Geochemical Characteristics And Tectonic Significance Of Late Paleozoic Intrusive Rocks In Bijigaibulusitai-Daijingquequeque Area, Southern Margin Of Mid-Tianshan Mountains, Xinjiang

The Bijigaibulusitai-Daijingquekeque area is located in the small Bulusitai station in the middle of the suture zone in the southern margin of the Middle Tianshan Mountains.The Late Paleozoic intrusive rocks are developed in the area,which is an important area for studying the tectonic evolution of the Middle Tianshan Mountains.The intrusive rocks in the area are generally distributed in the northwest-south direction.The Bijigaibulusitai rock mass is mainly composed of quartz monzonite,diorite,granodiorite and granite;the Yikaibulusitai rock is composed of diorite,monzonitic granite and quartz diorite;the Daijingquekeque rock mass is composed of diorite,monzonitic granite,granite porphyry,and syenite granite.Zircon U-Pb dating shows that the formation age of quartz diorite of Bijigaibulusitai intrusion in the southern margin of the Middle Tianshan Mountains is 341.8±2.1 Ma,The formation age of quartz diorite in the Ikaibusitai intrusion is 309±3.8 Ma.and the age of monzonitic and granite porphyry in the Daijingquekeque intrusion is 394.9±2.9Ma,and 298.3±2.2Ma,which reveals the Devonian to Late Carboniferous magmatism in the southern margin of the Middle Tianshan Mountains.Geochemical analysis of Daijingquekeque monzonitic granite shows that it is mainly a set of peraluminous rocks of high-potassium calc-alkaline series,with strong enrichment of light rare earth elements and strong depletion of heavy rare earth elements,indicating the characteristics of high-potassium calc-alkaline I-type granite in active continental margin arc.Similarly,the Devonian diorite in Yikaibulutai belongs to calc-alkaline series of quasi-aluminous rocks,which are the products of arc magmatism related to oceanic subduction environment.Ocean subduction,representing the region,has at least begun in the Devonian basin.The Early Carboniferous quartz monzonite is mainly a set of alkaline-calcium-alkaline quasi-aluminous rocks with high?La/Yb?_N=9.7-11.94,and weaker Eu Abnormal or no abnormality??Eu=1.02-1.53?,trace elements appear as relatively large enriched lithophile elements,and high field strength elements are relatively depleted,which is characteristic of subduction-related I-type granites.The Late Carboniferous quartz diorite in the Bruce station also shows that the rare earth element distribution type is obviously right-dip type,the light rare earth is strongly enriched,and the heavy rare earth is strongly depleted,which is formed in the active continental margin island arc environment related to subduction.It may represent that the region's southern Tianshan Ocean continues to subduct in the Late Carboniferous.The formation of the late Carboniferous in the late Carboniferous but the granite porphyry w?Al₂O₃?=13.96-14.30,A/CNK=1.02-1.08<1.1,the molecular volume of corundum is less than 1%,showing the genetic characteristics of the I-type granite.The DI index reaches 94.5,which is formed after the high-differential evolution of the low-temperature calc-alkaline type I granite slurry related to the collision environment.The magma is mainly derived from the shell-source calc-alkaline series.Geochronology and geochemistry of the granitoid system in the Bijigaibukusitai-Daijingquekeque area,prove that the oceanic basin on the Southern Margin of Mid-Tianshan Mountains began to subduct northward in the Ordovician,continued to subduct to the Late Carboniferous,closed at the end of the Late Carboniferous,and entered the post-collision stage in the Permian.