Alkali basalt from Seifu Seamount in the Japan Sea: post-spreading magmatism in a back-arc setting

We present geochemical and 40Ar/39Ar age data for a peridotite xenolith-bearing basalt dredged from Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Japan Sea. A 40Ar/39Ar plateau age of 8.33 ± 0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it was erupted shortly after the termination of back-arc spreading in the Japan Sea. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element 20 enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Japan Sea. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an EM1-like component. The SSM basalt was formed in a post-back-arc extension setting by low-degree partial melting of upwelling asthenosphere that had 25 previously been associated with the main phase of back-arc magmatism.