Comparative petrology of the Hotaka and Akagi volcanoes

Abstract

The path of fractional crystallization in the magma reservoir is different in the Akagi and Hotaka volcanoes, both of which are frontal volcanoes of northeastern Japan arc. Estimated magmatic temperature decreases by about 130°C from 1050 to 920°C during the compositional change of liquid from 54 to 70 wt % SiO₂ in the Akagi volcano. In the Hotaka volcano, however, the temperature decreases by only about 60°C from 1050 to 990°C during the same compositional change. This difference is also reflected on the early appearance of hornblende and common occurrence of groundmass hypersthene in the Akagi volcano. Al₂O₃, content of the magma of the Akagi volcano is higher than that of the Hotaka volcano. These differences can be well accounted for by the different water contents in the parental magmas; the magma of the Akagi volcano had a higher water content.
They also show different chemical trends on FeO^*/MgO-SiO₂ variation diagram; the constant FeO^*/MgO ratio with increasing SiO₂ in the Akagi volcano in contrast to the iron-enrichment trend in the Hotaka volcano. Least-squares mixing calculations show that the different iron-enrichment is caused by the different proportions of plagioclase in the fractionated minerals; higher plagioclase proportion in the Akagi magma than in the Hotaka magma.