ABSTRACT

The Koyama intrusive complex consists of varying rock-types from olivine pyroxenite to aplite as a consequence of differentiation of gabbroic magma. The whole-rock δ¹⁸O values of the main differentiates are +6.2 to +6.6‰ (in SMOW) for olivine rich members and are invariably around +7.8‰, from quartz gabbro to aplite. It is suggested that the Koyama magma differentiated without any significant isotopic fractionation. The isotopic difference between the two groups is largely due to the concentration into the former of olivine and pyroxenes which are isotopically lighter than plagioclase. The results support the view that the major rock-types are formed by the gravity sorting and accumulation of olivine, pyroxenes and plagioclase. Among the minor members of the complex, “granular” gabbro and a quartz diorite dyke are depleted in ¹⁸O by 2 to 3‰ compared to the major rock-types, whereas the veins of augite and of plagioclase-hornblende exhibit higher ¹⁸O content. It is suggested that a simple contamination of the residual magma by wall rock basalt can not produce such isotopic anomalies.