THE STRATIGRAPHICAL AGES,DIVISION AND CORRELATION OF THE EARLY PRECAMBRIAN IN THE TAIHANGWUTAI AREA,THE YANSHAN AREA AND THE EASTERN SECTOR OF THE YINSHAN MOUNTAINS
Wang Qichao (Hebei Regional Geological Survey Party, Langfang)
According to whether the protolith formation is composed mainly of sedimentary clastic rocks-carbonate rocks or of volcanic rocks-pyroclastic rocks, the Precambrian strata in the region may be divided along the Fengning-Zanhuang line into the stable stratigraphic area in the west and the active stratigraphic area in the east. An integration of the isotopic age data obtained by various methods available and combined with geological factors indicate that there were five significant metamorphic thermal events in the early Precambrian of the region, which took place respectively at 2800 Ma (between the Fuping Supergroup and Wutai Super-group or between the Qianxi Group and the Zunhua Group), 2500 Ma (between the Taihuai Group and the Gaofan Group or after the formation of the Zhuzhangzi Group), 2350 Ma (between the Gaofan Group and the Doucun Group), 1850 Ma (between the Dongye Group and the Guojiazhai Group), and 1700 Ma (between the Guojiazhai Group and the Changcheng System). According to the geological ages, stratigraphical or structural unconformable relationships and the history of the crustal evolution, the above-mentioned strata may be divided into three large stratigraphic units——the Middle and Upper Archaean and the Lower Proterozoic——and units at the group level therein, and a group-level stratigraphical correlation between the stratigraphic areas has been made. On that basis the stratigraphic system of the stable stratigraphic area has been adjusted to comprise the Middle Archaean Fuping Supergroup (including the Chenzhuang Group and Wanzi Group), Upper Archaean Wutai Supergroup (including the Longquanguan Group, Shizui Group and Taihuai Group), Lower Archaean Hutuo Group (including the Gaofan Group, Doucun Group, Dongye Group and Guojiazhai Group). The Strata of the active stratigraphic area have been adjusted to include the Middle Archaean Qianxi Group and the Upper Archaean Zunhua Group, Dantazi Group and Zhuzhangzi Group. Among them, some divisions (e.g. the boundary of the Middle and Upper Archaean is placed below the Longquanguan Group) and correlations (e.g. the correlation between the Fupping Supergroup and the Qianxi Group) are exploratory in nature.