The Discussion Of Characteristics And Tectonic Settings Of Adakites In Northern Daxin'anling Area,Inner Mongolia

This paper firstly summarizes the research status of the adakite,defines adakite and discusses its controversy,concludes the progress in the field of experimental petrology,the tectonic settings and genesis mechanisms of adakite and its connection with deposits.Additionally,this article discusses the characteristics and its tectonic implications of the adakites,which were found in three different ages in Northern Daxing'anling.The quartz diorite adakite formed at the age of 457±12Ma.Its Si O₂,Al₂O₃,Total REE,Sr,Yb,Y,?Eu contents range 56.95?58.64%,15.35?16.91%,110.27?174.34ppm,332?756ppm,2.8?3.3ppm,25.71?22.96 and 0.82?0.92,respectively.It's characterized by low magnesium contents.The research results show it formed in the subduction environment due to the partial melting of the ocean crust,which indicates that till the late Ordovicianthe the Songnen block was moving to the northern Xing'an block.Late Carboniferous granitoids adakite formed in two periods(321Ma?330Ma and 306Ma?314Ma).The early intrusion contents Si O₂,Al₂O₃,Total REE,Sr,Yb,Y,?Eu rang 53.20?68.42%,15.36?21.19%,55.83?121.84ppm,589.4?985ppm,1.02?2.12ppm,8.63?15.80ppm,0.90?1.0,respectively.The late intrusion contents Si O₂,Al₂O₃,Total REE,Sr,Yb,Y,?Eu rang64.16?69.28%,14.31?18.86%,79.48?150.70ppm,341?597.6ppm,0.81?3.05ppm,4.22?16.83ppm,0.76?0.99,respectively.It's characterized by high magnesium contents due to the contamination of the mental wedge.It formed in the subduction environment due to the partial melting of the ocean crust,which indicates that till the middle-late Carboniferous the Xiang'an block subducted northward to the Ergun block.The granitic diorite U-Pb isotopic ages are between 148.96?132.74Ma and quartz monzonite U-Pb isotopic ages are between 132.5?142.51Ma,which both belong to early Cretaceous.The granitic diorite contents Si O₂,Al₂O₃,Total REE,Sr,Yb,Y,?Eu rang 66.28?68.82%,4.46?15.43%,131.61?179.80 ppm,427.3?593.0ppm,1.42?1.78 ppm,12.01?16.69 ppm,0.69?0.89,respectively.The quartz monzonite contents Si O₂,Al₂O₃,Total REE,Sr,Yb,Y,?Eu rang 62.82?66.98%,15.54?18.29%,128.44?171.96ppm,515.4?1039ppm,1.36?1.87ppm,12.28?17.19ppm,0.89?1.22,respectively.It's characterized by low magnesium contents and shows the C type adakite geochemical characters.It derived from the melting of the basaltic invasion in lower crust,which may implicate the beginning of the crustal thinning process.The tectonic environment was the post-collision stage,which suggests that Mongolia–Okhotsk orogenic belt began to collapse.