Evolution of the calc-alkaline magma feeding system of the Komakusadaira pyroclastics in Zao volcano, NE Japan

Abstract

We examined the evolution of the magma feeding system at Zao volcano (NE Japan) during seven episodes of Komakusadaira pyroclastic activity (ca. 33–12.9 ka). The rocks, which are calc-alkaline and medium-K basaltic andesite to andesite (54.9–59.5% SiO₂), are herein suggested to have been derived from two main magmatic end-members: silicic andesite magma (59~60% SiO₂, 940~970°C, cpx-opx-plg as phenocrysts) and basaltic magma (52% SiO₂, ~ 1085°C, Fo_78~80 olv-plg as phenocrysts). These two end-members retained their compositional integrities throughout all seven eruptive episodes. The presence of Mg-rich (Fo_{> 80}) olivine phenocrysts suggests the basaltic end-member was derived from fairly primitive magma (> 1100°C), which differentiated during ascent. By examining temporal variations in the chemical compositions of the phenocrystic minerals, three evolutionary pathways that produced the erupted products are defined; these are reflected in seven sequential episodes: (1) Crystal-rich silicic andesite magma was repeatedly withdrawn from the reservoir by the forced injection of basaltic magma (episode 1). (2) Crystal-poorer intermediate magma was formed by effective mixing of injected basaltic magma. Subsequent eruption of the intermediate magma was triggered by a new injection of mafic magma. The percentage of intermediate magma involved in the eruptions decreased over time (episodes 2–4). (3) By repeated fairly primitive to basaltic magma injections, the percentage of the mobilized part of the silicic andesite magma involved in the erupted magma gradually increased over time (episodes 5–7).