[Abstract]

The eruptive activity of Torishima volcano is divided into three stages; the stratovolcano stage, the caldera-forming stage and the central cone stage, respectively. Volcanic rocks of the stratovolcano stage are characterized by basalt containing abundant plagioclase phenocrysts. The chemical variation of this stage in oxide-oxide diagrams can be explained basically by addition of plagioclase and moderate removal of mafic minerals. Volcanic rocks of the caldera-forming stage are characterized by olivine-bearing dacite. Their whole-rock compositions lie on a straight line between the rhyolitic glass and the stratovolcano basalt with MgO = 8 wt.%. They also show different Ba/Zr and Nb/Zr ratios from stratovolcano-stage basalts, indicating contamination of crustal materials. So they were most likely to be formed by magma mixing of basaltic magma and rhyolitic melt formed by partial melting of mafic lower crust. Volcanic rocks of the central cone stage are classified into two groups; Komochi-yama basalts and Io-yama basaltic andesites, respectively. Io-yama basaltic andesites make a tholeiitic trend with stratovolcano-stage basalt in SiO2 vs FeO*/MgO diagram and have higher TiO2 and FeO* contents than other volcanic stages. On the other hand, they lie on a mixing line between caldera-forming-stage dacite and stratovolcano-stage basalt in Zr vs Ba/Zr and Nb/Zr diagrams. Therefore, their magmatic evolution can be explained by two processes; (1) TiO2 and FeO* enrichment by additional fractional crystallization of stratovolcano-stage basalt, (2) magma mixing between caldera-forming-stage dacite and differentiated magma with higher TiO2 and FeO* content.

Key words:

Torishima volcano, Izu islands, magmatic differentiation, magma mixing, fractional crystallization