THE DISINTEGRATION OF THE "DANTAZI GROUP" IN NORTHERN HUBEI——A DISCUSSION

It has been found in recent studies that the protoliths of the lower part （the Yanwopu Formation and the Baimiao Formation） of the "Dantazi Group" are oceanic tholeiite intermediate-basic volcanic rocks and tuffs with argillo-arenaceous rocks and siliceous-ferrugious rocks. The metamorphism attains higher amphibolite-facies grade. The deformation is dominated by NE-trending brachy-folds. They have an isotopic age of 2450±20Ma. On the other hand, the protoliths of its upper part （the Fenghuangzui Formation and Liuying Formation） are island-arc tholeiite with argillo-arenaceous-calcareous sediments and very thick argilo-arenaceous sediments. The metamorphism only attains the higher greenschist-facies grade and in a few cases the lower amphibolite-facies grade. The deformation is characterized by the E-W-trending tight linear folds. The isotopic age is 2332Ma. The above-mentioned differences obviously reflect that the paleogeo-grphical environments in which they were formed and the tectono-ther-mal events that hey underwent were quite different. To sum up, the "Datazi Group" is the product of different stages of crustal evolution. According to this, it may be disintegrated into two parts: the lower part is called the late Archean Dantaze Group, and the upper part is assigned to the early Proterozoic Hongqiyingzi Group.