Geological Features And Geological Significance Of Late Paleozoic Depositional Stratum In The Lintan Area Of The West Qinling Orogen

Sedimentary rock is the most widely distributed rock type in the crust surface.The study of sedimentary rock is the key to solve a series of geological problems.The West Qinling Orogen is characterized by extensive Late Paleozoic sedimentary stratum.In this paper,four sets of Late Paleozoic depositional stratum in the Lintan area of the West Qinling Orogen is used as research objects,and the lithology,rock association,depositional environment,provenance tectonic setting,main provenance areas,regional tectonics and basin revolution of four sets Late Paleozoic sedimentary stratum in the Lintan area of the West Qinling Orogen are comprehensively studied by means of sedimentary petrology,statistics of clastics skeleton,age spectrum of detrital zicon,regional stratigraphic correlation and achieved the following research findings and cognitions:1.Based on the features of lithology,rock association,depositional structure,etc,four sets Late Pelozoic stratum in the Lintan area(Dacaotan Formation of Upper Devonian series,Badu Formation of Lower Carboniferous series,Xiajialing Formation of Upper Carboniferous series,Shilidun Formation of Permain system)are divided into different members and recovery Paleo-depositional environment preliminarily.Dacaotan Formation is divided into three members,the lower member is green sandstone,which is semi-deep to deep lake depositional environment;the middle member is dark red sandstone,which is flood plain-shallow lake depositional environment;the upper member is gravel sandstone,which is braided river-netted river depositional environment.Badu Formation is divided into two members,the lower member is black-gray clastic rock and slate,and the upper member is black-gray clastic rock and limestone.There are a large number of marine fossils in both members.The overall depositional environment should be coastal facies of land-sea convergence.Xiajialing Formation is divided into two members,the lower member is gray-black thick-bedded massive limestone,reflecting the depositional features of carbonate platform facies;the upper section is thin-bedded limestone and slate cyclic deposition,which is shallow sea shelf facies deposition.Shilidun Formation is divided into three members,the lower member is clastic rock;the middle member is sandy slate,clastic rock and small amount of limestone gravel;the uppermember is small clastic rock,slate and limestone gravel;Typical Bauma Sequence and slip depositional features of turbidite developed in this formation,combining with previous studies on this set of stratum paleontological relics,Shilitdun Formation should be semi-deep sea to deep sea slope facies turbidite.2.The clastic skeleton statistics of sandstone slices of Dacaotan Formation,Badu Formation and Shilidun Formation show that the sandstone types of Dacaotan Formation are mainly lithic feldspar sandstone,a few are feldspar lithic sandstone;the sandstone of Badu Formation is mainly feldspar lithic sandstone,a few are lithic feldspar sandstone;the sandstone of Shilidun Formation is mainly feldspar sandstone,lithic feldspar sandstone and feldspar quartz sandstone.Dickinson discriminant diagram indicates that the tectonic setting of the provenance area of Dacaotan Formation is dissected arc;the provenance area of Badu Formation is not as concentrated as that of Dacaotan Formation,and there are dissected arc,mixed areas and transitions to recycled orogen;while the points of Shilitun Formation samples mainly fall into the transitional continent and craton interior.The difference of tectonic background in the source areas reflects the change of provenance from "island arc relativity" to "continent relativity" from Devonian to Permian.3.From the morphology of detrital zircons,the psephicity of detrital zircons in Dacaotan Formation is the worst,which reflects the most obvious features of adjacent deposition.Zircons from Shilidun Formation are the smallest in comparison with those from Dacaotan Formation and Badu Formation.The uniqueness of zircons from Shilidun Formation is also reflected in the age spectrum.The peak ages of two samples in Dacaotan group is 509 Ma?820 Ma?985 Ma and 446 Ma?911 Ma respectively.The peak ages of two samples in Badu Formation is 426 Ma?885 Ma?957 Ma and 452 Ma?812 Ma respectively.The peak ages of two samples in Shilidun formation is 1873 Ma?2518 Ma and 1890 Ma?2506 Ma respectively.The detrital zircon ages of Dacaotan Formation and Badu Formation are mainly Caledonian and early Neoproterozoic ages,while those of Shilidun Formation are mainly Paleoproterozoic-late Archaeozoic ages.According to the age spectrums of detrital zircons and the features of geological body ages and magmatic activity periods in the adjacent area,it is analyzed that the Middle-Eastern part of Qilian Orogen and the northern margin of West Qinling Orogen are the main provenances of DacaotanFormation and Badu Formation,and the old basement material in the southwestern margin of North China Plate is the main provenance of Shilidun Formation.4.By reading the relevant literatures,comparing the object investigated with the contemporary stratum in the southern zone of the West Qinling Orogen and the Qilian Orogen,and combining with the previous understanding of the tectonic evolution of the study area and its adjacent areas,this paper puts forward the viewpoints of tectonics and basin evolution which is in line with the geological facts of the study area.Under the background of collision between Yangtze plate and North China plate at the end of Caledonia,Dacaotan Formation was deposited in the foreland basin,which is the depositional response of Hercynian continuous orogeny in the Western Qinling Orogen.Under the background of the expansion of the Eastern Paleo-Tethyan Ocean,the Mianlue Ocean reached its maximum expansion stage in the Carboniferous.The Badu Formation and Xiajialing Formation were deposited in the rift basin under the influence of the expansion of the Mianlue Ocean.In Permian,the special period of Qinling Orogen evolution,the turbidite series of Shilidun Formation is more unique than the other three sets of stratum in terms of lithology,rock association and detrital zircon ages.This paper holds that the deep-seated internal cause of this uniqueness is the northward subduction of Mianlue Ocean in the middle and late Permian,and the transformation of depositional basin from original rift basin to compress basin.