富山県南部の中新統岩稲累層のアダカイト質安山岩，高マグネシア安山岩，カルクアルカリ系列安山岩およびソレアイト系列安山岩の成因

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タイトル別名
  • Genesis of adakitic andesite, high-magnesian andesite, calc-alkaline andesite and tholeiitic andesite in the Miocene Iwaine Formation, southern part of Toyama Prefecture, Japan.
  • トヤマケン ナンブ ノ チュウシントウ イワイネ ルイソウ ノ アダカイトシツ アンザンガン コウ マグネシア アンザンガン カルク アルカリ ケイレツ アンザンガン オヨビ ソレアイト ケイレツ アンザンガン ノ セイイン

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説明

The Early Miocene volcanic rocks (the Iwaine Formation) are widely distributed in the Hokuriku Green Tuff Province. The volcanic rocks in the Iwaine Formation in the southern part of Toyama Prefecture can be divided into two groups in the FeO*/MgO-SiO2 diagram ; andesite group I has a tholeiitic trend showing enrichment in FeO*/MgO ratio from mafic to felsic andesite, whereas andesite group II has a calc-alkaline trend on which SiO2 content increases from about 54 to 68 wt.% despite a little variation in FeO*/MgO.Petrographic and chemical data indicate that andesite group I was formed from tholeiitic basalt magma by fractional crystallization of its phenocryst minerals.Andesite group II is composed mainly of calc-alkaline andesite with subordinate amounts of high-magnesian andesite and adakitic andesite. Most phenocrystic plagioclases in calc-alkaline andesite are dusy and show reverse zoning in terms of Ca, suggesting that the andesite was of magma mixing origin. Adakitic andesite is characterized by higher Sr and lower Y contents, and higher Sr/Y ratio than common andesites in island arcs, and is also characterized by higher MgO, Ni and Cr contents compared with typical adakite. The high-magnesian andesite shows lower K2O and Rb contents, and higher Sr/Y ratio than the Setouchi high-magnesian andesite and boninite. Adakitic andesite may result from modification of felsic (adakitic) magma generated by partial melting of the subducted oceanic crust, due to interaction with the overlying mantle peridotite or mixing with mafic magma during its rise to the surface. The high-magnesian andesite seems to have a genetical close relation to adakitic andesite.K2O content of basaltic magmas in the Iwaine Formation tends to increase from the study area (eastern margin of the Hokuriku Green Tuff Province) to the western area. Thus, it is possible to consider that the study area occupied the volcanic front in the Early Miocene Hokuriku Green Tuff Province.

収録刊行物

  • 地質学雑誌

    地質学雑誌 105 (11), 789-809, 1999

    一般社団法人 日本地質学会

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