Petrogenesis of Mafic Inclusions in Rhyolitic Lavas from Narugo Volcano, Northeastern Japan

Mafic inclusions present in the rhyolitic lavas of Narugo volcano, Japan, are vesiculated andesites with diktytaxitic textures mainly composed of quenched acicular plagioclase, pyroxenes, and interstitial glass. When the mafic magma was incorporated into the silica-rich host magma, the cores of pyroxenes and plagioclase began to crystallize (>1000°C) in a boundary layer between the mafic and felsic magmas. Phenocryst rim compositions and interstitial glass compositions (average 78 wt % SiO2) in the mafic inclusions are the same as those of the phenocrysts and groundmass glass in the host rhyolite. This suggests that the host felsic melt infiltrated into the incompletely solidified mafic inclusion, and that the interstitial melt composition in the inclusions became close to that of the host melt (c. 850°C). Infiltration was enhanced by the vesiculation of the mafic magma. Finally, hybridized and density-reduced portions of the mafic magma floated up from the boundary layer into the host rhyolite. We conclude that the ascent of mafic magma triggered the eruption of the host rhyolitic magma.