抄録
We measured trace element abundances and 87Sr/86Sr ratios of andesites from Yufu-Tsurumi volcano group. The results and mineralogical data indicate that the andesite magma is a product of magma mixing and that mixing end-members are represented by dark inclusions from Yufu-Tsurumi volcano group and dacite from adjacent older volcano (Jissoji volcano).
Trace element abundances vs. SiO2 plots of the andesites from a single variation trend between the inclusions at the mafic end and the dacite from the old volcano at the felsic end. 87Sr/86Sr rations of the andesites range from 0.7039 to 0.7044 and increase with increasing SiO2 contents. In contrast, the inclusions have a narrow range of 0.7038 to 0.7040. Phenocrysts of the andesites are petrographically divided into two groups as follows; 1) olivine, Mg-rich clinopyroxene, pargasite and An-rich plagioclase, and 2) Mg-poor orthopyroxene, magnesio-hornblende, An-poor plagioclase and quartz. Phenocrysts of group 1) are derived from mafic magma, whereas those of group 2) from felsic magma. Plots of highly incompatible element concentration vs. highly/moderately incompatible element ratios show consistency between observed data and the mixing curves, which was calculated from the inclusions from Yufu-Tsurumi volcano group and the dacite from Jissoji volcano. As inferred from the bulk chemistry of andesites, proportions of the mixed basaltic magma increased with time. This temporal change agrees with modal compositions.
Magma mixing of Yufu-Tsurumi volcano group did not occur under static condition within a closed magma chamber but under dynamic condition during magma's ascent from a deeper chamber. Our proposed mixing model in Yufu-Tsurumi volcano group is that the basaltic and dacitic magmas commingled during ascent from a deeper magma chamber, and hybridized in a shallow chamber, then the hybrid magma erupted.
