Abstract
87Sr/86Sr ratios of the volcanic rocks from the Pliocene Misasa Group and the andesite from the Quaternary Daisen volcano in San'in district were determined. The alkaline basalt and the calc-alkaline andesite from the Misasa Group have a restricted range of 87Sr/86Sr ratios (basalt 0.7051-0.7052; andesite 0.7052) and Rb/Sr ratios (basalt 0.03-0.04; andesite 0.04) with low Rb (basalt 19.4-34.5ppm; andesite 29.2ppm) and Sr (basalt 631-851ppm; andesite 729ppm) contents. These results may indicate that the calc-alkaline andesite may not have been derived from the alkaline basaltic magma by fractional crystallization or assimilation of older sialic materials. An analysis of the Daisen andesite yields a 87Sr/86Sr ratio of 0.7050, which is almost identical with that of the Misasa andesite. In 87Sr/86Sr ratios, the Misasa alkaline basalt, calc-alkaline andesite and the Daisen calc-alkaline andesite are very similar to the Oki-Dogo alkaline basalt (0.7048) which carries ultramafic inclusions and is possibly of the upper mantle origin (Kaneoka et al., 1978). This similarity in Sr isotopic ratios suggests that both the Misasa and Daisen volcanic rocks may have been derived from the upper mantle similar to the source material of the Oki-Dogo alkaline basalt.
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