Geochronology,Geochemistry And Geological Significance Of Late Mesozoic Volcanic Rocks Of Deerbuer Area In Inner Mongolia

Deerbuer is located in Genhe city,Inner Mongolia,belong to the northern segment of the Great Xing'an Rang.It has experienced a series of geological tectonic activities since the Paleozoic,forming a variety of volcanic rocks.Through the field geological surveying to get the regional geological background in this area.Based on the study of petrography,chronology and geochemistry,this paper discusses the magmatic activity,rock genesis and tectonic background,combined with previous research,restore the tectonic evolution process of the Great Xing'an Rang.The main results are as follows:?1?Tamulangou Formation is mainly composed of andesite,andesitic tuff,pyroxene andesite and basaltic andesite.LA-ICP-MS zircon U-Pb dating show that the most zircon of Tamulangou Formation are captured zircon,based on the analysis of stratigraphic contacts,it is inferred that Tamulangou Formation was formed in the Middle Jurassic.The age of captured zircon ranged from?1625±36?to?2661±27?Ma,indicating the existence of Precambrian basement.Rocks rich in aluminum?Al₂O₃=16.68%¹6.91%?and magnesium?Mg^#=50.37⁵2.34?,belong to high-K calc-alkaline series,quasi-aluminous volcanic rocks.It is similar to the distribution pattern of continental island arc elements which intense enrichment in LILEs?Ba,Sr?,depletion in HFSEs?Nb,Ta?and weak negative?Eu??Eu=0.92⁰.96?.The Tamulangou Formation were formed in the tectonic setting of subducting active continental margin,result from partial melting of continental lithospheric mantle by subduction fluid replacement.?2?The Manketouebo Formation is mainly composed of rhyolite,spherulite rhyolite,rhyolitic tuff and rhyolite volcanic breccia.The LA-ICP-MS zircon U-Pb dating show that the age of Manketouebo Formation is 155±1.8Ma,which is Late Jurassic.The Manketouebo Formation is composed of peraluminous high-silica rhyolite,which is enriched in silicon?Si2O=74.7%⁷5.44%?and K?K₂O=4.97%⁵.85%?,depleted in Na?Na₂O=0.1%⁰.12%?and Ca?CaO=0.14%⁰.16%?.The volcanic rocks of the Manketouebo Formation have the characteristics of S-type granite which the Rb/Sr ratio is 4.1¹0.3,the La_N/Yb_N ratio is 13.98²0.83,enrichment of Zr,Cs and Rb,depletion of Ba,Nb,Ta and P,strong depletion of Sr and Ti,obvious negative anomaly of?Eu??Eu=0.59⁰.7?.It is product of further evolution of magma under MUSH model in magma chamber,which originally formed by the partial melting of thickened lower crust.During the late Middle Jurassic,the tectonic setting of the regional continental collision.?3?The Manitu Formation is mainly composed of andesite,andesitic tuff and dacite tuff.The LA-ICP-MS zircon U-Pb dating show that the age of Manitu Formation is between 122.8±1.3Ma and 124.1±0.8Ma,which is Early Cretaceous.HREE and LREE are obviously fractionated,enriched Zr,Rb,Th and U,defective Sr,Ba,Nb,Ta,P and Ti,and obviously negative Eu abnormal??Eu=0.65⁰.76?.The volcanic rocks have the characteristics of A-type granite with the ratio of LaN/YbN is 9.22¹0.36 and the Rb/Sr ratio is 0.31¹.21?average value is 0.58?,it was formed by crustal decompression melting in extension background.?4?The age of volcanic diagenesis is characterized by a gradual youth from north to south.Combined with previous researches,it is concluded that the tectonic environment in the Great Xing'an Range was that the Mongolia-Okhotsk Ocean crust experienced southward subduction in Middle Jurassic.In the Late Middle Jurassic,the Mongolia-Okhotsk Ocean was closed with continental collision environment.In Early Cretaceous,the Great Xing'an Range was in a post orogenicextensional tectonic environment.It is concluded that the Late Mesozoic volcanic activities in the Great Xing'an Range were mainly controlled by the Mongolia-Okhotsk Ocean tectonic domain.