A strontium isotopic study of the volcanic rocks of the Myoko volcano group, central Japan

Abstract

Thirty-one selected volcanic rocks from the Myoko volcano group which comprises a volcanic chain of four independent volcanoes of Quaternary to Recent age are analyzed for 87Sr/86Sr ratios. The rocks of the lizuna volcano, the oldest among the Myoko volcano group, have higher 87Sr/86Sr ratios and show a larger scatter ranging from 0.70437 to 0.70556 than those of other volcanoes. The Kurohime volcanic rocks have a restricted range of 87Sr/86Sr ratios (0.70403∼0.70435). 87Sr/86Sr ratios of the Myoko volcanic rocks are almost the same in average to those of the Kurohime volcanic rocks, although somewhat varied ranging from 0.70378 to 0.70461. A single analysis of the Yakeyama volcanic rock yielded a 87Sr/86Sr ratio of 0.70427. A characteristic pattern in 87Sr/86Sr ratios is observed through the volcanic activity of the Myoko volcano group; 87Sr/86Sr ratios are high in the early stage of the volcanic activity and then decrease to low values, the late eruptives being characterized by constant 87Sr/86Sr ratios. The negative correlation between 87Sr/86Sr and Rb/Sr, and positive correlation between 87Sr/86Sr and Sr found in the rocks of the Iizuna volcano are interpreted to show the occurrence of contamination by materials with high 87Sr/86Sr ratios (>0.7056), low Rb/Sr ratios ( 300 ppm). Sialic crustal contamination may have played only a minor role.