Petrogenesis of the Bashisuogong bimodal igneous complex in southwest Tianshan Mountains, China: Implications for the Tarim Large Igneous Province

Abstract

The Bashisuogong (BSSG) complex is located in the tectonic transition zone between South Tianshan Collisional Belt (STCB) and Tarim Block (TB). The complex is composed of a mafic unit (mainly including gabbro and diabase) and a felsic unit (mainly composed of quartz syenite and alkali granite), both of which are crosscut by several diabase and alkali granite dykes. Here we present LA-ICP-MS zircon U–Pb data which show that the two units were emplaced coevally at ~276Ma. The rocks from the two units display a wide SiO2 gap, implying a typical bimodal magmatic feature. The petrographic and geochemical evidence such as mafic microgranular enclave (MME) within the syenite, negative linear correlations between SiO2 and some major elements, and a wide range of (87Sr/86Sr)i suggest that the syenite formed via magma mixing process. The positive εNd(t) values (+5.42 to +5.66 for gabbro and +4.70 to +5.02 for diabase) and OIB-like geochemical features of the mafic unit indicate that the parent magma was derived from asthenospheric mantle or mantle plume. The felsic unit shows higher contents of SiO2, K2O and total alkalis. Their trace element patterns are characterized by Rb, Y, Zr and Hf enrichment, and high 10,000Ga/Al ratios, indicating an A1-type affinity. The syenite shows εNd(t) values in the range of −0.15 to +0.30 and zircon εHf(t) values of +1.68 to +5.10, whereas the alkali granite has εNd(t) values of −2.10 to −1.92 and εHf(t) values of −4.10 to +0.32. The two stage Hf isotope model ages of zircon grains in the syenite are older than 1.0Ga, whereas those of the alkali granite are even older (>1.3Ga). Our results suggest that the alkali granite was generated by partial melting of a Neoproterozoic gabbroic source. Zircon grains in the A1-type felsic intrusions yield high Zr saturation temperature (728–983°C). Although the OIB-like affinities and high temperature zircon grains of the BSSG complex suggest a genetic link with the Tarim mantle plume, considering the Late Carboniferous–Early Permian A-type granites commonly occurring in the STCB, Altay and circum-Junggar region as narrow linear belts, we correlate the magmatism with post-collisional setting rather than a mantle plume event. The effect of Tarim mantle plume might not have reached beyond the north of Tianshan.