NEW Rb-Sr ISOTOPE DATA OF LOW-GRADE METAMORPHIC ROCKS IN EASTERN HEBEl AND THEIR GEOLOGICAL SIGNIFICANCE

Low-grade metamorphic rocks in eastern Hebei are mainly developed in the Shuangzishan area, Qinlong County, Hebei Province, and belong to the product of greenschist facies-lower amphibolite facies metamorphism. Four groups （24 pieces） of samples were collected from the low-grade metamorphic rocks of the main suite for isochron age determination. The results are as follows: blasto- pillow plagioclase amphibolite of the Luzhang Forination, 2217 Ma; cements of mataconglomerates of the Zhangjiagou Formation, 2228 Ma; two-mica granulitite of the Poluotai Formation, 1824 Ma; the quartz diorite massif intruded into the Ciyushan Formation, 2422 Ma. According to these results, the authors discuss the age assignment of the low-grade metamorphic rocks in the Shuangzishan area, the nature of metaconglomerates of the Zhangjiagou Formation and the Archean-Proterozoic boundary in eastern Hebei. The authors consider that the conglomerates of the Zhangjiagou Formation at the base of the original Zhuzhangzi Group exhibit the nature of mantle source. belonging to volcanic conglomerates. In terms of time, the Luzhangzi Formation and Zhangjiagou Formation are continuous, and unconformities are not possible to exist. So the authors suggest that the connotation of the original Zhuahangzi Group should be broadened so as to incorporate the Luzhangzi Formation and that the Archean-Proterozoic boundary in eastern Hebei should be placed below the Luzhangzi Formation and above the Ciyushan Formation, i. e. the new defined Zhuzhangzi Group should be of early Proterozoic age and the Shuangzishan Group is Archean in age.