Abstract
The Oligocene Omoto pluton in the Ishigaki-jima Island, southern Ryukyus, is divided into six rock types : Miyaragawa, Chayama, Ishigaki, Naguragawa, Otake and Omoto. These rock types that make up zoned plutonic sequence are younger and more felsic inward. The most of the rocks are adamellite and alkali feldspar granite, but the outermost rock facies is quartz diorite, and innermost one is porphyrytic granite. The clearly boundaries between each rock type indicate that zoned mechanism of the Omoto pluton is multiple intrusion.
The Omoto type that is innermost and most felsic rock facies shows characteristics of S-type granitoids, while the other types exhibit those of I-type granitoids. Cr contents of the Omoto type are similar to those of more basic rock facies in this pluton. These matters suggested that the Omoto type and the other types were derived from different sorce material.
In conclusion, the Omoto pluton is composed of six rock types which are derived from two different magmas.
