Comparative Application of Detrital Zircon U-Pb Dating for Stratigraphic Delineation:The"Taojinhe Formation"of the Neoproterozoic Fanjingshan Group as an example
[Objective]Precambrian stratigraphy has undergone multiple tectonic events,complex tectonics,severe deformation,and metamorphism,making it inconvenient and difficult to compare the strata and establish an objective stratigraphic sequence.[Methods]This study selected the Taojinhe Formation,the oldest outcrop of the Neoproterozoic Fanjingshan Group in the western part of the Jiangnan orogen as a research object,and we collected sandstone samples carry out comparing detrital zircon morphology,U-Pb dating,rock,and mineral identification and analysis of the material origin and sedimentary geotectonic settings of this formation,as well as for multidimensional scaling analysis and comparison of the stratigraphy of the Fanjingshan Group.[Results]The sample rocks are mainly composed of detrital particles and fillers;the detrital particles are mainly quartz,feldspar and cutting and sub-angular to sub-rounded with average sortability.The fillers are mainly mica,silica,and iron,and the mica is mainly sericite and white mica.The zircon grains of the"Taojinhe Formation"are angular,representing short-distance transport,and the source rocks are granite,diabase,Syenite/Monzonite,and basalt,with detrital zircon peak ages of 875 Ma,1 862 Ma,and 2 513 Ma.Based on the zircon geochemical data analysis,the"Taojinhe Formation"was deposited in a convergent background.U/Nb values of"Taojinhe Formation"zircon range from 25-700,with one mantle-sourced zircon and 98 arc-sourced zircons.Using U/Yb values to compare with Hf,all zircons were in the continental source range,and the Sc(average value 385.1)and Sc/Yb values(average 1.1)also show island arc magmatic features.Conversely,the zircons have high U/Nb,Sc/Yb,and U/Yb values,relatively low Nb/Yb values,and U/Nb>20,suggesting that the zircons were formed in a subduction environment.In contrast,the zircons of the Yujiagou Formation are mostly elliptical,suggesting long-distance transport and deposition in a disseminated background,and the source rocks are granites,diorites,basalts,alkalites,and rarely kimberlites,with two distinct age peaks at 1 845 and 2 325 Ma.[Conclusions]The"Taojinhe Formation"and the Yujiagou Formation have different source rocks representing different material source areas,whereas the tectonic background is also different,and the maximum depositional age of the"Taojinhe Formation"is younger than that of the overlying Yujiagou Formation,which is not consistent with the stratigraphic sequences of the regional geological survey.Therefore,the original stratigraphic division may be erroneous.Comparison of zircon age diagrams shows that the Taojinhe Formation is similar to the Xiaojiahe Formation above the overlying Yujiagou Formation and has the same material source.Combined other stratigraphic features of the Fanjingshan Group,it is suggested that the"Taojinhe Formation"and Xiaojiahe Formation in the Taoshulin area may be the same stratigraphic units.
万方数据
