伊豆大島火山の岩石学的発達史(<特集>火山のモデル(II))

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A petrologic model of Izu Oshima volcano is proposed on the basis of petrological investigations of rock samples collected systematically from various stratigraphic levels. Izu Oshima is divided into three groups ; (1) the Senzu group, which is the oldest, (2) the Older Oshima group and (3) the younger Oshima group. All three groups consist of many lava flows and scoria fall deposits. Rock types of the lavas and scoria vary from olivine-basalt through pyroxene-basalt and aphyric basalt to subordinate aphyric or magnetite bearing andesite. SiO_2 contents range from 50 to 58 wt%, although most of the rocks have 50 to 54 wt% SiO_2. They are low-K tholeiite and show typical tholeiitic trend with strong Fe enrichment. The bulk rock Mg# generally tends to decrease from the oldest to the youngest and is concordant with the change of phenocryst assemblages. These variations can be explained by fractional crystallization of olivine, two pyroxenes and plagioclase, and is consistent with experiment at 1 atm under dry conditions. It shows that, in Izu Oshima volcano, the fractionation appears to have taken place in a shallow magma reservoir under almost dry conditions. The first supplied relatively primitive (Mg# = 50 ; modal plagioclase phenocryst is less than 10%) basalt of Izu Oshima volcano exhibit high Al_2O_3 content (〜18 wt%). Because estimated H_2O content of the Izu Oshima magma is low(>0.5 wt%), the high Al_2O_3 character of the magma may be attributed to the Pl-free high pressure fractionation. Between the Older and the Younger Oshima groups, during which time the present caldera was formed, bulk Mg# abruptly increased whereas K_2O increased. Phenocrysts in the rocks of the transitional period show reverse zoning, and eruption volume is large in the earlier stage of the Younger Oshima Group. From these observations, it is inferred that a new, relatively high-Mg primitive magma with higher K_2O was supplied to the magma reservoir of the Izu Oshima volcano during the time of caldera formation.
特定非営利活動法人日本火山学会の論文
1991-07-15

著者

川辺禎久
地質調査所

伊豆大島火山の岩石学的発達史(<特集>火山のモデル(II))

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