Late Paleozoic-Mesozoic Granitoids From South Chifeng Area Of Inner Mongolia:Zircon U-Pb Geochronology,Geochemistry And Their Tectonic Significance

In this thesis,the Late Paleozoic-Mesozoic granite outcropped in the southern part of Chifeng City,northern margin of the North China Plate,is selected as the research targets.Based on the results of LA-ICP-MS zircon U-Pb chronology,major and trace elements geochemistry and zircon Hf isotopes,the chronological framework,genetic types and source properties of these granitic rocks are systematically discussed.Finally,combined with the characteristics of igneous petrotectonic assemblage,element geochemistry and and other regional geological data,the tectonic evolution process in southern Chifeng and northern margin of North China plate is discussed.The main conclusions of this thesis are as follows:1.Geochronological framework of the Late Paleozoic-Mesozoic magmatic activities in the Southern Chifeng area as well as the north margin of North China plateBased on LA-ICP-MS zircon U-Pb dating of granitic intrusive rocks in the southern part of Chifeng,and combined with regional magmatic rock distribution characteristics,stratigraphic unconformity and chronological data published by predecessors,the Late Paleozoic-Mesozoic magmatic activities in the southern part of Chifeng,including the northern margin of the North China plate,can be divided into three stages:Early Carboniferous to Middle Permian?356²65 Ma?,Late Permian to Late Triassic?260²05Ma?and Early Jurassic to Middle Cretaceous?201⁹8 Ma?.2.Early Carboniferous to Middle Permian:Southward subduction of the Paleo-Asian Ocean PlateThe Early Carboniferous to Middle Permian magmatic rocks in southern Chifeng area are represented by Kelidai biotite monzonitic granite.They belong to quasi-aluminous-weak peraluminous,calc-alkaline-high potassium calc-alkaline type I granite.They are rich in large ion lithophile elements and light rare earth elements,depleted high field strength elements and heavy rare earth elements,and zircon?_Hf?t?values and the two-stage model ages T_DM2 are of-14.13^-8.93 and 2.2-1.9 Ga,respectively,indicating that they could be mainly originated from partial melting of the Paleoproterozoic crustal material and were subducted by dehydration fluid of subduction plate.Combined with the constraints of contemporary sedimentology,paleontology and paleomagnetic evidences in the northern margin of the North China Plate,it is proved that the study area was in an active continental margin tectonic environment formed by the subduction of the Paleo-Asian Ocean during the Early Carboniferous to Middle Permian.3.Late Permian to Late Triassic:Extension after the collision between Siberian Plate and North China PlateLate Permian to Late Triassic magmatic rocks,represented by Sishijia biotite syenite granite,Jianping monzonite and Zhuanshanzi quartz syenite,belong to high-K calc-alkali-potassium basalt series,quasi-aluminous-weak peraluminous granite,zircon?_Hf?t?values range from-13.65 to+1.50,with a principal part of negative value,and the two-stage model ages T_DM2M2 ranges from 2.1 to 1.2 Ga.Rock types are mainly I-and A-type,porphyraceous textures and miarolitic structures are widely developed,indicating that they could be originated from the partial melting of Paleoproterozoic crust and intruded into an extensional tectonic environment.Combined with the constraints of paleontology and tectonic geology evidences in the northern margin of the North China Plate,it is proved that the final clouse of Paleo-Asian Ocean occurred from the Late Permian to the Late Triassic,and the study area was in a post-orogenic extensional tectonic environment.4.Early Jurassic-Middle Cretaceous:alternating effects from subduction of the Paleo-Pacific Plate and closure of the Mongolian-Okhotsk OceanEarly-Middle Jurassic magmatic rocks are represented by Shaoguoyingzi monzonitic granite which are rich in silicon and alkali,but poor in magnesium and calcium.They belong to the quasi-aluminous,calc-alkaline-high potassium calc-alkaline series and have high values of 10,000*Ga/Al and Zr+Nb+Ce+Y,can be classified as A-type granites.In combination with the transformation of magmatic activity in this period from calc-alkaline series in eastern Jilin-Heilongjiang to bimodal series in Zhangguangcai Mountain-Lesser Xingan Mountain area,it is indicated that the study area may be in a back-arc spreading tectonic environment caused by Paleo-Pacific subduction at this stage.Jurassic Oxfordian-Creataceous Hauterivian magmatic rocks are represented by the Duimiangou monzogranites and the Anjiayingzi granodiorites.They are rich in Sr and light rare earth elements,depleted in heavy rare earth elements,and have high Sr/Y and?La/Yb?_N ratios,belonging to the adakitic granites.Combined with the two main tectonic episodes of the Yanshanian movement in the Late Jurassic and Early Cretaceous,it is considered that the study area may be in the tectonic background of crustal compression and thickening during the Yanshanian period caused by the final closure of the Mongolian-Okhotsk Ocean.The magmatic rocks from the Barremian to Cenomanian in Cretaceous are a series of A-type intermediate-acid igneous rocks of high-K calc-alkaline-potassium basalt series,which represent the tectonic background of extension.The possible mechanisms that trigger this extensional background include the disintegration of the thickened lower crust during the late Jurassic-early Cretaceous period,the retreat of the subduction oceanic crust of the Paleo Pacific,or alternatively,the superposition of these two mechanisms.