Radio-metric ages of some Cenozoic volcanic rocks from Ryukyu Islands

Abstract

The radio-metric ages on 13 volcanic rocks from the Ryukyu Islands have been measured by one of fission track methods (viz. arranged fixed area population method, external-surface external-detecter method and external-surface internal-detecter method) or K-Ar method. The ages obtained are as follows: Takarajima propylite/1.4±0.4Ma (KAG-001), 5.05±0.38Ma (KAG-002), and 1.1±0.3Ma (KAG-003); Tairajima andesite/4.79±0.96Ma (KAG-004); Suwanosejima andesite/0.64±0.16Ma (KAG-006); Nakanoshima andesite/1.75±0.22Ma (KAG-008), and 0.14±0.06Ma (KAG-009); Okinawajima andesite/16.6±1.4Ma (OKA-011); Agunijimadacite/6.24±0.46 (OKA-021); Kumejima andesite/12.8±2.1Ma (OKA-023), and 15.1±2.6Ma (OKA-024); Ishigakijima rhyolite/44.1±1.8Ma (OKA-033), and 43.5±1.8Ma (OKA-035). Of these, two samples of KAG-001 and KAG-003 were measured by the whole-rock K-Ar method. The age determination has resulted in the recognition of four stages of volcanism in the Ryukyu Islands: middle Eocene Green-Tuff, middle to late Miocene Green-Tuff, late Miocene to early Pleistocene Older-Volcanic Islands, and early Pleistocene to Recent Younger-Volcanic Islands.