Geochemistry and zircon U-Pb geochronology of the oxidaban intrusive complex: Implication for Paleozoic tectonic evolution of the South Tianshan Orogenic Belt, China

Abstract

The South Tianshan Orogenic Belt (STOB) occupies the southwestern margin of the Central Asian Orogenic Belt (CAOB), and is the key region to understanding the Paleozoic evolution of the southern part of the Palo-Asian Ocean. Here we present an integrated study of zircon U–Pb ages, whole-rock major and trace elements and Sr–Nd isotopes on Silurian and Late Carboniferous intermediate-felsic intrusions exposed in the Oxidaban area located in the South Tianshan region in Xinjiang, NW China. The Silurian intrusion is composed of quartz diorite, granodiorite and monzogranite, whereas the Late Carboniferous rocks include granite porphyry and granite porphyry enclaves. LA–ICP–MS zircon U–Pb dating yields ages of 426.8 ± 4 Ma for quartz diorite and 424.5 ± 4 Ma for monzogranite, respectively. The quartz diorite shows middle–K calcic and metaluminous affinity, and the granodiorite exhibits similar features, but peraluminous. They display enrichment in large-ion lithophile elements (LILEs) and depletion of high field strength elements (HFSEs) together with negative Eu anomalies, broadly comparable with typical arc-type rocks. They have similar Sr–Nd isotopic compositions, with a narrow range in (87Sr/86Sr)i of 0.70578 to 0.70644 and εNd(t) of −0.95 to −2.16. Based on geochemical and isotopic characteristics, we infer that the granodiorite is produced by fractional crystallization of quartz diorite rocks, which were dominantly produced by partial melting of medium to high–K basaltic protoliths. Besides, the monzogranite also exhibits arc-type geochemical signatures, which we correlate to partial melting of middle-lower juvenile crust and subsequent contamination by ancient crust during ascent and emplacement. Zircon LA–ICP–MS U–Pb analyses yielded crystallization ages of 300.4 ± 1.5 Ma for the granite porphyry and 304.8 ± 2.3 Ma for the granite porphyry enclave. These rocks are dominantly shoshonite and peraluminous, and are likely derived from partial melting of the greywacke mixed with minor pelitic components. The low Al2O3/TiO2 (<100) ratios suggest a high temperature condition, which may be attributed to heat input from underplated mantle-derived magmas in the post-collision extension setting. Base on these scenarios, we propose that the bi-directional subduction of the South Tianshan Ocean occurred during the Early Paleozoic and the subduction polarity changed to northward during Middle Devonian, followed by the collision between Yili-Central Tianshan Block and Tarim Craton in the Late Carboniferous.