THE LOWER PRECAMBRIAN OF CHINA

Resumo

The Lower Precambrian of China consists of Archean and Lower Proterozoic formations formed probably prior to ca. 1,800-1,900 Ma. They are exposed chiefly in the North China Platform. Archean rocks are composed mainly of gneisses, granulitite and plagloclase-amphibolite of the amphibolite facies, with the lower part containing pyroxene-gneiss and granulite of the granulite fades. The parent rocks were not well differentiated sedimentaries and volcanics, forming two volcano-sedimentary cycles. During the Archean (before 2,500-2,600 Ma), the tectonic environment over an extensive area, was quite uniform yet fairly active. Towards the end of Archean there prevailed median- to high-grade metamorphism often accompanied by rather intensive migmatization. In the first Early Proterozoic epoch, a thick sequence of volcano-sedimentaries were accumulated in some marine troughs regarded as eugeosynclinal and developed on the Archean sialic basement, such as the Wutai Group. The protoliths were lhe rather widespread volcanics, the "semipelitic" and pelitic types and turbidites, mainly of greenschist fades and partly amphibolite facies, occasionally accompanied by migmatization probably not later than 2,300 Ma. After that, a stratigraphic pile accumulated in the miogeosynclinal basins or troughs as represented by lhe HutuoGroup, and was composed of coarser clastics, pelitics and stromatolite-bearing Mg-rich carbonates which show rhythmic deposition. Their greenschist metamorphism probably occurred during ca. 1,800-1,900 Ma and this marked the end of the Early Precambrian history.