THE AGE OF THE HONGQIYINGZI GROUP─ A FURTHER DISCUSSION

The Hongqiyingzi Group,equivalent to tbe upper part of the Dantazi Group（sensu lato）,isrepresented by a suite of metapyroclastic rocks of low amphibolite facies. The plagioclase gran-ulitite of the Liuying Formation in the upper part of this group was determined to have a single-grain zircon U-pb age of 2423±23 Ma; the zircon U-pb age of granites intruded into this groupis close（or slightly older） than the above age value. Dating of amphibolite in the upper part ofFenghuangzui gave whole-rock Sm-Nd isochron ages of 258±178 Ma and 2639± 174 Ma （thecorresponding TDM ageas are 2788 Ma and 2714 Ma respectively）.The samples were taken from different horizons or rock types, the rocks underwent late-stage metamorphism and deformation and hydrothermal modification,and methods and objectsof determination and different all these will give rise to notable differences and contradicto-ry phenomena;so the authors analyze and discuss the geological significance of these age values（referring to whether they represent formation or metan1orphism）.Finally the authors holdthat from an analysis of substantial isotopic data the above clastic zircon ages are not terrige-nous inherited ages but should be metamorphic ones. The time range for the formation of theHongqiyingzi is between 2650 and 2450 Ma;thus its age should be late Neo-archean.