Late Miocene volcanic rocks (Geiyo volcanic rock) and early-middle Miocene volcanic rocks (Setouchi volcanic rocks) from the Geiyo Islands in the western Setouchi region, southwest Japan.

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  • SENO Mamoru
    College of Liberal Arts and Science, Kurashiki University of Science and the Arts
  • MATSUURA Hirohisa
    Geology Department, Geological Survey of Japan

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Other Title
  • 瀬戸内海西部，芸予諸島の後期中新世火山岩類（芸予火山岩類）及び前‐中期中新世火山岩類（瀬戸内火山岩類）について
  • 瀬戸内海西部,芸予諸島の後期中新世火山岩類(芸予火山岩類)及び前-中期中新世火山岩類(瀬戸内火山岩類)について
  • セトナイカイ セイブ ゲイ ヨ ショトウ ノ コウキ チュウシンセイ カザンガンルイ ゲイ ヨ カザンガンルイ オヨビ ゼン チュウキ チュウシンセイ カザンガンルイ セトウチ カザンガンルイ ニ ツイテ

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Miocene volcanic rocks were discovered in the Geiyo Islands, western Setouchi region of southwest Japan. These occur as dikes or volcanic necks with marginal pyroclastic rocks. The volcanic rocks are composed of high-alkali tholeiitic olivine basalt, olivine andesite and bronzite andesite, in which the olivine andesite from Oge-shima chemically corresponds to high magnesian andesite.<br> Whole rock K-Ar age determinations were carried out on olivine basalts, olivine andesites and bronzite andesites. An olivine andesite from Osakishimo-shima and two bronzite andesites from Okamura-shima yielded late Miocene ages of 8.2±0.7 Ma, 7.8±0.4 Ma and 8.0±0.4 Ma, respectively. It follows that subalkaline volcanism distinctively occurred at late Miocene age in the western Setouchi region. Accordingly, we give a name of “Geiyo volcanic rocks” to these volcanic rocks, which include late Miocene high-alkali tholeiitic olivine basalt reported from the adjacent area.<br> On the other hand, olivine basalts from Okubi-shima and Ko-oge-shima give middle Miocene ages of 12.8±0.6 Ma and 15.4±0.8 Ma, respectively. In addition, an olivine andesite from Oge-shima has been dated as 17.4±0.9 Ma, indicating early Miocene age. On the basis of the radiometric ages, petrography and whole rock chemistry, these volcanic rocks are considered to be a member of the Setouchi volcanic rocks.

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