Geological Evolution Of Early Precambrian In Yun Zhongshan Area

The North China Craton is one of the oldest cratons in the world,it's also the oldest and the largest land mass in China,its geological evolution history can be traced back to~3.8 Ga,recording a long-term continuous geological evolution history of 3.8 Ga~1.8 Ga,and it has witnessed almost all the important geological evolution events on the earth.Because of its unique geological conditions and geographical location,the North China Craton has become an important area for China and even the whole world to study the geological characteristics and tectonic evolution of the early Precambrian.The main body of the study area spans the eastern part of Yun Zhongshan mountains in Lvliangshan mountains and most of the southern part of Xinding basin,it is located in the key part of the junction of Lvliang and Wutai.The division,attribution,era and evolution of the Precambrian metamorphic basement have been the focus and hot spot in the field of geology.Different experts,scholars and geologists have published relevant scientific research literature and achievement reports,but most of them cover a wide range.In this paper,the metamorphic igneous rocks of the Wutai Group are taken as the research objects,combined with the previous research results.The field observation,petrographic characteristics analysis,U-Pb dating of LA-MC-ICP-MS zircon and analysis of main trace elements in whole rock are comprehensively used to discuss the forming age,diagenetic environment and tectonic evolution of Wutai Group.In addition,the Hutuo Group in the district is discussed and analyzed in the light of previous research results.It is of great significance to further study the evolution history of the North China Craton.The main points are as follows:1.The eastern end of Yun Zhongshan mountains is the survey area,and the eastern end of Yun Zhongshan Mountains is exposed in the area of Tunwa.The exposed area is about 220 km~2.The overall geological characteristics of the Wutai Group in the area are basically consistent with those of the Wutai mountains in the classical area.The strata of Wutai Group and the gneiss alternate with a series of open backward shaped folds,and they are in structural contact,the direction of the structural line is that the north-east is east and the near east is west.Folds are developed in Wutai Group,and refolds are often seen.Through field geological survey and indoor thin section observation,it is concluded that Wutai Group is mainly a set of volcano-sedimentary rock series subjected to low amphibolite facies metamorphism.2.The zircon U-Pb dating of metamorphic igneous rocks in Wutai Group shows that the zircon age of protomagma is 2.5~2.6 Ga,representing an important magmatic thermal event in the late Neoarchean to 2.5 Ga,which is an important development period to the formation of the North China Craton during the Neoarchean.3.The metamorphic basic volcanic rocks of Wutai Group are transitional from old to new and from island arc to orogenic belt.The diagenetic environment of metamorphic basic volcanic rocks of Wutai Group belongs to island arc type.Based on the analysis of the main trace elements in the samples of Wutai Group,it is considered that the source of the metamorphic igneous rocks in the Jingangku and Zhuangwang Formation may be partial melting of the upper mantle.4.According to the field study and indoor thin section analysis of Hutuo Group in this region,the lithology is basically the same as that in Wutai region.The Hutuo Group in the study area should be the western extension of the strata in the Wutai region.Based on the analysis of the ages of the Hutuo Group by predecessors,it is concluded that the age of the bottom boundary of the Hutuo Group in this area should be about 2500 Ma,and the age of the top boundary should be larger than 1759 Ma.According to this,the lower limit of Hutuo Group is 2500 Ma and the upper limit is 1800 Ma.The era of Hutuo Group belongs to Paleoproterozoic.5.Doucun subGroup is composed of alluvial fan system-predelta-delta front facies-upper tidal mud flat,lagoon,carbonatite platform sediments with shallow seawater and less energy and continental clastic rocks;The main body of the Dongye subGroup is subtidal,intertidal and supratidal sedimentary environments,and part of it is delta facies,indicating that the sea water is consistent with the basin evolution from shallow to deep to shallow;Guojiazhai subpopulation is the sediments of inland basin formed in the process of the Hutuo basin fold uplift.