The Middle-Upper Proterozoic Section of Jixian is well known at home and abroad. The section contains four systems with the Changcheng System as the lowerest one. The Changzhougou Formation belonging to the Changcheng System is composed of conglomerate, feldsparthic sandstone and sandstone.At present, the lower limit age of Changcheng System is largely diversified from 2000 Ma to 1800 Ma reported by the different laboratories. One of the main reasons responsible for the discrepancy may be the specimen of whole rock of shale used for Pb-Pb isochron dating.The biotite seperated from metamorphic rock belonging to Pre-Changcheng System has been dated by 40Ar/39Ar method and a plateau age of 1782?3 Ma was obtained, which means that the lower limit age of Changcheng System should be younger than this age. Based on the recently reported data the 40Ar/39Ar isochron age of vien quartz grayal from Changzhougou Formation is 1744?97 Ma, and the falling age of cosmic dust from middle-upper part of the Changzhougou Formation is 1670?17 Ma by 40Ar/39Ar dating. Summing up what is mentioned above it can be concluded that the age of the bottom of the Changcheng System may be about 1700?20 Ma.
Wang Songshan,Sang Haiqing,Qiu Ji et al. THE METAMORPHIC AGE OF PRE-CHANGCHENG SYSTEM IN BEIJING-TIANJIN AREA AND A DISCUSSION ABOUT THE LOWER LIMIT AGE OF CHANGCHENG SYSTEM[J]. Chinese Journal of Geology, 1995, (4): 348-354.