Abstract
Rb and Sr concentrations and Sr isotopic ratios (Sr87/Sr86) have been determined for some mafic and ultramafic rocks from Ogawara, Kurouchi-yama and Higashiakaishi-yama bodies. Rb and Sr concentrations of mafic rocks are nearly comparable to those of oceanic tholeiites, those of ultramafics being within the range shown by ultramafic inclusions in alkali basalts. These rocks have Sr87/Sr88 ratios ranging from 0.7053 to 0.7072. No significant differences in these ratios are observed among these bodies and different rock types. These ratios are similar to those of Cenozoic continental basaltic rocks, and ultramafic inclusions in kimberlites and alkali basaltic rocks, while they lie in the lower part of the range measured in many ultramafic rocks in orogenic belts and oceanic regions. Considering from the results of present study and previous petrographical studies, it may be the most probable interpretation that these ultramafics are derived by early fractional crystallization of some basaltic or picritic magma.
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