TWO MAJOR PRECAMBRIAN GEOLOGICAL EVENTS OF NORTH CHINA BLOCK (NCB):CHARACTERISTICS AND PROPERTY
PENG Peng, ZHAI Mingguo(Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029,China)
We analyzed 236 data of ages older than 1.6 Ga from North China Block (NCB).There are two data peaks which are 2 550 to 2 475 Ma and 1 900 to 1 750 Ma. The two peaks are termed 2.5 and 1.8 Ga geological events respectively. We divided the age data into two groups: diagenesis data and metamorphism data. We find that both groups have two peaks at 2.5Ga and 1.8Ga, but there is difference. For 2.5Ga geological event, major diagenesis took place before major metamorphism. The metamorphism is mostly granulite facies metamorphism. While for 1.8Ga geological event, major metamorphism took place before major diagenesis. The metamorphism is mainly retrograde hornblende facies metamorphism. The rocks formed during this time are mostly magmatic rocks. In this paper, we argue that the characteristics of the metamorphism and the diagenesis apex epochs are related to the property of the geological events and the crustal evolution. During an amalgamation process, the diagenesis may take place before metamorphism. For before and during this process, there are many magmatic events because of heat flow and fluid, but after this process, many rocks may be metamorphosed because of the change of the PT and other conditions. While during an expansion process, the metamorphism may take place before diagenesis. For before and during this process, many rocks may be retrograde metamorphosed and exposed because the changing of P T, and other conditions, while after this process, magmatism takes place to balance the crust. Basing on petrology and chronology, we have a discussion on the property of 2.5 and 1.8 Ga geological events. The characteristics of the 2.5 Ga geological event are:forming of huge granulite belt,happening of magmatictectonicthermal events, beginning of sedimentary formation and changing of environment, and major diagenesis taking place before major metamorphism. It may relate to an amalgamation process in NCB. The characteristics of the 1.8 Ga geological event are: movements of anorogenic magmatism and rift style volcanomagmatic events, strong retrograde metamorphism, and major metamorphism taking place before major diagenesis. It's likely to be represented a rift event in NCB.
【CateGory Index】： P597
【CateGory Index】： P597