A Proposed Archean Subdivision of China
The item of Archean means that the geological history is a very long period before 2.5 Ga.It is characterized by, the intensive crust-mantle differentiation, widespread remobilized.and reworked crusts, and complex metamorphism and deformation. Because it had undergonestrong tectonic movements, the geqlgical records of the earliest history of the earth werenearly all destroyed, and only in a few areas are poorly preserved. Same as in the world,Archean rocks in China are also dominantly high-grade granulite-gneiss assemblages aridmiddle-low-grade granite-greenstone assemblage. We generally agree to the scheme of foursubdivision of Archean which was recommended by the internationa Subcommittee of Precambrian Stratigraphy in its ninth session. But according to the geology situation of China,we prefer to select the ages of 3.8 Ga, 3.3 Ga and 2. 9 Ga as the boundary times forEras. The geological period before 3. 8 Ga is named the Eoarchean Era. It is the earliest eraand alsq the least geolgical records in the earth history.The Paleoarchean is the era between 3.8 Ga and 3. 3 Ga.The Paleoarchean crustal remnants in China have been only discoved intab areas so far.One is in the Eastern Hebei and the other is in Anshan of LiaoningProvince.In addition to the above mentioned two areas,the Mesoarchean rock records (between 3. 3 Ga and 2.9 Ga) occurred as crustal remnants in varying scale have been found inthe Long gang Cbmplex-Group in Liaoniang-Jinn provicea, in the Sanggan Complex withinthe juncture of Hebei-Inner Monglia-Shanxi provinces,and in the Jiaodong Complex-Group ofShandong Provice.The Neoarchean is an era in which the Archean rock records arewidespread,and the granite-greenstone belts are also well developed in China.According tothe relatively abundant geolgical data and isotopic ages, the Neoarchean can be reasonablysubdivided into two stages;and the age of boundary is suggested at 2. 65-2. 7 Ga. About thetop boundary of Archean,we use the age of 2. 5 Ga, which was determined by the international Committee of Stratigraphy.