Detrital Zircon U-Pb Chronology And Paleomagnetism Of The Neoproterozoic Zhitang Formation From Jiangshan Of Zhejiang Province

The South China Yangtze plate developed relatively complete Neoproterozoic sedimentary strata.The field exploration revealed that the Neoproterozoic Zhitang Formation?Z₁z?was developed in the Jiangshan area of Zhejiang Province.The lower part of the formation is sandstone of fluvial deposition,and the upper part is a set estuary facies sedimentary sandstone.The Zhitang Formation is in an unconformity covering volcanic rocks of the Shangshu Formation?Pt₃s?,and is overlyinged by Leigongwu Formation?Z₁l?which was mainly tillite.The Zhitang Formation is a sedimentary strata before the South China Ice Age,and it is continuous with the glacial stratum.Due to the lack of geochronological study for the Zhitang Formation in the Jiangshan area,the issue of age affility and stratigraphic division has been debated for long time.In addition,the Neoproterozoic Zhitang Formation is of great significance for studying the evolution of the climatic environment before the ice age,the initiation mechanism of the"snowball earth"and the paleoclimatic change process.In view of this,this M.S.thesis has obtained the following new understandings and achievements through the comprehensive study of chronology and paleomagnetics in the Neoproterozoic Zhiyu Formation in Jiangshan area:?1?According to the study of detrital zircon via LA-ICP-MS U-Pb geochronology in the top sandstone of the Zhitang Formation,the youngest group was 721±7.6⁷33±7.7 Ma,and the age-weighted average was 727.5±5.9Ma.Therefore,it can be considered that the sedimentary age of the Zhitang Formation is less than or equal to 727.5±5.9 Ma,which is consistent with the age of the top stratigraphic strata studied by other scholars;therefore,the Zhitang Formation is determined to be located in the middle and late Neoproterozoic sediments below the glacial formation.The age of the top boundary of the Zhitang Formation and the neoproterozoic?before Nanhua glaciation?and its corresponding strata in south China also tended to be 720Ma,which was basically consistent with the starting age of Sturtian glaciation?about 720Ma?in the world,indicating that the bottom boundary of Nanhua glaciation was about 720Ma.?2?This paper conducts rock magnetic and paleomagnetic studies on three sampling locations?a total of 54 directional cores?in the Zhitang Formation in Jiangshan,Zhejiang.The results of rock magnet experiments show that the magnetic bearing minerals in the samples are mainly magnetite and hematite.The sample is thermally demagnetized systemly.The result illustrated that different samples have different blocking temperatures.Some cores can separate the high and low barrier temperatures.After bedding correction,the high temperature components are showed that Ds=325.6°,Is=40.4°,?95=8.1°,and it pass the fold test.They are primary.It reveal ancient latitude of the sample locations of the Jiangshan Zhitang Formation is 23°N.The results are very consistent with the paleomagnetic results of the predecessors in other parts of the Neoproterozoic strata in the southeastern margin of the Yangtze block.In addition,this result is consistent with the South China block at⁷20Ma at the middle and low latitudes.Previews paleomagnetic studies on the South China Neoprotozoic to the Sinian?820⁵40Ma?of South China block indicated it experienced a process of moving from high latitude to low latitude,a transition to a cold climate,it is so called the"snowball earth".Our low latitude palomag result is the response of the ice age in South China.