Late Carboniferous closure of the Junggar-Balkhash Ocean: Insights from the early Permian post-accretionary magmatism in the Barleik Mountains of West Junggar, NW China

Abstract

Early Permian magmatic rocks in the West Junggar Terrane (WJT, NW China) contain crucial information about the final closure of the Junggar-Balkhash Ocean (JBO) in the southwestern Altaids, whereas their tectonic settings are inconclusive. In this study, we conduct an integrated study incorporating the geochronology, geochemistry and zircon Hf isotopes of a set of volcanic and intrusive rocks from the northwestern Barleik Mountains in the central WJT. Zircon UPb ages of eight volcanic and felsic–intermediate intrusive samples are ∼292–281Ma, corresponding to a regional magmatic flare-up in the central WJT. The volcanic samples include rhyolitic tuff, andesitic tuff, and rhyolitic andesite, characterized by high-K calc-alkaline to shoshonitic I-type granite features. They yield positive εHf(t) values with a narrow range of 9.40–13.52 and two-stage model ages from 421 Ma to 672 Ma. Above isotopic features, in combination with their relatively low Mg# values of 25–35, demonstrate that they were generated by partial melting of the juvenile lower crust. Felsic intrusive rocks include peraluminous I-type granitic porphyry and A2-type K-feldspar granite, and their low Mg# values (21–35) and trace element ratios suggest the partial melting from lower crustal rocks. By contrast, intermediate intrusive rocks include monzonite, diorite and a dioritic dyke, which represent fractionated partial melts sourced from the underplated depleted mantle-derived magma. Based on the occurrence of I-type and A2-type rocks, together with the coeval extensional strike-slip fault systems and a late Carboniferous angular unconformity, we infer that the early Permian magmatism in the central WJT resulted from an extensional post-accretionary setting. Taken together with similar geological evidence from the WJT and East Kazakhstan, the final closure of the JBO is suggested to occur in the late Carboniferous.