In the Kabato district, central Hokkaido, the Aoyama Basalt which represents a volcanic on-land product of the early phase of Cenozoic magmatism, is locally exposed. The basaltic lava flows unconformably cover the Eocene Kabato Formation and are overlain by the Middle Miocene Ponsubetsu Formation showing an angular unconformable relationship. The Aoyama Basalt consisting of basalt and basaltic andesite belongs to the tholeiitic rock series. The patterns of MORB-normalized incompatible elements show that the Aoyama Basalt is enriched not only in LIL elements but also relatively in HFS elements, being similar to those of some island arc tholeiites from NE Japan Arc and active continental margin rocks. The abundance of the incompatible elements is higher than the island arc tholeiites. The Aoyama Basalt has higher ⁸⁷Sr/ ⁸⁶Sr and lower ¹⁴³Nd/ ¹⁴⁴Nd ratios, which are distinguishable from the Pliocene volcanic rocks in the same region. In comparison with the NE Japan Arc magma and the Pliocene magma in the same region, the source mantle and the relationship to the spreading of the Japan Sea basin were disscussed.

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