Geological Characteristics And Source Analysis Of The Triassic Strata In The Lintan Area,Western Qinling

By analyzing the mutual evolution between sediment source area,transportation mode and sedimentary facies sequence,the sedimentary basin prototype and its evolution process are analyzed,and the basin-mountain coupling relationship is analyzed to explore the basin evolution,orogeny and even the deep process,which is the current geological study of orogenic belt.One of the important ideas.Based on the theory of plate tectonics,this study used three sets of sedimentary strata of the Early Paleozoic Longwuhe Formation,Guanggaishan Formation and Daheba Formation in the Lintan area of ??the Western Qinling Mountains as the research object to conduct rock stratigraphic units.The classification,analysis of sedimentary characteristics,stratigraphic sequence,sedimentary facies and sedimentary environment and basin filling sequence,and then study the sedimentary age,sediment source,basin prototype and structural properties,and finally explore the tectonic setting of the Western Qinling Triassic,The geological evolution process provides new information.And made the following progress and preliminary understanding:1.Through the study of sandstone petrography,three sets of measured geological sections are controlled by three sets of Early Mesozoic sedimentary strata in the research area.According to the difference of sedimentary rock combination and sedimentary environment,the Longwu River is composed of three lithologic sections.It is a semi-deep sea turbidity deposition of a continental slope.The Guangcaishan component is divided into two lithologic sections,which are identified as deep-semi-deep sea sedimentary environment;the Daheba dam is composed of three lithologic sections,and the sedimentary environment should be shallow sea shelf-marine sedimentary sequence.The minimum detrital zircon U-Pb ages of the Longwuhe Formation,Guanggaishan Formation and Daheba Formation were 254 Ma,250 Ma and 244 Ma,respectively,which may represent the lower sedimentary limits of each group.The sedimentary ages of the Longwuhe Formation,the Guanggaishan Formation and the Daheba Formation were determined as the Early Triassic,Middle Triassic and Late Triassic,respectively,through the collection of paleontological data,ground-covered relationships,and tectonic magmat events.2.10 pieces of Longwuhe Formation,Guanggaishan Formation and Daheba Formationwere selected respectively.A total of 30 sandstone rock sheets were analyzed by Dickinson-Gazzi point method for debris skeleton in the study area.In the sandstone end-class classification diagram,the Longwuhe Formation samples basically fall into the lithic feldspar sandstone area,with a small number of sites falling into the feldspar lithic sandstone and feldspar sandstone areas;the Guanggaishan group samples are in the cuttings The feldspar sandstone and feldspar lithic sandstone regions have a relatively even distribution;the Daheba Formation sandstone samples mainly fall into the lithic feldspathic sandstone and feldspar lithic sandstone areas,and a small part falls into the feldspar sandstone area.In the rock QFL diagram,the Longwuhe group samples mainly fall into the mixed area,and some of the points fall in the transitional continental area;the Guanggaishan group samples mainly fall in the transitional continental area,and a small amount falls in the mixed area and the internal environment of the craton;The samples of the Daheba Formation mainly fall in the transitional continental and craton interior areas.It can be seen that the rock genesis tend to increase from the Longwuhe Formation to the Daheba Formation to the transitional continental and craton.3.LA-ICP-MS U-Pb geochronology study of six detrital zircon samples from the Triassic in the study area shows that one zircon sample in the third section of the Longwuhe Formation is concentrated in 254~407Ma,1829.~2869Ma age group;one zircon sample in the second section of the Longwuhe Formation is concentrated in the 1820~2677Ma age group;the two zircon samples in the third section of the Longwuhe Formation are concentrated in the 1832~2835Ma,1890~2702Ma age group;One zircon sample from the Guanggaishan Formation was concentrated in the 250~274Ma and 1837~2525Ma age groups;one zircon sample from the Daheba Formation was concentrated in the 244~362Ma,1852~2801Ma age group.The source is generally from the northern part of the basin.The ancient basement rocks of the North China block,the North Qinling orogenic belt and the Qilian orogenic belt are the main source areas of the Longwuhe Formation,the Guanggaishan Formation and the Daheba Formation;the North Qinling orogenic belt and the Qilian orogenic belt Caledonian magmatism A small amount of provenance is also provided for the Longwuhe Formation and the Daheba Formation;the magmatic events related to the geological background of the Late Paleozoic and the extensional rifting in the North Qinlingand the Middle Qinling are also the Longwuhe Formation,the Guanggaishan Formation and the Daheba Formation.A small amount of source.4.Comprehensively and read relevant literatures,combined with previous understandings of the tectonic evolution of the Western Qinling and adjacent areas,the Late Paleozoic-Early Mesozoic tectonic evolution divided into the late Paleozoic Paleo-Tethys ocean basin expansion stage and the Early Mesozoic ocean basin subduction The two parts of the subtraction and land-continent collision orogeny stage are subdivided into: 1).the Devonian-Middle Permian Mianlue structural belt;2).Late Middle Permian-Early Middle Triassic Mianlue slightly ocean Closed,Yangtze plate subduction collision,Qinling orogeny stage;3).Late Triassic West Qinling entered intracontinental evolution stage...