Crystal clots from dome lavas (dacites) of Usu Volcano, Hokkaido

Dacitic dome lavas of Usu volcano contain crystal clots (aggregate of coarse crystals) consisting of strongly zoned plagioclase, hypersthene, titanomagnetite, cristobalite, and pale brown glass. The cores of plagioclase in clots are remarkably calcic, and those of hypersthene are magnesian than phenocrysts. Since the mineral assemblage of the clots is rather uniform, the clots may possibly be a breakdown product of such a phenocryst mineral as amphibole. However, their microscopic texture and bulk chemical composition never correspond to breakdown of amphibole phenocryst at early stage of crystallization. The crystal clots in Usu dacites were probably derived from a cogenetic mafic magma that mixed with rhyolitic one before the eruption in 1663 AD; that was the first activity of felsic magma in Usu volcano. After that the silica contents of the historic lavas are decreasing gradually. This change should be caused by an increase in mixing ratio of mafic magma to felsic one.