阿蘇中央火口丘群西側斜面において新たに発見された流紋岩〜デイサイト質溶岩流

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(3) ,**- 0 . ,**. . / . Rhyolite to Dacite Lava Flows Newly-discovered on the Western Slope of Aso Central Cones, Southwestern Japan Naoaki MASUDA, Kazunori WATANABE and Yasuo MIYABUCHI Three lava flows : Nakahono, Nagano, Otogase lava flows and Otogase debris avalanche deposit were newly discovered on the western slope of Aso central cones, southwestern Japan. The Nakahono lava is biotite rhyolite (SiO,1- wt.ῌ) and is distributed on only a floor of the Otogasegawa River. Occurrence age of the Nakahono lava is older than that of ACP- or ACP/. The Nagano lava is pyroxene dacite (SiO,02 wt.ῌ) and comprises a pyroclastic cone (ca. 0** m diameter). The Otogase lava is pyroxene rhyolite (SiO,1- wt.ῌ) and forms a lava plateau (ca. + km long and ,**῍-** m wide). The lava is interbedded between the Aira-Tn ash (AT, ca. ,3 cal ka) and the Kikai Akahoya ash (K-Ah, ca. 1.- cal ka). The Otogase debris avalanche deposit is possibly generated by a collapse of lava flow or dome on the vent of the Otogase lava. Based on the stratigraphic relationship with tephra deposits, the eruption age was estimated at ca. ,* cal ka. In the period of younger stage than approximately -* cal ka, intermittent eruptions producing three rhyolite to dacite lava flows provide important information about eruptive history and magma evolution in the western part of Aso central cones. Key words : lava, rhyolite, dacite, debris avalanche, tephra. +. . <2A 2J=

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(10) +32/ A1~©.. [5\V]^_9kl<=N~FàmZM O). VRQZM. ª203῍,,,/ «¬®¯° +,00 d± Aso Junior High School, Kurokawa +,00, Asomachi, Kumamoto 203῍,,,/, Japan. ª20*῍2/// «²°³ ,῍.*῍+ «d´dp Faculty of Education, Kumamoto University, Kurokami ,῍.*῍+, Kumamoto 20*῍2///, Japan.. ª20*῍*20, «²°³ .῍++῍+0 µ¶·¸¹º»¼¹ Kyushu Research Center, Forestry and Forest Products Research Institute, Kurokami .῍++῍+0, Kumamoto 20*῍*20,, Japan. Corresponding author : Naoaki Masuda e-mail : msdn_*[email protected]. 3{ H O.

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