Voluminous basaltic rocks with arc geochemical signatures were produced in the Akita-Yamagata area, the back-arc side of NE Japan, in the Middle Miocene. temporally straddling the opening of the Japan Sea. Eleven of these rocks were studied for trace elements; among these seven whole rocks and five clinopyroxene separates were analyzed for 87 Sr 86 Sr and four clinopyroxene separates were analyzed for 143 Nd 144 Nd . The results suggest that the lithosphere has been partly broken up and eliminated or thinned beneath the back-arc side of NE Japan since ∼ 16 Ma. The new analyses and existing data collectively show that the basaltic magmas have decreased in 87 Sr 86 Sr and increased in 143 Nd 144 Nd since ∼ 16 Ma. Coincidently the magmas have changed their geochemical signature from a continental margin type to an oceanic arc type on average. The basaltic rocks younger than 16 Ma are likely to be products of varying degrees of mixing between subduction-related basaltic magmas with low 87 Sr 86 Sr (0.703–0.7035) and partial melts of subarc lithospheric mantle with high 87 Sr 86 Sr (>0.7055 at least in part) and arc tholeiitic geochemical characteristics. The basaltic rocks older than 16 Ma seem to have been more intensely contaminated by crustal material.