長野県岡谷市〜下諏訪町に分布する赤渋沢層のK-Ar年代

抄録

The Akashibuzawa Formation is distributed in the Okaya-Shimosuwa area, Nagano Prefecture, central Japan, and consists mainly of basalt and basaltic andesite lavas and some subordinate pyroclastic units. The Akashibuzawa Formation unconformably covers the Latest Miocene Tozawa Formation consisting mainly of volcanic rocks of quartz andesite, and is, in turn, unconformably overlain by the Enrei Formation of Early Pleistocene in age. A wide range of K-Ar ages between 1.54 ± 0.07 Ma and 2.24 ± 0.13 Ma has been previously reported from the Akashibuzawa Formation. Really fresh samples of basalt and basaltic andesite have been collected from the Akashibuzawa Formation in order to determine more precise timing of the volcanic activity. Whole-rock K-Ar ages of three samples are obtained to be 2.31 ± 0.04 Ma, 2.77 ± 0.06 Ma and 2.82 ± 0.05 Ma. These are significantly older than the ages previously reported, and likely represent a more accurate age of the volcanic activity of the Akashibuzawa Formation.