ABSTRACT

On the basis of Sr and Nd isotopic data for Tertiary to Quaternary volcanic rocks, the isotopic characteristics of the upper mantle beneath the NE Japan arc and genesis of Middle Miocene bimodal volcanism are discussed. Basaltic rocks from the back-arc side, erupted from 35 Ma to present, can be divided into two different groups based on initial Sr isotopic ratios (Sri), initial εNd values, and ages. One group has higher Sri ratios (∼0.7040–∼0.7058) and lower εNd values (∼0–∼+3) than the other (∼0.7029–∼0.7040 and ∼+3–∼+8). Basaltic activity of the high Sri group took place continually from 30 to 10 Ma, whereas activity of the other group occurred mainly in two periods; i.e., at around 34–35 Ma and after –15 Ma. A similar pattern is also observed in basaltic rocks from the transitional zone and the trench side, where basaltic activity characterized by higher Sri ratios took place continuously during ca. 15–0 Ma, whereas eruption of lower Sri basalts occurred at around 15 Ma and after about 3 Ma. One interpretation of these features is that the sub-continental mantle beneath the NE Japan arc may be layered; comprising upper isotopically undepleted (Sri = ∼0.7040–∼0.7058, εNd = ∼0–∼+3) and lower depleted (Sri = ∼0.7029–∼0.7040, εNd = ∼+3–∼+8) parts. Basaltic magmas with an undepleted isotopic signature are considered to have formed from the upper layer, while those with a depleted isotopic signature are from the lower layer. The sharp increase in basaltic activity with a depleted isotopic character after –15 Ma in the back-arc side may be due to the upwelling of hot depleted layer from a deeper region in the mantle (asthenosphere) to the upper undepleted layer beneath the back-arc side of the NE Japan arc during the opening of the Japan Sea. This asthenospheric upwelling might have also caused melting of the lower crust to produce the Middle Miocene acidic volcanic rocks in the back-arc side and the transitional zone. Rhyolitic rocks from Tobishima Island and the Tsugawa-Tadami area in the back-arc side show significantly higher Sri ratios (0.7055–0.7072) and lower εNd values (∼0–∼-5) than the associated basaltic rocks, indicating a lower crustal origin for these rhyolitic rocks. On the other hand, the similarity in εNd values (∼+4) of dolerites and rhyolites from the Kakudate-Rokugo area of the transitional zone suggests derivation from a common mantle source with an isotopic signature between the upper undepleted and lower depleted layers.