Study On The Tectonic Setting Of The Mugagangri Formation Of The Middle-lower Jurassic Rongmanan In South Rongma?nima County,Tibet

The paper relies on the 1:50,000 regional geological survey project of Rongmanan,Nima County,Tibet undertaken by China University of Geosciences(Beijing).The tectonic background of the formation of rock groups is studied.The paper analyzes the sedimentary facies and sedimentary environment of the LowerMiddle Jurassic Mugagangriyan Formation by combining field surveys,profile surveys,thin section laboratory identification statistics,and other existing regional background data.The siliceous rock samples collected in the Mugagangriyan Formation were geochemically tested to analyze the content characteristics of major elements,trace elements and rare earth elements in the samples,and to study their relationship with the structural background of the Mugagangriyan Formation.Therefore,the structural background of the Mugagangriyan Formation is comprehensively inferred.The Lower-Middle Jurassic Mugagangriyan Formation of Dangma Lower-Middle Jurassic in Rongmanan District,Nima County,Tibet is a set of semi-deep-deep-sea submarine fan sedimentary assemblages.After studying the rock characteristics,rock assemblage characteristics,and sedimentary environmental indicators of the Mugagangriyan Formation,a total of 9 rock types and 8 rock combinations were identified in this set of geological bodies,and fan central subfacies and fan ends can be divided subfacies,and braided channel microfacies.The semi-deep sea-deep sea environment with a water depth of 1500 meters or deeper is used to determine the sedimentary environment of the rock group based on the semi-deep sea representative ancient network trace fossils found in the Mugagangriyan Formation.According to the discriminant diagram of the main elements of the siliceous rock,the structural background of the siliceous rock of the Mugagangriyan Formation is the background of the continental margin and the origin of the hydrothermal fluid.The average Th/U ratio is 2.13,which is a low oxygen environment,The average value ofCe/Ce* is 0.67,and the low content of Ce element is a loss.Hydrothermal fluids generally originate from the extensional tectonic background.Therefore,we believes that the structural background of the Mugagangriyan Formation is most likely to be an active continental margin with extensional tectonic properties.The sandstone on the top of the Mugagangriyan Formation contains the terrestrial neo-aloe fossils transported into it.The structural maturity and compositional maturity of the entire Mugagangriyan Formation are relatively high.Through comprehensive judgment,this paper believes that the structural background of the Mugagangriyan Formation is most likely to be the back-arc basin in the active continental margin,that is,during the early-middle Jurassic period,the area where the study area is located is in a stretched tectonic background state.