A DISCUSSION ON THE ISOTOPIC GEOCHRONOLOGY OF THE MOTIANUNG MASSIF
Si Shi (Peking No. 3 Research Institute)
Six zircon smaples were collected from the Motianling massif. According to the characteristics of zircons, they were divided into three types and that of type Ⅲ were separated for isotopie age determination. Through calculation and graphical analysis of the isotopie data on each zircon sample, true zircon ages were obtained: 28(50 m.y. for type 1, 7(50 m.y. for type Ⅱ and 510-580m.y. for type Ⅲ. It is evident from geological environments of the Motianling massif, in conjunction with the characteristics of the zircons, that the first type formed earlier than the massif, the second one was contemporary with the massif, thus representing the age of the latter, and the third came into being due to metamorphism to which the massif had been subjected after its formation. Prom the above facts it is reasonable to believe that the Motianling massif is formed by granitization during the Xuefeng movement from the Sibao Group and the Banchi Group in the studied area. Later, it once again suffered metamorphism (luring the Caledonian movement and thus giving rise to the massif we observed at present.