Rock types of Miocene two petrographic provinces in the Pacific side region of Northeast Japan.

Two petrographic provinces are classified in the Miocene volcanic rocks widely exposed in areas to the east of the Quaternary volcanic front of Northeast Japan. The first is that of the island arc tholeiite in the Kitakami river area, where rocks are basalts, andesites and dacites, all being of the island arc tholeiite or calc-alkali rock series. On the other hand, the second is a mixed province of the oceanic tholeiite and the island arc tholeiite series, in which occur, in association with the island arc tholeiitic rocks, basalts and andesites occasionally rich in TiO2 and FeO*, icelandite-like andesites and dacites similar in composition to rocks of the oceanic regions. Its area extends from Sendai southwards to the Abukuma mountains district. The boundary between the two provinces corresponds to the Matsushima-Honjo line defined by Oide and Onuma (1960).