| On the basis of recent investigation in north continental margin of North China Platcr'orm, the author consider that there is a tectonic morbile belt in north continental margin, which was formed in Middle Proterozoic era. This tectonic active belt extends from the western of Inner Mongolia to the west of Liaoning Province and Jilin province, the overall length is over 2000km and the wide is about 100km. It consists of low—grade metamorphic rocks, plutons which intruded in Middle proterozoic era and varied tectonites. The tectonic active belt is bounded on the south by North China ancient continental, the bound line between the two units lies from Wuyuan, Jining in Inner Mongolia to JiuMiao in the west of LiNing Province, Showing a liner gravity anomaly belt. Geological field work indicatcs that one main faulted zone coincide with the linear gravity anomalies.There are many metamorphic sedimentary stratigraphies in the tectonic active belt of ancient continental margin, including zhaertay group,Baiyun'ebo group, Weijiaguo rock group and Seluohe group.Zhaertay group and Baiyun'ebo group occurred in Inner Mongolia, Weijiaguo rock group in the west of Liaoning province,Seluohe group in JiLin province. According to primary sedimentary formation, tectonic setting and isotopic ages, those metamorphic stratigraphies can be divided into three types. The first kind of statigraphyies developed in the ancient rift which occurred in continental margin, which consists of low—grade metamorphic clastics with some carbonate rocks. Some volcanic rocks exist in the bottow of rift basin,which are composed of alkali basalts, trachyandesites,basalts and rhyodacites, collectively about 635m thick.Some picrite inclusions are found in mafic lavas The plotting of the vocanics in TiO2—K20—P205 diagram shows they were erupted in the continental Margin rift and belong to the weak—to the strong—alkaline series.Along the rift a series of intrusions of alkali mafic rocks, aegirinite, and syenite are scattered in the middle Inner Mongolia and the northern Hebei province. The carbonatites containing monazite near Baiyun'ebo give a isotopic age of 1678Ma,showing the age of rift formed. There it is suggested that the northern margin of North China Platform shared the same environment of tensile tectomism within the plate in early stage of the middle proterozic era.The second kind stratigraphy developed in passive continental set ting,which is composed of clastic rocks,si 1iceous 1imestone,dolomitic limestone, and argillite. The primary formation characterized by extensively distributing from Inner Mongolia to the west of LiaoNing province and JiLin province. The U—Pb age is about 1.6—1.44Ga for the formation of primary rock. Without volcanics developed during passive development, Showing crust stable.The third kine stratigraphy developed in active continental margin, being further divdided into two types sedimentary formation, island arc volcanic formation and ophiolite. Island arc formation extensively distributed from Inner mongolia to the west of Liaoning province and JiLin province, including Bainaimiao group, Weijiaguo rock group and Seluohe group, showing volcanic island arc chain from west to east in Middle proterozoic era. The plot tings of volcanics in Na2O+K20-Si02 and SiO2—Fe*/MgO diagram show that they belong to calc-alkaline series, in logt -logo and TiO2-SiO2 diagram show that they were erupted in active continental margin of orogenic belt setting. The U-Pb age is 1.4—l.OGa for those volcanic formation.Wendurmiao ophiolite lies in Inner Mongolia.ophvolite suit consits of metamorphic harzburgite, gabbroic cumulate, sheeted diabase, mafic pillow lava, and abyssal sedimentary rocks. Thses rocks occur generally in the form of detached structurae blocks. The ophiolite and abyssalsilliceous rocks are unconformably overlain by palaeozoic stratigraphy. Wendurmiao group have been metamorphosed to form various schists of greeschist and blueschist facies. In all greenschist facies rock., stilpnoraelane is discovered. The blueschists of Wendurmiao region contain glaucophane,lawsonite,and other minerals.The metamorphic age of the bluschist, presumably related to a subduct ion,zone,is about 1.OGa.According to crust deformation mechanism, deformation condtions, structural elements overprinting, and the properties of magma action, the crust tectonic deformation in the ancient continental margin of North China Plateform, can be divided into two phases. The fist is characterized by crust extensional deformation, and occurred during early middle proterozoic era. Beausc of effect of this deformation, downfaulted basins and synsedimentary faults had been formed along ancient continental margin, they controlled local sedimentary stratigraphy, sedimentary enviroment, and mineralization.The second phase crust deformation took place in late middle proterozoic, and was strong orogenic continental movement. The driving force for orogenesis were caused by ancient Asia ocean subducted to North China Plateform. The evidence associated with sudduction is only remained as some ophiolite and high — pressure metamorphic belt in Wendurmiao region in Inner Monglia The second phase crust deformation can be divided into three stages of deformation based on structural elements overprinting and deformation conditions. The fist stage of deformation is characterized by bedding glide under high greenschist facies. Its structural elements include bedding rheid folds, bedding schistosity Slf and bedding ductile shear zones. The second stage of deformation is crust compression deformation, which causted the metamorphic stratigraphy to fold. Some big tight folds and inverted folds developed during the second stages of deformation, their axial lines extend in west-east direction. Basides folds, the structural elements of this deformation stage include axial schistosity S2iinineral stretching linerations, and ductile shear zones. The last stage of deformation is thrust-naprnng deformation, which had formed some largethrust-mapping ducttile shear zones.According to Sedimentation, magamatism, tectonism and metamorphimz described above in the area, the evolution model of north ancient continental margion of North China plateform in middle proterozoic era is established, and discusses relationship between tectonic enviroraer and mineralization.
|
PDF Full Text Request