森吉火山における安山岩 : デイサイト質マグマ生成モデル(<特集>火山のモデル(II))

抄録

The magma system of Moriyoshi volcano, northeastern Japan is discussed, according to geological, petrographical and geochemical data. More than 90% of the total eruptive products are andesite and dacite, accompanied by a small amount of basalt. Judging from major and trace element chemistry and Sr isotopic ratios, the basaltic magma cannot be parental to the andesite and dacite by a process of fractional crystallization. In addition, the lack of correlation between the Sr isotopic ratios and silica and the absence of basaltic andesite indicates that assimilation of basaltic magma with crustal material did not take place. The calculated magmatic temperature, abundance of incompatible elements (LIL, HFS and REE), and the LIL/HFS ratios of the andesite and dacite systematically change with time. The most plausible model which accounts for the observed results involves hydrous crustal anatexis, whereby the basaltic magma played an important role in supplying heat for crustal melting. During the initial stage, hydrous partial melts may have been generated by consumption of hydrous minerals within the source crustal materials. Following migration of the melt, the solidus temperature of the source would have increased. The variability in the solidus temperature and the hydrous state of the source region would control not only the magmatic temperature but also the stability of accessory phases, containing HFS and REE. Thus both abundance and ratio of those incompatible element wolud change.