Fission track ages of some Cenozoic acidic intrusive rocks from Ryukyu Islands

Abstract

The fission track ages of zircon have been measured in nine acidic intrusive rocks from the Ryukyu Islands by the arranged fix-area population method and the external-surface external-detecter grain-by-grain method. The ages are as follows: Kuchinoerabujima xenolith (granodiorite), 15.0±1.2Ma; Okinoerabujima granodiorite, 32.9±2.7 Ma; Okinawajima tonalite, 30.0±4.7Ma; Okinawajima quartzporphyry, 15.7±1.3Ma, 15.3±1.5Ma, and 15.8±1.7Ma; Ishigakijima Omoto granite, 28.7±0.9Ma, 29.9±0.9Ma and 29.9±1.1Ma. These fission track ages indicate that the acidic intrusive rocks were emplaced in late Oligocene and early to middle Miocene times.