousuquan belt to the north and the Aqishan-Yamansu belt to the south. The youngest Yuhai pyroxene diorite was likely formed under a post-collisional extension setting after the Dananhu-Tousuquan and Aqishan-Yamansu belts had collided.