Triassic magmatism and tectonic setting of the eastern Tianshan, NW China: Constraints from the Weiya intrusive complex

Abstract

The Weiya intrusive complex, located in the eastern part of the middle Tianshan terrane can provide crucial constraints on the tectonic evolution of the eastern Tianshan during the Triassic period. Here, we perform detailed zircon U–Pb chronological and Hf–O isotopic as well as whole-rock geochemical and Sr–Nd isotopic analyses on the hornblende gabbro, syenogranite, and granodiorite of the Weiya intrusive complex. The hornblende gabbro with a zircon U–Pb age of 241.6 ± 1.0 Ma is an alkaline gabbro and characterized by enrichments in large iron lithophile elements with depletions of Nb and Ta. It has uniform initial 87Sr/86Sr ratios (0.707173–0.707211) and positive εNd(t) values (+0.58 to +0.61) with zircon δ18O and εHf(t) values ranging from 6.0‰ to 7.6‰ and from +6.1 to +10.5, respectively. Geochemical modelling indicates that the mantle source of the hornblende gabbro was metasomatized by ~10% oceanic sediments associated with previous subduction events. The syenogranite (238.1 ± 2.0 Ma), emplaced nearly simultaneously with the hornblende gabbro, is an A-type granite with slightly metaluminous to peraluminous characteristics (A/CNK = 0.99–1.02), and it has high initial 87Sr/86Sr ratios (0.711740–0.713603) and negative εNd(t) values (−0.94 to −0.76). By contrast, the granodiorite (230.8 ± 0.9 Ma), which contains minor late Carboniferous dioritic enclaves (314.4 ± 3.3 Ma) belongs to the high-K calc-alkaline I-type granite with metaluminous characteristics (A/CNK = 0.95–0.98), and it has relatively low initial 87Sr/86Sr ratios (0.706731–0.707081) and negative εNd(t) values (−1.11 to −1.33). The δ18O and εHf(t) values of zircons from the syenogranite vary from 6.1‰ to 7.5‰ and from +2.8 to +5.3, respectively, while those of zircons from the granodiorite vary from 7.6‰ to 9.0‰ and from −8.1 to −3.4, respectively. These geochemical and isotopic data imply that the syenogranite formed by partial melting of Neoproterozoic calc-alkaline granitoids at high temperatures, whereas the granodiorite was derived from partial melts of (meta)-igneous rocks with additions of supracrustal sedimentary materials. Taking account into all available geological data, we suggest that the eastern Tianshan was in an intra-continental setting during the Triassic period, and the Weiya intrusive complex was generated related to lithospheric extension and thinning.