Textural and compositional evidence for magma mixing and its mechanism, Abu volcano group, southwestern Japan

Abstract

Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2 content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.