九州中北部に分布する火山岩のＫ‐Ａｒ年代とフィッション・トラック年代の比較検討 火山構造性陥没地の形成開始年代

Abstract

In central-north Kyushu, a large volume of Pliocene-Quaternary volcanic rocks are distributed in and around the east-west elongated graben (40×30 km), which is considered to be a volcanotectonic depression (VTD). The radiometric ages of the volcanic rocks are isochronally zonated in concordant with the outline of the VTD, so that the ages increase from the center to the rim of the VTD. In this study, K-Ar ages were determined upon four volcanic rocks from the northern rim of the VTD. K-Ar ages of the volcanic rocks from the Hikosan area (northwest part of the VTD) were determined as 5.0±0.4, 4.0±0.2 and 4.7±0.3 Ma, which show close agreement with the respective FT dates (Watanabe et al., in press). In the Yamaga area (northeast part of the VTD), the andesite lava from the Yabakei Formation, whose FT age is reported as 12.1 Ma (Hayashi et al., 1983), was dated by K-Ar method as 5.7±0.3 Ma. The K-Ar ages of the Usa Group which is overlain by the Yabakei Formation near the Yamaga area show 4.8±0.4, 5.3±0.3 and 4.9±1.1 Ma (Kamata, 1985b). Accordingly the previous FT age (12.1 Ma) for the Yabakei Formation is apparently too old, which may account for corrections made for thermal alteration of the specimen and/or contamination of accidental particles. On the other hand, the K-Ar age (5.7 Ma) is considered to be close to the eruption age of the Yabakei Formation in this area. These results indicate that the oldest volcanic rocks, which are distributed on the rim of the VTD, were erupted 5-6 Ma ago. Therefore it is suggested that the VTD has been formed since 5-6 Ma ago in centralnorth Kyushu.