ABSTRACT

K, Rb and Sr concentrations and Sr isotopic composition were determined for eight mafic inclusions found in Cenozoic alkali basalt of the Uta-jima (Uta Island) in the Japan Sea, Southwest Japan. The mafic inclusions are composed of gabbroic and granulitic rocks of presumably lower crustal origin. The mafic inclusions have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.70457 to 0.70612. Most of them are isotopically distinguishable from the host basalt (0.70360–0.70440), while one mafic inclusion is isotopically indistinguishable from the host, suggesting isotopic exchange. Data for separated minerals from one gabbroic inclusion define a crude isochron, yielding a young age of approximately 100m.y. From the results of the present study and those reported for ultramafic inclusions from the Oki-Dogo Island, another island in the Japan Sea, Southwest Japan, it is inferred that the materials constituting the lowermost crust and the uppermost part of the mantle beneath the Southwest Japan arc have relatively high ⁸⁷Sr/⁸⁶Sr ratios from 0.705 to 0.706 and have relatively young ages.