THE EVOLUTION OF LATE PALEOZOIC TECTONIC - DEPOSITIONAL ENVIRONMENT IN SIKESHU SAG, JUNGGAR BASIN, NW CHINA

The Late Paleozoic was a critical period for the closure of the ancient Tianshan Ocean. During this period, the Sikeshu Sag on the southern margin of the Junggar Basin experienced a complex process of ocean-continent transition. The determination of the tectonic and sedimentary environment in the Late Paleozoic is helpful to research. the evolution process of the Central Asian tectonic domain. Therefore, based on previous geological research, through detailed field geological survey and profile survey, this paper uses zircon SHRIMP U-Pb isotopic dating to constrain the age of volcanic rocks exposed in the piedmont. According to its lithologic type and geochemical characteristics, the Paleozoic sedimentary environment of Sikeshu Sag has been systematically studied and analyzed. The trace elements of the Devonian volcanic rocks show that they are enriched in LREE, LILE and depleted in Nb and Ta. It is speculated that they were formed in the continental margin arc environment where the oceanic crust is subducted. The positive anomalies of rare earth elements Ba and Eu in Carboniferous volcanic rocks and Th / Zr (> 0.01) and Nb / Zr (< 0.2) indicate that volcanic activities are frequent in the island arc environment. The negative anomalies of transition metal trace elements Cr, and the positive anomalies of Co, indicating that the shallow part of the crust was invaded or sprayed by deep magma. The trace elements characteristics of Permian volcanic rocks (Rb, Nb, Y, Ce, Ga) indicate the depositional environment at the convergent edge of the plate. The volcanic-sedimentary rock assemblages on the field profile includes, basalt, granite, volcanic breccia and tuff with OIB-type geochemical characteristics, indicating that the volcanic activity is gradually weakening and the range of volcanic activities is gradually decreasing, which also reflects the Sikeshu sag. The transition of the Late Paleozoic sedimentary environment from the Devonian remnant ocean-island arc are environment to the Permian active continental margins, also indicates the closure of the ancient Tianshan Ocean to a certain extent, and recorded key information in the evolution of the Central Asian tectonic domain and the formation of the Tianshan orogenic belt.