THE ISOTOPIC AGES OF THE HONGQIYINGZI GROUP IN NORTHERN HEBEI AND ITS AGE ASSIGNMENT

The Hongqiyingzi Group in northern Hebei lies unconformably on the Archean folded metamorphosed basement rock series.Divergent views still exist as to its age assignment.Through an intensive analysis of more than 30 isotope age data obtainedby using different methods combined with a study of the geological setting,it is considered that the single grain detrital zirconU-Pb age of 2450（2434±23）Ma represents the maximum lower-limit age of formation of the Hongqiyingzi Group,while the UPb ages of metamorphosed zircon and zircon in granite are all about 2000（1978-2026）Ma,which is the upper-limit age of this group. Thus the authors hold that the time for the formation of the Hongqiyingzi Group in northern Hebei is in the range of 2450-2000 Ma;so the group should be assigned to the Lower Paleozoic.