Pb, Nd, and Sr isotopic constraints on the origin of Miocene basaltic rocks from northeast Hokkaido, Japan: Implications for opening of the Kurile back-arc basin

Late Miocene (7-9 Ma) basaltic rocks from the Monbetsu-Kamishihuro graben i n northeast Hokkaido have chemical affinities to certain back-are basin bas alts (referred to herein as Hokkaido BABB). Plo-, Nd- and Sr-isotopic compo sitions of the Hokkaido BABE and are-type volcanic rocks (11-13 Ma and 4-4. 5 Ma) fr om the nearby region indicate mixing between the depleted mantle a nd an EM II-like enriched component (e.g. subducted pelagic sediment) in th e magma generation. At a given Sr-87/Sr-86; Hokkaido BABE have slightly low er Nd-143/Nd-144 and slightly less radiogenic Pb-206/Pb-204 compared with a ssociated al c-type lavas, but both these suites are difficult, to distingu ish solely on the basis of isotopic compositions. These isotopic data indic ate that while generation of the Hokkaido BABB involves smaller amounts of the EM II-like enriched component than do associated are lavas, Hokkaido BA BE are isotopically distinct born basalts produced at normal backarc basin spreading centers. Instead, northeast Hokkaido BABB, are more similar to ba salts erupted during the initial rifting stage of back-ape basins. The Monb etsu-Kamishihoro graben may have developed in association with extension th at formed the Kurile Basin, suggesting that opening of the basin continued until late Miocene (7-9 Ma).