日光火山群男体火山の１２ｋａ噴火の推移とマグマ供給系

Abstract

The 12 ka eruption of Nantai Volcano, central Honshu, Japan, produced a succession of pyroclastic deposits followed by the emplacement of a lava flow. The 12 ka eruption began with phreatic eruptions, forming a fine ash deposit (unit A). Explosive magmatic eruption followed forming a scoria-fall (unit B) and partially welded scoria-flow deposits (unit C). The scoria eruption was followed by a pumice eruption producing a pumice-fall deposit (unit D) and a consecutive voluminous pumice-flow deposit (unit E). The 12 ka eruption finished with the extrusion of a heterogeneous dacite-andesite mingled lava flow (unit F).The early scoriae are phenocryst-poor (∼3-15 vol.%) tholeiitic dacite and andesite with an anhydrous phenocryst assemblage (plagioclase+orthopyroxene+clinopyroxene+Fe-Ti oxides±olivine), whereas unit D-F eruptives are porphyritic calc-alkaline dacite and andesite with a hydrous phenocryst assemblage (plagioclase+hornblende+quartz+orthopyroxene+clinopyroxene+Fe-Ti oxides±olivine). This suggests that two magma types, firstly tholeiitic and secondly calc-alkaline, fed the 12 ka eruption. Although major and trace elements are broadly similar in all dacites erupted between unit D and F, there are considerably differences in phenocryst contents: unit-D and unit-E dacites have lower crystal contents (∼17 vol.%) whereas unit-F lava has higher crystal contents (∼50 vol.%). The unit D-E dacite (pumice) and the unit-F dacite (lava) may have been derived from different parts of a magma chamber, perhaps from inner melt-rich and marginal mushy parts, respectively.