Geochemical Characteristics Of Neoproterozoic Granites From The Xingxingxia,Eastern Tianshan,and Their Tectonic Significance

Numbers of variable types of the Neoproterozoic granites were identified in Xingxingxia area,eastern Tianshan orogenic belt.Xingxingxia area is located in the middle of Junggar and Tarim landmass,the middle of the Central Asian orogenic belt,where magmatic rocks are well developed.It is helpful to understand the collisional orogeny in the process of aggregation of the Rodinia supercontinent by studying the Neoproterozoic medium-grained monzonitic granite and fine-grained granodiorite exposed in Xingxingxia area through the methods of petrology,geochemistry and zircon U-Pb geochronology.Both types of granites from Xingxingxia area have weak gneissic structure,and the major element data shows that they are characterized by high SiO₂?>70%?,high Al₂O₃?>12%?,slightly rich in potassium?5%^?and less Ti and poor in P.The trace and rare earth element data show the characteristics of rich Rb,Th and U,poor Nb and Sr,and the ratio of Rb/Sr is larger?0.38-3.87?.In the rare earth element data,light rare earth elements??REE?are enriched and europium is depleted.In the normalized distribution curve of the rare earth element chondrites,the two types of rocks are right-leaning smooth curves.All these characteristics indicate that the Neoproterozoic granites in Xingxingxia area are quasi-aluminous-weak peraluminous calc-alkaline granites,and belong to I-type granite in genetic type,reflecting that the magma source area is crystallized after remelting of the lower crust.Applying LA-ICP-MS zircon U-Pb dating method,it is found that the age of intermediate-grained monzonite and medium-fine-grained granodiorite in the study area is 949.7±6.1 Ma and 927.6±7.5 Ma respectively.Both of them belong to Neoproterozoic.Based on the reliable zircon U-Pb dating results obtained in Pingdingshan,Kawabulake,Baluntai and Nalati areas around Xingxingxia,it can be inferred that the Neoproterozoic granites exposed in the East Tianshan orogenic belt were mainly formed at 960-910 Ma,and that the early Neoproterozoic magmatism widely existed in the Middle Tianshan Block was proved.Based on the geological characteristics of the Middle Tianshan Block and Tarim Plates and the dating data obtained,it is considered that the Neoproterozoic granites in Xingxingxia area should be formed in volcanic arc tectonic environment,which is related to plate subduction.In the early Neoproterozoic,the Middle Tianshan Block should be located on the outer edge of the Rodinia Supercontinent and influenced by the circum-Rodinia subduction zone.