Formation and evolution of silicic magma plumbing system: Petrology of the volcanic rocks of Usu volcano, Hokkaido, Japan

Abstract

The whole-rock composition of the historical rocks of Usu volcano, which has erupted nine times since AD 1663, has changed from rhyolite (SiO 2 = 75.8 in AD 1663) to dacite (SiO 2 = 69.1 in AD 2000). Temporal changes in the composition of the eruptive magma have been explained as due to repeated withdrawals from a single, zoned magma chamber. Nevertheless, new petrographical and geochemical data suggest that the juvenile materials are composed of three groups: Group-1 is nearly aphyric hornblende–augite-bearing hypersthene rhyolite (SiO 2 = 74.3–75.8) from the 1663 eruption; Group-2 is porphyritic augite–hornblende–quartz-bearing hypersthene rhyodacite (SiO 2 = 70.5–72.7) from four eruptions during the 18–19th centuries; and Group-3 is slightly porphyritic, augite-bearing hypersthene dacite (SiO 2 = 69.1–70.5) from three eruptions in the 20th century. All the rocks show wide compositional distributions of phenocrystic minerals and coexistence of normally and reversely zoned phenocrysts, which suggest that the rocks originated from magma mixing. Many variation diagrams of the whole-rock chemistry show distinctive mixing trends with possible compositional gaps. The groups differ—especially Group-3 from Group-1 and Group-2—not merely in terms of several oxide–oxide diagrams of matrix glass chemistry, but also in terms of SiO 2 vs. Sr isotopic ratios of the whole-rock chemistry. These geochemical features suggest that the end-member magmas of each group are different from those of the other groups. Felsic end-member magmas, which have been voluminous, could not have been formed from a single magma by fractional crystallization or assimilation and fractional crystallization (AFC) processes: the felsic end-member magma in the magma plumbing system must have been replaced twice, and the three resulting magmas could have been active ever since. Compositional variations of these felsic magmas could have been caused by partial melting of the same source by different degrees. During the 350 years of volcanic activity, the temperature and degree of melting of the magma progressively increased from Group-1 to Group-3, as reflected by successive melt segregations from partially melted crustal materials by a single heat source.