The K-Ar age and the Sr isotopic ratio of a pleistocene datong basalt, China

The basement of the main land of China is composed of Precambrian formation, and the volcanic activities in Pliocene and Pleistocene epochs are concentrated to Datong district, Shansi province. But the details of the age of activities are not clear. The determinations of the K-Ar age and the Sr isotopic ratio were carried out on the lava flow of Datong basalt. The results contribute to the estimation of the nature of magmatic substance from the volcanos of continent. The samples for measurement were collected from Datong district, and the specimens of about 1 cm3 were cut out from the center of sampled blocks. One part of the specimens was used for the Ar measurement, and the rest was pulverized to the powder under 200 mesh, and used for the quantitative analysis of K, Rb and Sr, and the measurement of Sr isotopic ratio. The obtained K-Ar age was 0.25+- 0.4 Ma, which is nearly the same as the geologically expected value. 87Sr/86Sr value was measured to be 0.70376 +-0.00002 which was similar to that of the arcuate islands of Japan, but the relation with the sinking of plate was different. Further studies are necessary to clarify the mechanism of generation and supply of magma for understanding the nature of volcanic activities. Comprehensive consideration based on 3He/4He ratio and other information is required. (Ishimitsu, A.)