Evolution of magma feeding system in Kumanodake agglutinate activity, Zao Volcano, northeastern Japan

Abstract

The Kumanodake agglutinate of Zao Volcano in northeastern Japan consists of pyroclastic surge layers accumulated during the early part of the newest stage of activity (ca. 33ka to present). Our petrologic study of this agglutinate based on systematically collected samples aims to reveal the evolution of magma feeding system. To understand the magma evolution, we have examined samples from the agglutinate by using petrologic data including, petrography, analysis of minerals (plagioclase, pyroxene, and olivine), glass compositions, and whole rock major element and trace element (Ba, Sr, Cr, Ni, V, Rb, Zr, Nb, and Y) compositions. Agglutinate are mixed, medium-K, calc-alkaline olv-cpx-opx basaltic andesite (55.2–56.2% SiO2). Results show that the magma feeding system comprised a shallow felsic chamber injected by mafic magma from depth. The felsic magma (59–62% SiO2, 950–990°C), which was stored at a shallower depth, had orthopyroxene (Mg#=60–69), clinopyroxene (Mg#=65–71), and low-An plagioclase (Anca. 58–70). The mafic magma is further divisible into two types: less-differentiated and more-differentiated, designed respectively as an initial mafic magma-1 and a second mafic magma-2. The original mafic magma-1 was olivine (Fo~84) basalt (ca. 48–51% SiO2, 1110–1140°C). The second mafic magma-2, stored occasionally at 4–6km depth, was basalt (1070–1110°C) having Foca. 80 olivine and high-An (Anca. 90) plagioclase phenocrysts. These two magmas mixed (first mixing) to form hybrid mafic magma. The forced injections of the hybrid mafic magmas activated the felsic magma, and these two were mixed (second mixing) shortly before eruptions. The explosivity is inferred to have increased over time because the abundance of large scoria increased. Furthermore, the erupted magma composition became more mafic, which reflects increased percentage of the hybrid mafic magma involved in the second mixing. At the beginning of activity, the mafic magma also acted as a heat source for activation of the felsic magma chamber, thereby suppressing the volume percentage of mafic magma in the mixing. As the activity proceeded thereafter, the shallow felsic chamber would become more mobile. Consequently, the hybrid mafic magma would be able to mix with felsic magmas more easily, resulted in higher percentage of the hybrid mafic magma in the mixing.