Volcanic geology of the Setouchi volcanic rocks from Kanonji and adjacent areas, northeast Shikoku, Japan.

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  • 四国北東部，観音寺周辺に分布する瀬戸内火山岩類の地質
  • シコク ホクトウブ カンノンジ シュウヘン ニ ブンプ スル セトウチ カザンガンルイ ノ チシツ

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Abstract

Geological investigation was carried out on the Miocene volcanic strata (Setouchi volcanic rocks) named the Sanuki Group constituting three volcanic masses, Shichihosan, Iyadanisan and Gahaishiyama masses, in Kanonji and adjacent areas, northeast Shikoku.These volcanic masses have the following common lithological characteristics and stratigraphic sequences. Volcanic strata unconformably overlie the Creataceous Ryoke granitic rocks and the Miocene Tonosho Group. These volcanic masses are lithologically divided into the lower unit consisting mainly of rhyolitic volcaniclastic rocks and accosiated rhyolitic lavas, and the upper unit composed mainly of andesitic and dacitic lavas with subordinate amounts of their volcanic breccias. The upper unit includes the composite lava flows formed by the mingling of andesitic and dacitic magmas in Iyadanisan mass. Rhyolitic volcaniclastic rocks from the lower unit are characterized by parallel lamination and slump structute and rubbles with water-chilled joints, suggesting the volcanism in subaqueous environment. Based on their phenocryst mineral assemblages and groundmass texture, andesitic and dacitic rocks constituting the upper unit were divided into twelve rock types. Among them, rocks with hornblende and/or biotite phenocrysts seem to have been erupted at the early stage of andesitic-dacitic activity. It is suggested that andesites and dacites of the respective masses erupted from several vents within each mass, because of the difference in constituent rock types among the three volcanic masses. Rocks from basalt through andesite to dacite constitute composite lava flows and dikes in Iyadanisan mass and many other volcanic masses in northeast Shikoku, but rhyolitic rocks are not constituents of such lavas and dikes. This feature is consistent with stratigraphic relationship between the rhyolite and basaltic-dacitic rocks, suggesting the difference in genesis between the both rock types. The Iyadani composite lava flows could be formed by eruption from compositionally zoned magma chamber.

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