ISOTOPIC AGES OF PRECAMBRIAN STRATA IN THE ZHONGTIAO MOUNTAINS AREA AND THEIR SIGNIFICANCE

The Zhongtiao Mountains area is located in the southern part of the North China platform. The Sushui Group complex constitutes the crystalline basement of the area, which is overlain by the Precambrian Jiangxian, Zhongtiao and Xiyanghe Groups. The whole-rock Sm-Nd isochron age of the ultrapotassic metavolcanic series of the Jiangxian Group is 2.63±0.10 （2σ） Ga; the zircon U-Pb isochron age of the host calc-alkali metavolcanic-subvolcanic series of the Tongkuangyu copper deposit and the host hydrothermal-sedimentary series of the Hu-Bi-type stratabound copper deposit are 1.778±0.027 Ga and 1.832±0.026 Ga respectively. The former represent the formation ages of the ultrapotassic volcanic series and calc-alkali volcanic-subvoleanic series, while the latter represent the age of regional metamorphism. By means of the data of the Rb-Sr isochron ages obtained by predecessors, the andesite series of the Xiyanghe Group has been determined to be formed 1.454±0.05 Ga ago.