Evolution Of Magmatism Of Hanwula Extensional Granitic Dome In East Ujimqin Banner,China-Mongolia Border Area And Its Tectonic Settings

The Late Mesozoic(Jurassic-Cretaceous)period was a critical period for the geological evolution of the earth and the crustal movement.The most prominent feature of the Late Mesozoic in the Northeast Asian continent was the large-scale crust extension and magmatism,and a large number of granitoid intrusions formed one of the largest granite provinces in the world.In order to understand the Late Mesozoic tectonic-magmatic evolution of the HanWula granitic dome of the China-Mongolian border,the study is carried out on the HanWula granite extensional dome in the northern of the Dong Ujimqin,and the petrology and the zircon U-Pb ages and the zircon Hf isotopes and the geochemical were studied for the Hamosaige and Shamai rocks in the Hanwula extensional dome,and discussed the formation age,magmatism evolution and the relationgship with the structural process,and dating for the tectonic evolution of the Middle Jurassic and early Cretaceous.The Hanwula dome is located in the Dong Ujimqin on the Sino-Mongolia border.Its main body is composed of Early Cretaceous Biotite monzogranite,biotite-bearing monzogranite,potassium granite and early emplacement of the middle-late Jurassic biotite monzogranites and syenite.The whole is distributed in the direction of northeasten 30~50 degrees,the North East and South West extending to Mongolia,and the total outcropping length is about 70~75Km in China.The structural study shows that the hanwula dome underwent the northward extension of the upper plate.The zircon U-Pb age of granitic mylonites and footwall weakly deformed granite in the extensional shear zone is 141~128Ma,and has high temperature deformation texture and syntectonic characteristics.Therefore,the Hanwula dome is the same extensional dome formed during the early Cretaceous intrusion and cooling uplift.The mid-Jurassic Haimosaige rock is mainly composed of biotite monzogranite and syenite,the U-Pb age of zircon is 165~163Ma.Hornblende Syenite rock belongs to the shoshonitic series with metaluminous and weakly peraluminous features and has the characteristics of A-type granites.Biotite monzogranite is a weakly peraluminous high-K calc-alkaline series with highly fractionated I-type granite;Early Cretaceous Shamai rock mainly consists of biotite monzogranite,biotite-bearing monzogranite and potassium granite,the U-Pb age of zircon is 141~128Ma,all belonging to weakly peraluminous high-K calc-alkaline series,characterized by I-A transitional granites.The zircon Hf isotopic compositions indicate that the two magmatic activities all show the characteristics of young crust sources,Haimosaige pluton have positive zircon ?Hf(t)values of +8.2~+10.3,and the model ages are 0.63~0.50 Ga;Shamai pluton the zircon ?Hf(t)values are +3.1~+13.5,and the model ages are 0.98~0.31 Ga.Combined with regional background,the Middle Jurassic syenite in the Haimosaige are formed that crustal thickened melting in the early stage of the crust extension thinning,Granitic rocks are formed by partially melting of lower crust during the early stage of the crust extension thinning,and the extension background is more likely to be related to the end of the subduction or beginning of the extension of the Mongolia-Okhotsk ocean;the early Cretaceous granitoid of the Shamai are produced by the mixture of the mantle magma and crustal magma under the extensional thinning background,and the extension background may be more close to the closure of the Mongolia-Okhotsk ocean,and superposed by the back-arc extension of the paleo Pacific subduction.In NE Asia,the Jurassic to Cretaceous granitic magmatism evolution in the Hanwula area is consistent with the crustal extension process,indicating the evolutionary process of the crust in the area from deeper extensional in early stage to large-scale extensional thinning in the peak period(late stage),it provides favorable conditions for the formation of the Sha Mo deposit in the area.This study shows that the early deep extension of the northeastern Asian region started about 163 Ma starting from the deep crust,and the 145~130Ma reached the peak and continued to 130~120Ma.