K-Ar DATING OF HANNUABA BASALTS AT ZHANJIAKOU
Wang Huifen Dai Tongmo Pan Sikun Yang Xuechang (Institute of Geochemistry, Academia Sinica)
The Hannuaba basalts are located in the area of the Mongolian Plateau. Research work on the basalts started in 1920's. Still no unanimous conclusion has been drawn on the age of emplacement.Samples were systematically collected from four representative sections in this re-gion. Whole-rook samples and some separated minerals, such as analbite and phlogopite were all collected from fresh unaltered or slightly altered basalts and determined by K-Ar method.The 38Ar isotopic dilution method was used to determine K-Ar ages of the samples on an MM-1200 mass-spectrometer. 20 apparent ages were obtained. After isochron-treated these ages fall into two groups: 24m.y. and 18 m.y., corresponding to Early Miocene and Middle Miocene, respectively. The results are in good agreement with stra-tigraphic evidence.