Characteristics And Tectonic Environment Of Qiaolunengeci Granite In Ejina County

Ejina area,Inner Mongolia,is located in the middle of the Central Asian orogenic belt,Which is belonging to Beishan orogenic belt and is the key part of connection the western part of the Altai-Xingmeng orogenic belt(Xinjiang,China)and the eastern section(northeast China).So it is an important area for studying the tectonic evolution of the ancient Asian Ocean.Starting from the study of granite,this paper provides evidences for magmatic rocks in order to restore the Triassic tectonic evolution process in this area.The previous study of magmatic rocks in the area is very weak,and is only classified as Hercynian granites.This seriously restricts the understanding of the evolution process of the ancient Asian Ocean.In this paper,through the comprehensive study on the field and indoor,considered that Qiaolunengeci granite is a composite intrusive rocks,containing monzonitic granite,porphyritic monzonitic granite and syenogranite monzonitic granite.Porphyritic monzonitic granite have clear facies belts,and the central and marginal facies are divided respectively.The rock types include fine grain,medium grain,coarse grain monzonitic granites,medium grain,coarse grain porphyritic monzonitic granites and syenogranite.Petrochemistry is characterized by potassium rich,aluminum rich,but poor magnesium and calcium.It belongs to a high-K calc-alkaline series,which is a quasi aluminous transition to peraluminous rock.Geochemical characteristics show the granites have properties of arc granite.The magma originated mainly from the thickened crust.Geological characteristics and Zircon U-Pb isotope dating results indicate that the granite belong to the Middle Triassic,and its formation age is 236.8±2.1Ma(MSWD=1.7).The granite is formed in the post collision tectonic environment.These new evidences reveal that the ancient Asian Ocean had been closed in the Ejina area before the the Middle Triassic.