Abstract
The dacitic lava dome group (DLDG), located on the NNW and SSE flanks of Rishiri volcano, northern Hokkaido, has been considered to be the Tertiary basement or early volcanic products of the Rishiri volcano. Newly obtained K-Ar ages of the rocks from the DLDG are 0.133±0.008 Ma (Peshimisaki lava dome), 0.078±0.005 Ma (Fujino lava dome) and <0.1 Ma (Ishiyama lava dome). These ages suggest that the DLDG are the products of flank eruptions during the early stage of activity of Rishiri volcano, and that the structure of the volcanic eddifice must be revised from those of previous studies. In addition, the chemical compositions of volcanic rocks from adjacent regions has changed from low-K tholeiitic during the Tertiary to alkalic (high Na20) during the Quaternary.
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