Late Paleozoic Magmatism And Its Constraints On Regional Tectonic Evolution In Linxi-Dongwuqi Area, Inner Mongolia

NE China and its adjacent area belong to the east segment of CAOB(Central Asian Orogenic Belt) on geotectonic setting,which is located between the Siberia plate,the North China plate and the Pacific plate.This region is adjacent to Siberia plate by Mongolia-Okhotsk suture belt to the west and north,adjacent to North China plate by Xar Moron River-Yanji suture belt to the south,and adjacent to Pacific plate by Sichote-Alin subduction belt to the east.CAOB is one of the regions that underwent significant Neoproterozoic-Phanerozoic continental crust growth in the world.This region experienced multi-stage geological evolution and strong reconstruction,the crust here has complicated substance composition,texture and structure,meanwhile the region possesses abundant and various mineral and oil-gas resources,so it is the ideal region for the research on dynamic evolution of continental lithosphere and mineralization.Magmatism is the product of the movement of upper mantle or deep crust on the surface or shallow crust of the earth,which recorded abundant information of the evolution of crust and mantle and their interaction in geological history.It is one of the important research contents to study rock assemblages,especially igneous rock assemblage or series to retrieve paleo-tectonic environment and discuss geodynamic processes of deep lithosphere.Linxi-Dongwuqi area is situated between the famous Hegenshan ophiolite belt and Xar Moron suture belt.This area experienced subduction of paleo-Asian plate,collisional orogenesis between North China plate and "Jiamusi-Mongolia Block".and subsequent extensional collapse.Different igneous assemblages formed in different tectonic environments,which recorded the tectonic evolution history of the CAOB.In this paper,the tectonic evolution history of SE part of CAOB would be studied through the research on late Paleozoic igneous petrotectonic assemblages combined with previous research achievements in regional stratigraphy, paleontology and tectonic geology,and further basic geological constraints on the formation and distribution of mineral and oil-gas resources will be provided. It is the basic work to carry out accurate geochronology on magmatic rocks to do further researches on the evolution of regional tectonic environment,so 13 zircon U-Pb ages of late Paleozoic volcanic rocks of Benbatu formation,Dashizhai formation and typical intrusive bodies are obtained in this paper.Combined with chronological data published recently,the magmatic activities during late Paleozoic and early Mesozoic could be divided into five periods:1.Late stage of Early Carboniferous-Early stage of Late Carboniferous(329～309Ma);2.Early stage of Early Permian(298～293Ma);3. Middle-late stage of Early Permian(286～270Ma);4.Early Triassic(252～241Ma);5. Late-Middle Triassic(239～204Ma).Tectonic evolution since late Paleozoic in Linxi-Dongwuqi area would be discussed on the geochronological framework.During late stage of Early Carboniferous and early stage of Late Carboniferous,the Suzuoqi-Xiwuqi magmatic arc was formed on the south margin of "Jiamusi-Mongolia Block" by the northward subduction of the paleo-Asian oceanic slab along Xilin Hot subduction zone.The magmatism of the arc is distributed in Suzuoqi,Xilin Hot,Xiwuqi and Zhaluteqi in the direction of northeast by east.The magmatic rocks comprise tholeiitic basalts and calk-alkaline volcanic rocks of Benbatu formation,and diorite and quartz diorite of Baiyinbaolidao,Daqihundi,Jinxing,Mishengmiao and other plutons.Associated with the magmatic arc,neritic clastic sedimentary rocks of Benbatu formation deposited in fore-arc setting to the south.Magmatism tended to be quiet in late stage of late-Carboniferous,and Amushan formation composed mainly by marine carbonate is formed during this period.Because of the relative motion of North China plate and "Jiamusi-Mongolia Block", the paleo-Asian oceanic slab rolled back during early stage of early-Permian,which caused the asthenosphere upwelling in Dongwuqi area and the formation of Hegenshan SSZ-type ophiolite in the back-arc extensional setting.The heat brought by the upwelling asthenosphere caused the lower continental crust to the north of Hegenshan area to melt. forming the Baiyinwula- Dongwuqi alkaline and alkali-feldspar granite belt represented by Hanbuyinbudun pluton.Following the magmatism of this period,the marine strata of Gegen'aobao formation occurred in the nearshore setting of the small Hegenshan Ocean. while the continental strata of Baoligaomiao formation occurred to the north with the alkaline and alkali-feldspar granite belt in time and space closely.Because of the continual subduction of paleo-Asian oceanic slab,the Hegenshan Ocean disappeared before long causing the emplacement of the Hegenshan ophiolite during its closure and the absence of early Permian sedimentation in Dongwuqi area. Because of the continual and quickly subduction of paleo-Asian plate during middle-late stage of Early Permian.intensive magmatism represented by the Dashizhai formation formed in the region.In the south part of Xar Moron River to Linxi area, occurred the tholeiitic and calk-alkaline assemblages of basalt,basaltic andesite and other rock types associated with subduction.Meanwhile or later,the breaking-off of subduction slab caused the upwelling of asthenosphere.Bimodal assemblage of igneous rocks occurred near the upwelling center in the north part of Xilin Hot-Xiwuqi area.The basic end-member comprised