Geochemical characteristics and petrogenesis of Late Carboniferous A-type granites in the middle section of Xiemisitai Mountain, West Junggar

A large number of Late Carboniferous A-type granites developed in the middle of Xiemisitai Mountain, West Junggar.The main lithologies are quartz syenite-syenogranite-orthoclase granite porphyry-monzonitic granite.The LA-ICP-MS zircon U-Pb dating results show that the weighted mean ages of orthoclas granite porphyry, quart syenite, syenogranite and monzonitic granite are 301.7±2.5 Ma, 302.1±3.6 Ma, 301.6±3.1 Ma and 305.3±2.5 Ma.The rocks belong to the high potassium calc-alkaline and shoshonite series, they have high silicon, rich alkal, peraluminous, low calcium, low magnesium, high TFeO/MgO value(>3.45)and other characteristics.The rocks are relatively enriched in light rare earth elements(except Eu)and large ion lithophile elements(Rb, K), relatively depleted in Ba, Nb, Ta, and strongly depleted in Sr, P, Ti, which is consistent with the characteristics of typical aluminum A-type granite.The ε_Hf(t)of the rocks is positive(8.93~14.82, average 12.64), and the two-stage model age(average 452.01 Ma of syenogranite and average 565.17 Ma of monzogranite)is similar to the formation age.Accordingly, it is considered that the A-type granite in this area is formed in the post-collision extension background.Due to the upwelling of the asthenosphere, the melting of the lower crust previously metasomatized by the subduction-related fluid occured.It has experienced the crystallization differentiation of minerals such as plagioclase, pyroxene, apatite and sphene, and showed volcanic arc imprinting(A₂ type)in the early stage of magmatic activity.With the further extension of the plate, the characteristics of intraplate rift(A₁ type)appear, which provides a new basis for regional geological evolution.