The Definition Of The Paleoproterozoic Stratum And The Prospecting Significance In The Yimen Area,Central Yunnan Province

The Yimen area in central Yunnan is located in the central and southern section of the Kangdian axis on the southwest margin of the Yangtze platform.It is caught by two regional faults near the north and south,the Luzhujiang fault in the west and the Xiaojiang fault in the east.The structure is relatively developed in the study area,and the basic geological problems in the area are one of the most controversial basic geological problems in Yunnan.It restricts the progress of geological prospecting to a certain extent,and also restricts the further improvement of the geological research level in Central Yunnan.The regional geological survey of the 1:200000 Kunming block in Yunnan divided most of the strata in the study area into “Dongchuan Group” and“Kunyang Group”.There is no definite age basis,but only according to the characteristics of its rock assemblage.The stratigraphic sequence of “Dongchuan Group”and “Kunyang Group” in the Middle Proterozoic has long been controversial.This paper relies on the regional geological records of Yunnan Province(2nd edition,revised)and the regional geological survey of the 1:50000 Erjie?Mingyihe?Shangpubei and Dianzhong block?In recent years,with the development of the field work of the project,a large number of first-hand field data and related zircon dating data have been obtained,and a new understanding of the pre-Qingbakou shallow metamorphic stratigraphic sequence and age has been obtained in the central Yunnan area.There have also been some major new discoveries,and many breakthroughs have been made.From the analysis of the geological data obtained at present,the real Middle Proterozoic "Dongchuan Group" and "Kunyang Group" in the central Yunnan area are very small in area and limited in distribution.In fact,many areas belong to the Middle Archean to Paleoproterozoic,rather than to the Middle Proterozoic as previously thought.In the two projects,the geological data of the areas of Yuanjiang Cuoke,Yimen Tongchang,Dongchuan Niuchangjie and Shanmuqing area were synthesized,and the new lithostratigraphic units of the Paleoproterozoic,namely the Yimen Group,were established.Yimen group is divided from bottom to top into six lithostratigraphic units of Abudou formation,Luowadie formation,Liangshan formation,Yongjingshao formation,Xishancun formation and Shanmuqing formation,respectively.Because of the reconstruction of multi-stage tectonic events,it is difficult to find 6 successive outcrop sections.The nomenclature section of the Abudou formation and Luowadie formation is located in the area of the Cuoke in Yuanjiang County.The nomenclature section of Liangshan formation,Yongjingshao formation and Xishancun formation is located in the study area.The nomenclature section of the Shanmuqing formation is located in the area of Shanmuqing,Dongchuan.In this paper the study area exposed Yimen Group carried out studies on the part of strata is named,profile of the exposed is not complete.The named section of the Abudou and Luowadie formation is located in the Cuoke area of Yuanjiang County.Because the strata are not fully exposed in the study area,two LA-ICP-MS zircon U-Pb age samples from the named section are used to calibrate the formation age of the Yimen Group.In combination with four LA-ICP-MS zircon U-Pb age samples in the study area,by means of isotopic LA-ICP-MS zircon U-Pb dating method,the result of 2339±18Ma age represents the formation age of the Abudou formation.The age of 2286±41Ma,2290±76 Ma and 2320 ±44Ma represents the formation age of the Luwadie formation.The age result of 1862±27Ma and 1839±21Ma represents the formation age of the Shanmuqing formation.The formation age of Yimen group was about 2339 to 1839 Ma,it was Paleoproterozoic.The Yimen Group of Paleoproterozoic recorded many important geological events of Earth evolution,“Huron Ice Age”event,The earliest macrocell multicellular organisms,Lomagundi event,Large oxidation event,Columbia supercontinent event.The definition of the Paleoproterozoic Yimen Group not only improves the level of Precambrian geological research in central Yunnan,but also has certain guiding significance for geological prospecting in the future.