Study On The Sedimentary Environment And Provenance Of The Devonian Tapo Formation In The Lueyang Area,South Of Qinling Mianlue Tectonic Zone

Sedimentary rock is a representation of different temporal and spatial tectonic events,and contains a lot of geological information,which is of great significance for studying the paleo-tectonic setting and ocean-land conversion.Material composition and provenance analysis of sedimentary rocks is an important method to study the tectonic background and restore paleogeography.The Tapo Formation is exposed on the southern side of the Mianlue Tectonic Zone,recording important information about the formation and evolution of the Mianlue Tectonic Zone.In this paper,in order to ascertain the material composition and structural deformation characteristics of the Tapo Formation,restore the stratigraphic sequence of the Tapo Formation,and comprehensively discuss its basin properties and material sources,the author integrates various methods and theories,such as lithological methods(including sedimentary rock lithology assemblage,gravel composition analysis and clastic framework statistics),geochemistry and detrital zircon U-Pb dating.Based on the above analyses,we achieved the following results and proceedings:1.Lithofacies studies indicate that the Tapo Formation is characterized by a set of terrigenous clastic rocks,including mainly conglomerate,conglomerate sandstone,sandstone and siltstone(silty slate).According to the characteristics of lithological combination and sedimentary structures,we divide the Tapo Formation into three lithological sections from bottom to top:The lowest section(D_1-2t¹)represents alluvial fan-fan delta sedimentary system characterized by complex conglomerate,pebbly sandstone and gritstone,indicating rapid vertical deposition in the early Devonian.The middle part(D_1-2t²)mianly develops a gravity sedimentary combination of gravel sandstone,sandstone and siltstone with typical granular sequence bedding,flame-like structure,Bauma sequence(Tbcde),heavy-load model and other sedimentary structures,and developes different-scale slump blocks locally.It indicates that the sedimentary environment is a submarine fan of slope facie.The uppermost of the Tappa Formation(D_1-2t³)is developed with gravel sandstone and sandstone,with a small amount of phyllite and limestone,indicating a fan delta environment.2.The petrology and petrographic analyses indicate that there are granite,metamorphic and sedimentary rock debris in the Tapo Formation,and the sedimentary rock debris are mainly carbonate rock debris.The statistics show that the gravel of the Tapo Formation can be divided into two types,subangular-round granite,quartz,limestone and dolomite,and angular-sub-angular sandstone and slate gravel.There is a certain difference in the composition of the uppermost part and the lowest part.In the uppermost part,the proportion of granite conglomerate is relatively large,and the proportions of limestone and dolomite conglomerate are significantly reduced.There are many carbonate rock clasts and feldspathic clasts in the sandstone samples.The content of quartz and feldspar increases and the content of lithic clasts decreases from the bottom to the top.The clastic skeleton analysis results show that the provenance of the Tapo Formation is mainly sandstone from re-cycled orogenic and mixed orogenic.Both the statistics of gravel and the clastic skeleton analysis indicate a certain change in the source rock during the deposition.3.The sandstone whole-rock geochemical analysis indicates that the source rocks of the Tapo Formation are dominated by feldspathic rocks.The samples contain high LOI,Mg O and Ca O values,which also indicates that the samples contain a great amount of carbonate debris.Geochemical analysis shows that the tectonic setting of the source rocks is mainly continental island arc and active continental margin,and the Tapo Formation deposited in semi-arid environment.4.The results of LA-ICP-MS U-Pb dating of the Tapo Formation show that the ages can be divided into 2 groups:2706?2413 Ma and 2089?1967 Ma.The group of 2706?2413 Ma accounts for 33%of the total,with a peak at 2.48 Ga;the age group of 2089?1967 Ma account for 53%of the total,with a peak at 2.0 Ga.Based on aggregate gravel statistics,sandstone clastic skeleton statistics,geochemistry and isotopic chronology data,it is concluded that the Yudongzi Complex and the unexposed basement,as well as the Sinian Dendying Formation and Doushantuo Formation provide the main clastic materials for the Tapo Formation.In addition,there are differences in the source supply of the middle-lower and uppermost sections of the Tapo Formation.At the early stage,the Dengying Formation and the Doushantuo Formation provide more detrital materials,and the ratios of ancient basement materials increases significantly in the later period.5.Deformation characteristics of the Tapo Formation can be divided into three stages:the first stage is the main tectonic period,establishing the current structural framework of the Tapo Formation,which is related to the Indosinian orogeny in the Mianlue Tectonic Zone,and characted by NNE-SSW compression.The tectonic stress of the second stage intersects with the first stage at a certain angle,and is related to the late orogenic thrust of the Mianlue Tectonic Zone.The third stage is dominated by left-lateral slip.A series of small vertical folds were formed in this period.This stage of deformation is related to the intracontinental orogeny of the Mianlue Tectonic Zone.