Geochronology and geochemistry of the Jinghan granitic pluton in South Tianshan, Xinjiang and their tectonic significance

The Jinghan granitic pluton, located in the north of Heshuo County, Xinjiang Autonomous Region, bears important information for studying the tectonic evolution of the South Tianshan orogenic belt. This pluton consists mainly of monzonitic granite and less amounts of quartz monzodiorite and two-mica granite. LA-ICP-MS zircon U-Pb dating yields ages of(316.1±3.9)Ma～(298±4)Ma, indicating that the pluton was emplaced in Late Carboniferous. Geochemically, this pluton shows wide variation of chemical compositions, with SiO₂ contents ranging from 54.20% to 74.22%, and total alkali values(ALK)from 5.12% to 9.25%, and show characteristics for rocks of high-K calc-alkaline series. Granitic rocks from this pluton have A/CNK values of 0.77～1.07, thus can be grouped into metaluminous-weakly peraluminous granitoids. Total REE contents of 29.65×10^-6 to 298.50×10^-6 and(La/Yb)_N ratios of 1.83 to 29.16 show an obvious enrichment of light rare earth elements. Weakly negative Eu anomalies with δEu values of 0.54 to 1.04 display characteristic of weak depletion. Geochemical characteristics suggest that the granitic magmas likely originated from partial melting of crust materials, and also underwent magma mixing with mantle-derived mafic magmas during the formation process. Combined with the regional tectonic settings and the discrimination results of trace elements, it is concluded that the Jinghan pluton was generated during the late stage of the collision between the Tarim Plate and the Yili-Kazakhstan Plate under a collisional to post-collisional tectonic setting. These results also imply that the late Paleozoic residual oceanic basin in the South Tianshan Belt was finally closed in late Carboniferous.