basalt,basaltic andesite,mugearite,trachyte andesite and gabbro, which display the affinity of fluid-metasomatized lithosphere mantle with minor superimposition and reformation by asthenosphere in geochemical characteristics.The acidic end-member is composed of rhyolite,andesite,trachyte,monzonitic granite,and miarolitic granite,which are the products of partial melting of lower crust caused by the underplating of mantle-derived magma and fractionation afterward.Middle-late stage of Early Permian is the most intensive period of magmatism during late Paleozoic.After this period magmatism tended to be quiet,middle Permian marine strata of.Zhesi formation and late Permian continental strata of Linxi formation developed widely in Linxi-dongwuqi area.According to recent studies,there was also southward subduction of paleo-Asian oceanic slab towards North China plate from late Carboniferous to middle Permian.which caused the emplacement of nearly east-westward calk-alkaline granitoid belt in Longhua, Fengning,Luanping of north Hebei province and Wulatezhongqi of Inner Mongolia.The disappearance of late Permian marine strata indicates the beginning of collisional orogenesis between North China plate and "Jiamusi-Mongolia Block".Lacustrine and lagoon sedimentation of late Permian Linxi formation deposited on the south margin of "Jiamusi-Mongolia Block".while there was not sedimentation on north margin of North China plate in the same period.From the occurrence of strata,it is indicated that there wasn't large scale orogenesis in Linxi-Dongwuqi area during the syn-collision stage,and the sedimentary setting was similar to foreland basin.The syn-collisional orogenic belt was distributed along the north margin of North China plate.Because of the subduction of "Jiamusi-Mongolia Block" towards north margin of North China plate in early Triassic. Dayushan and Chegendalai plutons formed in the tectonic setting of crust thickening.The root of the southward subducted slab ruptured under shearing force during the collision process.Magma originated from asthenosphere and lithosphere mantle was injected into the fracture zone forming the basalt and gabbo-diabase dikes in Linxi and Xilin Hot area. while the heat brought by the magma caused the earlier residual oceanic crust to melt forming the Jianshetun pluton with the characteristics of O-type adakites.During middle-late Triassic,the southward subducted paleo-Asian oceanic slab broke off,which induced the upwelling of asthenosphere and partial melting of lower crust under North China Craton,and forming the east-westward huge alkaline rock belt and basic-ultrabasic complex belt in Yan-Liao and Yinshan area.The breaking-off of subducted slab also caused the convection of asthenosphere in adjacent region,which induced extension and thinning of the lithosphere under CAOB and the forming of the post-orogenic A₂-type granites and volcanic rocks.The occurrence of all these alkaline rocks,A₂-type granites and basic-ultrabasic complexes indicated the termination of post collision/orogeny stage of CAOB.CAOB is one of the regions where significant Phanerozoic continental crust growth had happened.Through the study on the relation between late Paleozoic magmatism and tectonic evolution in Linxi-Dongwuqi area,it is thought that late Paleozoic is also an important period in continental crust growth of CAOB,and the accretion mechanism included lateral accretion related to subduction and vertical accretion related to underplating of basaltic magrna.There were different accretion mechanisms in different period,lateral accretion dominated during late Carboniferous and early Permian,while vertical accretion was the main pattern in middle and late Permian.Besides,different areas could have different accretion mechanisms even in the same period.Lateral accretion dominated in the south region between Linxi and Xar Moron suture belt in early Permian, while vertical accretion happened in the north region of Xilin Hot and Xiwuqi area.It is indicated that the continental crust growth might be a multistage process through the whole evolution history of CAOB.Mineral and oil-gas resources are the products under specific tectonic environment in specific geological history.According to recent researches,there might be a large paleo-continental block,named "Jiamusi-Mongolia block",in NE China and adjacent area. The Jiamusi-Mongolia block was formed in Paleozoic and bounded by the Mongolia-Okhotsk Suture Belt to the north,Xar Moron River-Yanji suture belt to the south and Sichote-Alin subduction belt to the east.Linxi-Dongwuqi area is located on the south margin of this block,through studies on late Paleozoic strata,magmatism and tectonic evolution,it is indicated that the middle Permian Zhesi formation and late Permian Linxi formation are the products under the sedimentary environments of stable continental margin basin and foreland basin respectively.There are abundant fossils in the both formations.They are potential hydrocarbon source rocks in NE China,and should become the important new strata for deep hydrocarbon exploration.NE China is located in east segment of CAOB,where the land area is up to millions of square kilometers.Numerous mineral and oil-gas resources are buried in the area, which were the products of different stages in the evolutionary process of the paleo-Asian Ocean.It is of important significance to study the coupling relationship between the forming age and tectonic settings of the resources and the evolution of paleo-Asian Ocean in searching for new mineral and oil-gas resources in NE China.