LA-ICP-MS zircon U-Pb isotope age of granitoids in Baoertu area, Hoxud County, Xinjiang, and its geological significance

A lot of granitoids exist at Baoertu of Hoxud County in Xinjiang，lying in the middle segment of Middle Tianshan Mountains. Petrologic, mineralogical and geochemical investigations indicate that Wusutonggou pluton, which occurs in northern Baoertu area and belongs to Northern Tianshan Mountains, is composed of a suite of intermediate-acid intrusive rocks showing features of typical calc-alkaline volcanic arc granitoids which are enriched in LILE and depleted in HFSE and have genetic relation with the melting of the lower crust and the assimilation-hybridization of the upper crust resulting from the subduction of the ocean crust. The U-Pb LA-ICP-MS isotope age shows that the Wusutonggou pluton intruded at 358.9±3.5 Ma, belonging to the last stage of Devonian to the early stage of Lower Carboniferous. Wusutonggou granitoids resulted from the southward subduction of northern Tianshan Ocean beneath the middle Tianshan landmass, and could serve as the newest geological record of the subducting events in Devonian. Petrologic, mineralogical and geochemical investigations show that Hongshan pluton, which occurs in southern Baoertu area and belongs to Southern Tianshan Mountains, is a suite of acid intrusive rocks of typical calc-alkaline syn-collision granitoids, having typical characteristics of magma mixing and lower contents of Nb and Ta. Hongshan granitoids resulted from increasing thickness of continual crust and represented an important vertical development of the continual crust. LA-ICP-MS U-Pb isotope age shows that the Hongshan pluton intruded at 295.8±1.2 Ma, belonging to the last stage of Carboniferous to the early stage of Lower Permian. Studies show that granitoids in Baoertu area have recorded two important geological events and offered the important evidence for the tectonic evolution in Tianshan area.