Abstract
Abyssal tholeiites are found from Tobishima Island in Japan Sea, 40km northwest from Sakata, northeastern Japan. These basalts (Tobishima basalts), including lava flows, minor dikes, epiclastic lapilli tuff and tuff breccia, and hyaloclastite, were formed under water, and overlie acidic volcanic rocks. The basaltic lavas and dikes containing less than 10% of olivine and plagioclase phenocrysts are uniform in composition, but show various degrees of alteration. They are similar to abyssal tholeiites in composition of groundmass augite and coexisting phenocrysts of olivine and plagioclase. In addition, Tobishima basalts plot in the field of abyssal tholeiites in the tectonic discrimination diagrams of FeO*/MgO-TiO2, FeO*/MgO-FeO*, Y-Zr/Y, Y-Cr, TiO2-P2O5, Ti-Cr and Ti-Ni. The submarine and bimodal character of the volcanism at Tobishima suggests the possibility of Middle Miocene age instead of Early Miocene as was previously deduced from K-Ar dating.
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