Abstract
The Koyama Gabbro mass intruding into the Miocene sedimentary strata has various kinds of rock facies such as anorthosite, anorthositic gabbro, gabbro, diorite, aplitic granite and aplite. The Sr;87/Sr86 ratios for each rock facies including apiitic granit and aplite are well constant, and the average ratio (0.706) is slightly higher than that for Japanese basaltic rocks. This result indicates that it is unlikely that the aplitic rock was formed through assimilation of an appreciable amount of sialic crustal material relatively rich in Sr87.
