Etai-dake basalt complex, first named by Satoh (1961), forms a large lava platform (5×7km), situating the eastern flnak of Shokanbetsu volcano, in western Hokkaido. It consists of olivine bearing basalt and basaltic andesite lava flows of Pliocene age (Table 1).
Satoh (1961) published the description of the basalts from geological and petrological view points, but no petrochemical data were presented.
The basalts are rich in olivine but poor in plaigoclase phenocrysts, and contain little hypersthene. Sometimes xenoliths of the basement rocks are included.
In thin section, most of plagioclase phenocrysts have dusty inclusions in their inner zone, suggesting some events in crystallization process.
Chemical analyses of seven representative rocks (Table 3) indicate that the basalts have a little alkalic character and are slightly undersaturated or oversaturated in silica. And the basalts have a intermediate position between alkali olivine basalt and calc-alkaline andesite, in the silica-alkalis diagram (Figure 2).