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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

フォーマット:
論文(リポジトリ)
責任表示:
川畑, 博 ; 周藤, 賢治
出版情報:
日本地質学会, 2000-10
掲載情報:
地質学雑誌 — 地質学雑誌
ISSN:
00167630  CiNii Research  Webcat Plus  JAIRO
著者名:
巻:
106
通号:
10
開始ページ:
670
終了ページ:
688
概要:
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 adjacent areas, northea st Shikoku. These volcanic masses have the following common lithological characteristics and stratigraphic sequences. Volcanic strata unconformably overlie the Cretaceous 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.<br />四国北東部の七宝山体,弥谷山体,我拝師山体を構成する瀬戸内火山岩類は,下位に流紋岩質火砕岩・溶岩,上位に安山岩質~デイサイト質溶岩・火砕岩が累重する共通の層序を示す.流紋岩質火砕岩の特徴から,流紋岩質岩の活動時,本地域には水域が点在し,火砕岩は起伏のある基盤地形を埋めるように堆積したと推定される.一方,安山岩質~デイサイト質岩のうち,普通角閃石や黒雲母などの含水鉱物を斑晶に含む岩石は,これら火山岩類の活動時期のより初期に集中して噴出した.上記の3山体は,各山体内の火道からもたらされた噴出物が重なることで形成された複合火山といえる.また,瀬戸内火山岩類中の複合溶岩を構成する岩石は,玄武岩からデイサイトに限られており,流紋岩は複合溶岩の構成メンバーになっていない.弥谷山体にみられる複合溶岩は,岩脈状のマグマ溜まり内で成層していた組成の異なるマグマが混合することで形成された可能性がある. 続きを見る
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