A Study On The Late Paleozoic Tectonic Framework And Its Evolutionary Process In The Northern Margin Of The West Qinling

The clastic sediments of basin is not only the direct evidence and important symbol ofsedimentary basin and tectonic evolution, but also the organic combination of provenance andsedimentary basin controlled by regional tectonic background. The clastic sediments recordedthe evolution history of sea-land transformation, the formation of basin-mountain andinteraction between surrounding environment, through the analysis of clastic sediments, theorogenic process can be restored and then regional tectonic evolution and tectonic frameworkcould be discussed,which has become a hot topic today. Late Paleozoic is an importanttransformation stage that West Qinling microplate which inherited orogenic plate subductioncollision in early Paleozoic transformed to the next evolution stage. Thus, this thesis discussthe provenance and the nature of the basin,and finally determine tectonic deformationsequence and the ancient tectonic environment of formation through the research on Devoniansedimentary formation and some of the Carboniferous sedimentary formations includingShujiaba group, Dacaotan group and Xihanshui group in Tianshui area of northern WestQinling margin from several aspects such as sedimentary characteristics, clastic composition,petrogeochemistry, detrital zircon LA–ICP–MS U–Pb ages. And on this basis, combined withthe latest research results of regional geological background, the following implications havebeen achieved:1、The study on detrital component statistic shows that the detritus of the sandstonesamples from Shujiaba Group is given priority to with metamorphic rock detritus, followed bysedimentary rock detritus, volcanic rock detritus content is relatively low. According toDickinson triangle figure discriminant, the detrital rock provenance of Shujiaba Group iscutting type arc source and orogenic belt turning source. The characteristics anddiscrimination diagrams of petrogeochemistry primary quantity and trace elements exhibitpolyphyly and relative stability of detrital composition from Shujiaba Group, it also showsthat the source area of Dacaotan Group has close relationship with the continental island arcand continental margin environment, and there are obvious differences in some of the rockgeochemical characteristics with Shujiaba Group.2、The study of detrital zircon age shows that detrital zircons in sample TS182fromShujiaba Group has5age groups including425～541Ma、786～967Ma、1066～1175Ma、1597～2044Ma、2490～2613Ma. The423～541Ma group accounts for about20%of thetotal, the group of786～967Ma has25zircons, accounts for about42%of the total, is a agegroup which take largest overall proportion; The detrital zircons of TS192can be roughly divided into5age groups including413～523Ma and728～988Ma,1131～1396Ma,1574～1856Ma and1856～2090Ma. The age group of413～523Ma has25zircons, accounts forabout42%of the total,which takes over the largest overall proportion; Detrital zircons fromDacaotan Group have4age groups which included422～518Ma，756～887Ma，1100～1231Ma，2417～2736Ma.The age group of422～518Ma take over the largest proportion. Thesamples of two formations reflect information of4age stages(Caledonian, Neoproterozoic,Mesoproterozoic, Proterozoic–Archean), Dacaotan Group is given priority to with Caledonianperiod age group, while Shujiaba Group given priority to with Neoproterozoic andCaledonian age stages, and the difference in age distribution of Shujiaba Group’s samples alsoreflect that different sedimentary stages have different focus provenances.Combined withregional magmatic activities,the North Qinling orogenic belt and Qilian orogenic belt providethe main source for Shujiaba Group. The rock series of orogenic belt crystalline basement(part of the basement rock series with North China plate properties) are also the possiblesource. While the main source area of Dacaotan group is northernWest Qinling margin.3、According to depositional characteristics, Shujiaba Group has typical characteristics ofdeep-sea turbidite(Baoma sequence), the depositional environment of Shujiaba Group isbathyal to abyssal environment; From north to south,the palinspastic of Dacaotan Group basinis braided river sedimentary system, flood plain and fan bay, semi deep lake-deep lakedepositional environment,and the sdepositional formation as a set of lacustrine facies coarseclastic rocks of continental sedimentary combination, which has an obvious difference withShujiaba Group; Middle-late Devonian Xihanshui group is a set of neritic shelf facies,carbonate platform facies depositional formation; The Carboniferous formations includeCarboniferous in Mayanhe-Gaoqiao area, Carboniferous in Lixian area and Carboniferous inYanghe-Gucheng, the depositional environment of Carboniferous in Mayanhe-Gaoqiao areais carbonate platform and deep water basin, the depositional environment of Carboniferous inLixian area includes five types of depositional environment which are nearshoreshelf,offshore shelf, basin and slope,steep coast margin and transition facies of seacontinent,the depositional environment of Carboniferous in Yanghe-Gucheng consists of the debris flow-low density clastic turbidite deposit and neritic carbonate shelf deposit.4、Based on the comprehensive analysis about the deformation characteristics of the fourlate Paleozoic geological units and the boundary faults between the four geological unitsincluding Shujiaba Group,Dacaotan Group,Xihanshui Group and carboniferous in northernmargin of West-Qinling, the dominant tectonic framework of northern margin of West Qinling is the production of NWW trending folds and a series of thrust nappe structure superimposedon it, which formed in the comprehensive continent-continent collision orogenic process inIndosinian. And several phases of tectonic deformation are analyzed from each geologicalunit.5、Combined with regional geological background, the ancient flow and above evidence,the basin prototype of mid-Devonian Shujiaba Group is a fault basin which formed in thelocal area with the tectonic environment starts from extrusion to stretch after collisionorogenic, Shujiaba Group is a deep sea-bathyal facies sedimentary formation which depositedin the fault basin; Middle-late Devonian Xihanshui Group is a set of neritic shelf facies,carbonate platform facies depositional formation which deposited in the fault basin at thefurther evolution process; Late Devonian Dcaotan Group is set of lacustrine faciescoarse,clastic rocks of continental sedimentary combination which deposited on theaccretionary complex of North Qinling caledonian subduction collision orogenic, the basinbelongs to the intermountain basin; Carboniferous strata deposited in the local stretch faultbasin of Shujiaba group and Xihanshui group in the northern magrin of West Qinlingmicroplate.Comprehensive regional magmatic activity and events and the tectonicdeformation characteristics of the studied objects,the tectonic evolution of the study area candivide into five evolution stages, namely the Cambrian-Ordovician ocean basin opening-subduction stage, Silurian to early Devonian collision stage, middle Devonian to Permianintracontinental extension stage, Early-Middle Triassic collision orogenic stage, and Cenozoicintracontinental orogenic stage.