ABSTRACT

The Ryukyu Arc is a chain of islands stretching from the southern tip of Kyushu to an area northeast of Taiwan. Cretaceous to Tertiary plutonic rocks occur in the Arc. Four Rb-Sr whole rock ages and one mineral isochron age for plutonic rocks have been obtained. Whole rock isochron ages are 69.8 ± 8.0 Ma (Yonama body), 61.0 ± 6.7 Ma (Kanemi body), 39.3 ± 2.0 Ma (Omoto body), 40.7 ± 14.5 Ma (Chayama body) and the mineral age is 69.3 ± 2.6 Ma (Todoroki body). These ages are about 10 Ma older than the K-Ar biotite or fission track zircon ages for the corresponding rocks. Judging from the Rb-Sr isochron ages and published K-Ar and fission track ages, it is clear that plutonic activity in south Ryukyu migrated toward the Pacific Ocean side with time. Furthermore, plutonic activity terminated in the central Ryukyu Arc before opening of the Okinawa Trough.