Tectonic framework of the northern Junggar Basin part I: The eastern Luliang Uplift and its link with the East Junggar terrane

Abstract

The Luliang Uplift preserves relatively complete Carboniferous strata and is an important region to study the spatio-temporal framework and tectonic evolution of the Junggar Basin. Here we report new SHRIMP/SIMS U–Pb ages, whole rock Sr–Nd data, and in-situ zircon Hf isotopic compositions for Carboniferous strata samples in the eastern Luliang Uplift. The gravity and magnetic data suggest that the Carboniferous volcanics are distributed as NW-trending bands and also bring out N–S oriented parallel amalgamation. The sedimentary units and structural deformation of the eastern Luliang Uplift suggest that the basin evolution involved Late Carboniferous extensional graben stage and latest Carboniferous–Early Permian compressional stage. The Carboniferous Batamayineishan Formation is divided into the lower volcanic unit followed up sedimentary unit and an upper volcanic unit as inferred from the lithology and unconformities. The lower volcanic unit from the drill cores mainly consists of andesite, dacite, rhyolite and tuff intercalated with minor basalt. The middle sedimentary unit is made up of sandstone and mudstone with tuff and tuffaceous sandstone, and the upper volcanic unit is composed of basalt, rhyolite, tuff and breccia with minor clastic sedimentary rocks. Zircon SHRIMP U–Pb age (337.2Ma) for the tuff and SIMS U–Pb age (308.6Ma) for basalt suggest that the lower and upper volcanic units belong to Lower Carboniferous and Upper Carboniferous, respectively. The basalts from upper volcanic unit in the eastern Luliang Uplift show typical calc-alkaline affinity with low Nb/Y ratios (0.17–0.28) and SiO2 contents (47–54%) and moderate Th abundances (0.45–3.74ppm), enrichment in LREE and LILEs and depletion in Nb, Ta and Ti. They have low initial 87Sr/86Sr ratios (0.703571–0.705122) and positive εNd(t) values (5.5–8.6) and constant Sm/Yb ratios (1.56–2.03), and variable trace element ratios (e.g., Ba/Th, Ba/La and Ba/Nb), suggesting that the parental magmas were mainly derived from 2 to 8% partial melting of a spinel Iherzolite mantle source metasomatized by hydrous fluids and melts of subducted sediments. Furthermore, relatively high TiO2 (1.52–3.76%) and Zr contents (172–414ppm) indicate that they were formed in an extensional setting. Taking into account the occurrence of coeval island arc-related volcanics, we propose an intra-arc rift basin model for the eastern Luliang Uplift. The eastern Luliang Uplift and East Junggar terrane display different tectonic features during the closure of the Junggar Ocean. During the Late Carboniferous, continued northward subduction prevailed and generated a fault-controlled intra-arc basin in the eastern Luliang Uplift. Intra-arc basin contraction in Permian is probably indicative of the closure of Junggar Ocean in the northern Junggar Basin. Our study confirms the important role of subduction–accretion process associated with the Carboniferous crustal growth in the Junggar terrane.