Geochronology、Geochemistry And Tectonic Implications Of Dashizhai-Chagan Rock Mass In Keyouqianqi, Inner Mongolia

The study area is in the middle section of Da Hinggan Mountains.By thegeological mapping and profiling survey work of the area, this article choosesDashizhai-Chagan rock mass of the center of map sheet and makes a detailed studybased on petrology, isotope chronology and geochemistry.By means of LA-ICP-MS U-Pb dating, this research obtains2emplacement ageof Dashizhai-Chagan rock mass:217±1Ma and218±2Ma.And the isotope ageindicates that the invaded age of this rock mass is in Middle Triassic Epoch.The geochemical data shows the major elements of Dashizhai-Chagan rock massare rich in alkali and silicon, and lacking in aluminum, magnesium and calcium. Therock mass belongs to metaluminous-peraluminous granite and it features high-Kcalc-alkaline series.The partition pattern of rare earth presents right deviation seagulltype curve. The fractionation degree of LREE is more obvious than HREE.Theeuropium presents strong negative anomaly. Trace elements enrich in HFSE （highfield strength elements). The content of LILE （large ion lithophile elements） suchas Ba and Sr is low, while Rb is in high abundance. It indicates in the diageneticprocess, plagioclase, apatite and ilmenite experienced fractional crystallization. Andthe magma experienced strong evolutionary divergence.Dashizhai-Chagan rock mass belongs to A type granite. And it can be judgedfurther as A2type granite according to the tectonic conditions. The rock mass formedin the postorogenic extensional tectonic environment, when the North China plate andSiberian plate had sutured. By calculating saturation temperature of zircon,Dashizhai-Chagan rock mass belongs to granite of high temperature. How the rockmass forms may be related to underplating of mantle-derived magmas withlithospheric thinning in the extensional tectonic environment. Integrating with217-218Ma obtained in the area, the closing time of Paleo-Asian Ocean should be latePaleozoic. This article favors the opinions of Wu Fuyuan, Sun Deyou and etc.