長野県大門峠地域に分布する流紋岩質溶岩ドームのK-Ar年代 : 諏訪-八ヶ岳火山地域の流紋岩類の活動時期について

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The voluminous Quaternary volcanic rocks in the Suwa-Yatsugatake volcanic province (SYVP) of central Japan are known as Enrei volcanic rocks, Yabashira and Yatsugatake volcanic products. These volcanic rocks consist mostly of basalt and andesite, with a small amount of rhyolite. We report on the revised stratigraphy and K-Ar ages of rhyolitic lava domes in the Daimontoge area of the central part of the SYVP, and discuss the duration of rhyolitic volcanism in the SYVP. Our work clarifies the fact that the two rhyolitic lava domes studied were formed independently, ca. 0.24Ma (Utsukushimatsu lava dome) and ca. 1.07Ma (Daimontoge lava dome), and the age of the basement volcanic rocks predates ca. 1Ma. The duration of rhyolitic volcanism in the SYVP is divided into 1.2-0.6Ma and 0.3-0.2Ma, based on the findings of previous studies and this study. The previous studies show that volcanic activity had not occurred post-0.6Ma in the central part of the SYVP. However, our data show that the rhyolitic lava dome (Utsukushimatsu lava) was formed much later, ca. 0.24Ma. The spatiotemporal distributions and whole-rock composition of the rhyolite suggest that rhyolitic volcanism prior to 0.6Ma was related to the formation of Oiwake volcanic graben in the central SYVP. At the time of the effusion of the Utsukushimatsu lava dome, the activity of the Yatsugatake volcanoes that adjoined the eastern side of the Daimontoge area was the most vigorous in their history. Therefore, we suggest that the formation of the Utsukushimatsu lava dome in the Daimontoge area resulted from the increment and expansion of activity of the Yatsugatake volcanoes.
2009-04-30

長野県大門峠地域に分布する流紋岩質溶岩ドームのK-Ar年代 : 諏訪-八ヶ岳火山地域の流紋岩類の活動時期について

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