福井県大野市，前期中新世荒島岳コールドロンの地質と岩石

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  • Geology and petrology of the Early Miocene Arashimadake cauldron in Ono City, Fukui Prefecture, central Japan.
  • フクイケン オオノシ ゼンキ チュウシンセイ アラシマダケ コールドロン ノ チシツ ト ガンセキ

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The Arashimadake cauldron is a concentrically zoned volcano-plutonic complex (7.5×5 km in size) located in the southeastern margin of the Tertiary Green Tuff region along the Japan Sea coast. The cauldron is composed of a central pluton (quartz monzodiorite), surrounding andesitic and basaltic volcanic rocks, and outer ring dikes (two-pyroxene porphyrite), and is inset in the basement rocks such as Mesozoic sedimentary rocks, Mesozoic granite and Paleozoic gneiss. The volcanic rocks are composed of alternating pyroclastic rocks and lavas intercalated with minor clastic sedimentary rocks, and dip toward the rim of the cauldron. Dikes of plagioclase porphyrite and dolerite are abundant in the basement rocks surrounding the cauldron, in which those dikes are rare. The dike swarm may be feeders of the stratovolcano, which existed prior to the cauldron subsidence.K-Ar whole rock ages and geologic relationship indicate following history. The feeder dikes and stratovolcano formed around 20 Ma. Its central part subsided along a circular fracture, along which the ring dikes intruded. Resurgent dome uplift may have taken place in the subsided volcanic block in relation to the subsequent intrusion of the central pluton at about 18 Ma. The resurgent doming is characteristic of the Valles-type cauldrons, although this cauldron is smaller and lacks dacitic or rhyolitic welded tuff, which marks the collapse.The volcano-plutonic rocks mostly belong to the medium- and high-K calc-alkaline series. Their initial 87Sr/86Sr ratios (SrI) range from 0.705 to 0.707, which are higher and more variable than that of the Tertiary tholeiitic basalt in the Japan Sea and Noto Peninsula (<=0.704). The correlation between SrI and Sr content suggests mixing of the alkali basaltic and tholeiitic magmas of mantle origin as well as assimilation of high 87Sr/86Sr, low-SiO2 crustal material such as pelitic gneiss.

収録刊行物

  • 地質学雑誌

    地質学雑誌 106 (5), 313-329, 2000

    一般社団法人 日本地質学会

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