Sedimentary Environment Analysis Of Neoproterozoic Luoquan Formation In Luonan Area,southern Margin Of North China Plate

The study of the Luoquan Formation moraine strata on the southern margin of the North China Plate has an important indication of the Neoproterozoic glacial sedimentary environment.This paper is based on the field measured profile and sample collection of Luoquan Formation in Luonan area of Shangluo City,Shaanxi Province.Using the methods of hand specimen observation,microscopic thin section identification,main element test,trace element test and rare earth element test,from the perspective of petrology,sedimentology,petrogeochemistry and element geochemistry,the sedimentary facies,paleoclimate,paleoredox conditions,provenance and tectonic background of Luoquan Formation are studied and discussed in detail,and the paleosedimentary environment evolution of Luoquan Formation in Neoproterozoic is revealed.The observation of petrological characteristics and the identification of rock thin sections show that the bottom of Luoquan Formation is a set of light gray mixed accumulation of massive conglomerate,with high gravel content,large particle size difference,no sorting and poor roundness,and complex gravel composition mainly dolomite,quartzite and sandstone.The central part is a light gray weak layered conglomerate,with low gravel content and different sizes,poor sorting and poor roundness.The gravel composition is mainly dolomite and quartzite,and the rockfall structure can be seen.The upper part of the Luoquan Formation is light gray,gray gravel-bearing slate and sandstone.The fine content of gravel is low,the sorting and rounding are general,and the layered structure and falling stone structure are developed.Based on the comprehensive analysis of the above petrological and sedimentary characteristics,the lower part of Luoquan Formation is divided into moraine facies,and the middle and upper part is divided into ice water facies.The results of geochemical analysis show that the content of Si O₂ in the main elements of Luoquan Formation is the highest,Ca O and Mg O are obviously enriched compared with the average of the upper continental crust and the average shale in North America,and P₂O₅ is basically equivalent to the average of the upper continental crust and the average shale in North America.The standardized distribution curves of trace elements show that Ba,Th,U,Ni and V are relatively enriched,while Sr,Co,Cr and Cu are relatively depleted.The standardized distribution curve of rare earth elements shows that light rare earth elements are relatively enriched,the curve of heavy rare earth elements is flat,and the trend of light and heavy rare earth elements is consistent.The high values of LREE/HREE and（La/Yb）_N indicate that the source of Luoquan Formation sedimentary rocks is proximal deposition.The values of Fe O/Fe₂O₃,Si O₂/Al₂O₃ and Mg O/Ca O reflect that the formation of Luoquan Formation experienced a climate change from cold and dry to warm and humid.The values of U/Th,V/Cr,Ni/Co andδU indicate the oxidation conditions of the paleoenvironment during the deposition of the Luoquan Formation.The discriminant diagram of provenance and tectonic background shows that the provenance of Luoquan Formation mainly comes from the island arc of acidic magmatic rock combination and the felsic rocks of upper crust.The sedimentary location is on the continental margin and the tectonic background is continental island arc.The sedimentary environment evolution of the Neoproterozoic Luoquan Formation:In the early Ediacaran,glaciers migrated and eroded the underlying rock strata and mixed with the source debris of the upper crust,and rapidly accumulated on the continental margin to form the lower massive conglomerate.In the middle of the Ediacaran period,with the warming of the climate,the glaciers melted,and the debris was further deposited after the transformation of ice water to form a weak layered conglomerate in the middle;in the late Ediacaran period,the water body further deepened,and the ice raft carrying fine gravel fell into the ice water to form the upper conglomerate layer.In summary,the formation of the Neoproterozoic Luoquan Formation experienced a transition from the early Ediacaran glacial environment to the middle and late Ediacaran glacial water environment,and was affected by the’Snowball Earth’event at the end of the Neoproterozoic.It can be regarded as the geological response of the Ediacaran Gaskiers glacial event in the southern margin of the North China Plate.