Characteristic And Petrographic Mapping Of Mesozoic Volcanic Rocks In Mid-Eastern Da Hinggan Ling Of Inner Mongolia

The Mesozoic volcanic rocks are widely distributed in the Mid-Eastern DaHingganLing of Inner Mongolia,it can be mainly classified into:Manketouebo Formation,Manitu Formation and Baiyingaolao Formation,which the most well-development is Manketouebo Formation.The volcanic rocks is dominated by tuff and rhyolitic ignimbrite.Based on detailed field geological investigation,main elements,trace elements and isotope of U-Pb ages will be described,and the nature of magma source,petrogenesis and tectonic settings will be discussed in this paper.The result of SHRIMP ziron U-Pb dating of Manketou Formation,Manitu Formation and Baiyingaolao Formation are 141.5±2.3Ma?143.1±2.3Ma and 133.8±2.1Ma respectively.Major elements characteristics show that Mesozoic volcanic rocks in the study area are silica-rich and high content of alkali,poor in magnesium and short of calcium.Averge value of Rittman index is 2.4,belongs to calc-alkalin series.The conten of SiO₂ in Manketouebo Formation and Baiyingaolao Formation are obviously higher than Manitu Formation,and there is negative correlation between SiO₂ and TiO₂,TFe₂O₃,Al₂O₃,CaO,MgO,P₂O₅,however,the relationship between Na₂O,K₂O and SiO₂ is positively.The trace elements are characterized by enrichment in high field strength elements such as Ta,Th,U and depletion in large-ion lithophile elements like Ba and Sr.The chondrite-normalized REE paterns indicate that the volcanic rocks are enriched in light rare earth elements.The total of the rare earth elements?REE change from 69.52 to242.57ug/g,the content of light rare elements?LREE?change from 61.55 to 222.97ug/g,and the heavy rare earth elements?REE?content is between 7.06 to 22.08ug/g.The value of?La/Yb?_N range from 4.13 to 25.41,means the fractionation between the light rare earth and the heavy rare earth is obvious.Both the Manketouebo Formation and the Baiyingaolao Formation are middle-stron negative Eu anomaly,but this charecteristics is not found in the Manitu Formation.The lithofacies of the research area can be divided into explosive-sedimentary phase,explosive phase and volcanic vent phase and the explosive phase can be further divided into clastic flow accumulative phase,void accumulative phase and caving accumulative phase.The volcanic activity in the Mesozoic can be divided into the early cycle and the late cycle according to the stragraphic contact relationship in the research area.The early cycle include the Manketouebo Formation and the intermediate-acid rock in the Manitu Formation and the late cycle is chiefly composed of the Baiyingaolao Formation.The volcanic rocks in research area have the similar characteristics to the crustal magma,maybe formed by partial melting of the crust.The source rock of the Manitu Formation could be from the lower crust,and the Manketouebo Formation and the Baiyingaolao Formation maybe evolved from the shallow to middle crust.Combing the achievement of previous studies,it is concluded that the Mesozoic volcanic rocks in the Mid-Eastern DaHingganLing were formed by the effect of the extensional environment caused by the ancient Pacific subducting the Asian continent.