The formation age and volcanic rock geochemical study of Songshugou strata in the Sawuer Mountain

Abstract：Songshugou strata in the west of the Junggar Basin are made up of clastic sedimentary rocks and volcanic rocks. The volcanic rocks are mainly basalt, andesite and volcanic clastic rock, in which basalt belongs to calc-alkaline series and andesite belongs to porphyry series. Zircon U - Pb dating of these rocks shows that the age of zircon in andesite is （317.1±6.4）Ma. In 56 zircon samples, only one age is（389±9）Ma, suggesting the middle Devonian, whereas all other ages are concentrated in 319 ~359 Ma, with the youngest one being （319±6）Ma. It is thus held that the strata were probably formed at the early stage of the late Carboniferous rather than in the middle Devonian. Geochemical studies show that the basalt and andesite have similar REE partition curves as well as right-oblique REE patterns of rich light rare earth elements, and are enriched in high ionic elements such as K, Rb, Ba, Sr and depleted in high field strength elements such as Nb, Ta, Th, Zr and Hf, with Ta, Nb, Ti negative anomalies. It is concluded that the volcanic rocks were formed in an island-arc environment and the magma was derived from mantle wedge formed by metasomatism of subducting plate dehydration that produced fluid. The formation of island arc volcanic rock restricted the closing time of the zhaisang basin(later than 317.1 Ma).