Pre-caldera Volcanic History in Aira Caldera Area : K-Ar Dating of Lavas from Kajiki and Kokubu Areas in Northern Part and Ushine Area in Southern Part of the Caldera

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  • SUDO Masafumi
    Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University
  • ISHIHARA Kazuhiro
    Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University
  • TATSUMI Yoshiyuki
    Institute for Geothermal Sciences, Graduate School of Science, Kyoto University

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Other Title
  • 姶良カルデラ地域の先カルデラ火山活動史 : カルデラ北縁部加治木, 国分地域及び南縁部牛根地域の溶岩流試料のK-Ar年代測定
  • アイラ カルデラ チイキ ノ セン カルデラ カザン カツドウシ カルデラ ホクエンブ カジキ コクブ チイキ オヨビ ナンエンブ ウシネ チイキ ノ ヨウガンリュウ シリョウ ノ K Ar ネンダイ ソクテイ

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Abstract

Ten K-Ar ages of the pre-caldera lava fiows from northern (Kajiki and Kokubu) and southern (Ushine) areas of the Aira caldera were determined precisely in order to document the detailed volcanic history before the Aira pyroclastic eruption at 24.5 ka. The K-Ar dating method used is the sensitivity method and mass fractionation correction procedure for the argon analysis. The results combined with the previous data show the following time-space distribution of the pre-caldera volcanism after 3 Ma; (1) 3 to 1 Ma: eruptions of andesitic lava flows (Hokusatsu volcanic rocks) at the northern and southern areas, (2) 1 to 0.5 Ma: eruptions of andesitic lava flows (Yuwandake andesite) at the northern area and basaltic to rhyolitic lava and pyroclastic fiows at the western area, (3) 0.5 to 0.1 Ma: eruptions of basaltic (Ushine basalt) and rhyolitic (Okoga-shima rhyolite) Iava flows at the southern area, (4) 0.1 to 0.025 Ma: eruptions of andesitic lava (Shikine andesite) and pyroclastic fiows at the northern area and rhyolitic lava flows (Shimizu and Ushine rhyolites) both at the northern and southern areas. The most remarkable feature of the pre-caldera volcanism is the eruptions of similar rhyolitic lava flows, which contain characteristic osumilite, at 15 km apart opposite sides of the caldera rim during 0.04 to 0.03 Ma. This might imply that a large magma chamber has been present under the Aira caldera since at least 0.04 Ma.

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