The Evolution Of Permian Sedimentary Sequences In Zhen'an,Shaanxi,China,and Its Response To The Changes Of Permian Paleoclimate,Paleoenvironment And Paleogeography

Global paleogeography,climate,environment,and faunal composition underwent dramatic changes during the Permian,in response to the history of convergence,drift,and break-up of Pangea,the transition from deep icehouse to greenhouse conditions,eruptions associated with igneous provinces,and mass extinction events.These make Permian become a research focus among geologists in the past decates.However,there are many controversies about the Permian paleogeological evolution models,sea-level fluctuation models,the exact timing of the end of the LPIA?Late Paleozoic Ice Age?,the patterns and originals of mass extinctions,and the geochemistrical anomlies of oceans.The paper focuses on the Permian sedimentary records and tries to provide some available data to tackle above problems.Xikou area in Zhen'an,Shaanxi Province,China belonging to the South Qinling belt?SQB?,was separated by Mianlue Oeacn?the easternmost Paleo-Tethys?with the South China Block?SCB?during Permian.During that time,the study area was situated on the passage linking Paleo-Tethys Ocean and Panthalassa,which was located in the convergent zone between the North China Block?NCB?and SCB as well.Thus,the Xikou section has important paleogeographic and paleoenvironmental significance to contrast with other Permian profiles in South China and other places in the world and has a potential to become a classical Permian section in the world.The Xikou section deposited huge thick carbonate-dominated sequences from Upper Carboniferous to Lower Triassic,which makes it possible to record complete Permian events,such as mass extinction events,glacier evolutions of the LPIA and Kamura event,etc.Based on the study of paleontology,stratigraphy and sedimentology of the Permian Xikou section,this paper focuses on the evolution of Permian paleoclimate and paleoenvironment by combining the features of clay minerals and carbon isotope of the carbonate rocks.According to the results of detrial zircon chronology and Hf isotopic characteristics mainly,this paper also analyzes the Late Paleozoic tectonic evolution of Central China.Conclusions are as follows:1)This study analyzes 60 microfossil samples,from which 4 genus and 18 species of conodonts are identified.These conodonts distribute from Upper Carboniferous to Upper Permian,and mainly occur in Middle Permian Shuixiakou Formation,Upper Permian Xikou Formation,Yundoutan Formation and Longdongchuan Formation.According to the appearance of conodonts,13 conodont zones can be recognized on this profile,including Clarkina subcarinata zone and C.changxingensis zone of Changhsingian Stage,C.postbitteri zone,C.dukouensis zone,C.liangshanensis zone and C.orientalis zone of Wuchaipingian Stage,Mesogondolella posterserrata zone,M.shannoni zone,M.altudaensis zone and M.granti zone of Capitanian Stage,M.nankingensis zone of Roadian Stage,Sweetognathus whitei zone of Artinskian Stage,S.inornatus zone of Asselian-Sakmarian Stage.Based on the conodont zones mainly,a new scheme of biostratigraphic classification including 3 series and 9 stages is built,which can compare well with the global standard.2)Thirteen lithofacies and twelve microfacies are recognized from this section,which reveals the sedimentary evolution history from open carbonate platform?phase 1,from Cisuralian to Guadalupian Series?,to mixed carbonate-siliciclastic platform?from the end Guadalupian Series to the early Wuchiapingian Stage of Lopingian Series?,then to open carbonate platform?phase 3,from the late Wuchiapingian to the Changhsingian Stage of Lopingian Series?.The sea level change curves of Xikou area can be concluded as follows:?1?Cisuralian Series,sea level dropped slightly;?2?Guadalupian Series,sea level rose gradually;?3?at the end of Guadalupian Series,sea level significantly dropped;?4?Lopingian Series,sea level rose gradually and reached to the maximum at the middle Wuchiapingian Stage;?5?sea level gradually dropped until middle Changhsingian Stage;and?6?sea level rose slightly in late Changhsingian Stage.3)Permian clay mineral assemblages from the Xikou setion consist of Illite?70.9%?,Chlorite?8.6%?,Illite-smectite mixed layer?8.3%?,Kaolinite?7.9%?,and Smectite?4.4%?.The vertical changes on clay minerals reval five discrete climate cooling intervals,including the Asselian to Early Artinskian Stages?P1?,Late Artinskian to Early Kungurian Stages?P2?,Roadian Stage?P3?,Late Capitanian to Early Wuchiapingian Stages?P4?,and Middle to Late Wuchiapingian Stage?P5?.These recovered climate fluctuations are approximately comparable to the calibrated eastern Australian glaciations and recovered climate trends in North American Shelves and suggest to have global significance.The Kungurian Stage is characterized by a significant increase of kaolinite,and a dramatic decrease of illite in the Xikou sections that suggests a climatic warming interval representing the main phase of deglaciation of LIPA.Thus,the Kungurian Stage records an important phase of the Late Paleozoic climatic transition interval,and should receive more attention.Clay minerals suggest that the Capitanian Stage comprised an early phase of climatic warming followed by a later phase of climatic cooling,which is significantly different than previous interpretations that the Capitanian Stage was either one of either global cooling or warming.In particular,the climate cooling event during the Capitanian Stage represents the Kamura event in Eastern Paleo-Tethys.4)A complete Permian carbon isotopic stratigraphy were built on this section.The?¹³C_carb values vary between+0.3‰ and+5.71‰,reflecting seven positive shifts and seven negative shifts,and record several global/regional carbon isotopic anomalies at the boundaries of Artinskian-Kungurian Stages,the Guadalupian-Lopingian Series,and Permian-Triassic System,as well as a signal resembling the Kamura Event.Overall,the carbon isotope corresponds approximately to the climatic changes linked to glacial fluctuations during the Early to Middle Permian.During the Early to Middle Permian,the?¹³C_carb changes correspond approximately to the climatic fluctuations recognized by clay minerals in the study area.These correlations suggest that ?¹³C_carb values are a sensitive indicator of climatic fluctuations that occurred in eastern Paleo-Tethys,and support the interpretation that the waning and waxing of continental ice sheets often correlate with negative and positive ?¹³C_carb shifts,respectively.As the most complete carbon isotope composition profile,the Xikou section has important implications for the carbon cycle and climatology in the Paleo-Tethys and surrounding areas,and can be a reference for global carbon cycle and climatic reconstruction during Permian time.5)Results show the majority of Permian detrital zircons belong to 2100-1760 Ma age population?55.8%;peak at 1893 Ma?with ?Hf?t?values ranging from-8.1 to 7.3?average at-1.9?,following by age populations of 2750-2300 Ma?16.0%;peak at 2547 Ma?,1200-770 Ma?12.0%;peak at 1037 and 870 Ma?,and 512-256 Ma?10.0%;peaks at 450,324,and 270 Ma?,with ?Hf?t?values ranging from 0.6 to 5.5?average at 2.6?,1.4 to 9.4?average at 4.3?,and-18.0 to-4.3?average at-10.4?,respectively.The Permian sediments in the South Qinling belt were mainly transported from the southern margin of the North China Block,and the North Qinling belt,and no contributions from the Yangtze Block.There was no physical barrier between the North China Block and the South Qinling belt,and the North Qinling belt must have been largely eroded before the Permian.We propose that the South Qinling belt had been merged with the North Qinling belt and the North China Block before Permian.The South Qinling belt is part of passive margin of the united Qinling-North China Block during Permian.