Geochronology and petrogenesis of the composite Zuluhong Granite, North Xinjiang Province of China: Implications for the crust-mantle interaction and continental crustal growth in Western Tianshan Orogen

Abstract

The mineralogy, whole rock geochemistry, zircon U-Pb-Hf isotopic compositions, and Sr–Nd isotopes were analysed from the composite Zuluhong Granite at the Alataw Mountains in NW China. The aim is to determine the granite's nature and tectonic setting, and gain a better understanding of the continental growth of the Western Tianshan Orogen (WTO), which is commonly regarded as part of the Central Asian Orogen (CAO). The Zuluhong Granite is divided into a monzogranite and porphyritic granodiorite. The monzogranite yields a zircon U–Pb date of 320 ± 2 Ma interpreted as its emplacement age, and the porphyritic granodiorite yields a zircon U–Pb date of 309 ± 1 Ma interpreted as its emplacement age. The monzogranite is characterised by its high SiO2 content (72.7–74.4 wt%), moderate 10,000 × Ga/Al values(2.59–2.81), strongly negative Eu anomaly (0.39–0.43), and relatively low zircon saturation temperatures (Tzr) of 726–807 °C, which are indicative of a highly fractionated monzogranite. The combined arc-like geochemical signatures (e.g. enrichment in LILE and LREE relative to HFSE, negative anomalies in Nb and Ta), and relatively high positive εNd(t) values of +4.4 to +4.8, indicate that the monzogranite originated from the partial melting of arc rocks in the lower crust. In contrast, the granodiorite has higher Al2O3 assays averaging 16 wt% and Sr/Y ratios averaging 40, and lower Yb assays of 1.38–1.49 ppm with a weak Eu anomaly, which are characteristics of adakite-like rocks. The combination of the granodiorite's relatively lower SiO2 content of 63.7–65.3 wt%, higher Mg# values of 53–57, higher εHf(t) values of +10.9 to +13.4, and complex oscillatory zoned plagioclase phenocrysts are indicative of a source derived from material derived from the mantle and lower crust. Our data on the Zuluhong Granite, coupled with the results of previous studies of the Late Carboniferous to Early Permian magmatism in the WTO, indicate that a regional tectonic transition from compression to extension took place in the CAO during the end of the Early Carboniferous (ca. 320 Ma). Furthermore, substantial post-collisional reworking of the Precambrian rocks along the northeastern margin of the Yili Terrane is more pronounced than the West Junggar Terrane to the east. The generation of widespread magmatism and continental growth in the CAO in a post-collisional tectonic setting including the recycling of earlier juvenile-arc complexes, and underplating of mantle-derived magma.