Sr and Nd isotopic compositions of the magma source beneath north Hokkaido, Japan: comparison with the back-arc side in the NE Japan arc

Most middle Miocene to Quaternary basaltic rocks younger than about 12 Ma from north Hokkaido show a narrow range in initial 87Sr/86Sr (SrI) and initial 143Nd/144Nd (NdI) isotope ratios, which are slightly undepleted, compared to MORB. SrI ranges from 0.70296 to 0.70393 and NdI from 0.51288 to 0.51307, but there is no temporal variation in either SrI or NdI. An exceptional basalt from Ohmu area, which is more undepleted in terms of both SrI and NdI than other north Hokkaido (NH) basaltic rocks. Most of the NH basaltic rocks are isotopically similar to basaltic rocks found on the back-arc side of the NE Japan arc (BA basaltic rocks) younger than about 15 Ma, whereas basaltic rocks from the Ohmu area are isotopically similar to BA basaltic rocks older than 15 Ma. The generation of both NH and BA basaltic rocks appears to be closely related to mantle evolution. This involves upwelling of asthenosphere into the continental lithosphere beneath north Hokkaido and the back-arc side of the NE Japan arc, at the same time as spreading of the Kurile back-arc basin and Japan Sea back-arc basin. The magma source for the Ohmu area basalts and the BA basaltic rocks prior to 15 Ma may correspond to the subcontinental lithospheric mantle thinned by the upwelling of asthenospheric mantle material from which most of the NH basaltic magmas and the BA basaltic magmas younger than 15 Ma were derived. This asthenospheric mantle is slightly different from MORB source in terms of major- and trace-element characteristics such as TiO2/K2O and Zr/Y as well as Sr and Nd isotopes, which may be due to involvement of EM II-like component.