[Abstract]

Miyakejima, a volcano island, resting on the Izu-Mariana arc is an active basaltic stratovolcano. The eruptive activity is divided into five stages; (1) Ofunato stage (10,000 y.B.P.-4,000 y.B.P.), (2) Tsubota stage (4,000 y.B.P-2,500 y.B.P.), (3) Oyama stages> (2,500 y.B.P.-1,300 y.B.P.), (4) Oyama stages? (1,300 y.B.P.-A.D.1469), (5) Shinmio stage (A.D.1469-1983 eruption). Evolution of the magma plumbing system during the last 10,000years is discussed on the basis of petrological investigations of systematically collected rocks from various stratigraphic levels. All five stages consist of a number of lava flows and scoria fall deposits. SiO2 content shows a range from 48.6wt.% to 63.2wt.%. Since 4,000 years ago, whole-rock Mg# (=Mg/(Mg+Fe)qû100) systematically decreases at the initial eruption of each stage. These variations can be explained basically by fractional crystallization. On the other hand, mineral compositions and petrographical characters indicate that most of the eruptive products during the last 10,000 years contain three different types of phenocrysts (Type 1, Type 2, and Type 3), which can not equilibrate with each other. Different types of phenocrysts often exist within a single rock sample. Type 1 phenocrysts consist of calcic plagioclase (An 90) and magnesian olivine (Fo 80). Considering Fe-Mg partitioning between liquid and olivine, olivine of Type1 phenocrysts can not equilibrate with any whole-rock compositions. Type 1 phenocrysts occur in Ofunato stage and in the earlier period of each stage. Abrupt increase of whole-rock Mg# strongly suggests that relatively undifferentiated magma carrying Type1 phenocrysts was supplied into the preexisting magma plumbing system. Phenocrysts of Type 2 (plagioclase, olivine, and clinopyroxene) and Type 3 (plagioclase, clinopyroxene, orthopyroxene, and magnetite) are able to equilibrium with basaltic magma and andesitic magma, respectively. Type 2 phenocrysts occur since the eruption formed Hatchodaira caldera (2,500 y.B.P.). Type 3 phenocrysts occur in Tsubota and the later stage. Subsequently, whole-rock Mg# decreased by fractionation of phenocrysts of Type 2 and Type 3 since Oyama stage and in Tsubota stage, respectively. Keywords: Miyakejima volcano, magma plumbing system, evolution of magma, magmatic differentiation

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