1989 年 84 巻 6 号 p. 200-207
Sr isotope ratios are analyzed for voluminous acidic pyroclastics erupted at 1-3 Ma in five areas on the volcanic front of Northeast Japan. The initial values of 87Sr/86Sr ratios range from 0.7040 to 0.7055. There is no significant difference in ratios between 1-3 Ma acidic pyroclastics and 0-1 Ma andesitic volcanics in each area in spite of differences in age and in mean SiO2 content. On the other hand, the ratios in both of 1-3 Ma and 0-1 Ma volcanics vary along arc in the same manner. The changes of Sr and Rb contents in each area are consistent with systematic changes by fractional crystallization. The low 87Sr/86Sr ratios and chemistry suggest that 1-3 Ma acidic pyroclastics of Northeast Japan formed by a high degree of fractional crystallization from basic magma which is common in genesis with young andesitic volcanism. The mechanisms of the formation of the basic magma and the character of mantle source in each area have not changed for the past 3 Ma. Degrees of fractional crystallization changed with changes of the tectonic condition of shallow magma chamber from a weak horizontal compression stress field to a strong one.