The activity of Towada Volcano started about 200,000 years ago and more than 20 eruptive episodes have been geologically recognized. The Ninokura scoria group, investigated in this study, is a series of pyroclastic fall deposits (NK-a to NK-k) that erupted just after the latest caldera-forming Hachinohe pyroclastic flow (13,000 y. b.p.). In order to understand both the deep and shallow magma processes of the Ninokura stage, we have particularly examined the chemistry of glass inclusions enclosed in phenocrysts, in addition to whole-rock and mineral chemistry. Glass inclusions are useful because they are considered to represent magmatic melt present at the time of crystal growth. The bulk rock compositions of the Ninokura scoria belong to tholeiitic series. On the other hand, the compositions of glass inclusions range from tholeiitic series to calk-alkalic series, suggesting the mixing of both series magma.
The compositional change and supposed magma processes during the Ninokura stage are summarized as follows. In the earlier stage (NK-k to NK-i), each scoria has large heterogeneity in the composition of phenocrysts and glass inclusions, and the chemical compositions of magmas tend to be more mafic with time. The compositions of glass inclusions are on a mixing line formed by the Hachinohe magma and a basaltic magma. We interpret the compositional variation as a result of mixing of the residual felsic magma (Hachinohe) with the newly injected mafic magma. In the middle (NK-h to NK-e) and later (NK-d to NK-a) stages, the compositions tend to become SiO2-rich. The heterogeneity is minimum at the NK-h stage. The interpretation is that the residual magma was replaced by the mafic magma at the time of the NK-h stage. Magma of NK-g to NK-e can be explained by fractional crystallization. In the last stage (NK-b) remarkably SiO2-rich glass appears. It is probably crustal melt generated by the heat of repeatedly injected basaltic magma.
- Key words:
- Ninokura scoria, Towada volcano, glass inclusions, magma process