[SIT29-04] Mid-Cretaceous Tectonics of the Ryukyu arc reconstructed by geology of Kerama Islands, Okinawa
The bedrock of Kerama Islands is made up of a pile of three lithological units, greenstone, metasandstone, and pelitic schist in ascending order. Metamorphosed pelagic sediment is not observed. Greenstone generally contains epidote, actinolite, chlorite, and, hornblende as metamorphic minerals. Previous study shows the greenstone in Kerama Islands has geochemical features of N-MORB (Chinen et al., 2004). Our detailed observation of the contact between the top of greenstone unit and the bottom of sedimentary rock layer shows that the boundary records intrusion of basaltic lava, the protolith of the greenstone.
U-Pb ages of detrital zircon grains obtained from massive metasandstone and psammitic schist in Kerama Islands were analyzed. The measurement was carried out using LA-ICP-MS installed at the National Museum of Nature and Science. The concordant ages calculated from the result is ranging from 104 to 2565 Ma. The youngest age of zircon ranges 104-110 Ma (early Cretaceous) and no zircon grains with late Cretaceous age is obtained. The depositional age of original rocks of metasandstone in Kerama Islands is considered to be ~100 Ma.
We examined peak metamorphic temperature of pelitic schist in Kerama Islands. Raman spectra of carbonaceous materials in thin section of the samples are obtained using a Raman spectrometer with a 532 nm Nd-YAG laser installed at Nagoya University. Peak decomposition of Raman spectra was performed and peak metamorphic temperature was estimated following the method of Beyssac et al. (2002). The result shows that the bedrock of Kerama Islands suffered 470-525°C, which correspond to the temperature range of proposed metamorphic facies of greenstone characterized by the combination of epidote and actinolite (Maruyama et al., 1996; Hacker et al., 2003). This peak metamorphic temperature of Kerama Islands is higher than any reported temperatures in the Shimanto belt.
According to numerical model of Iwamori (2000), metamorphic P-T condition of rocks of Kerama Islands achieved by subduction of mid-oceanic ridge for several millions of years. Ridge subduction is also supported by the existence of N-MORB intruded into trench-filled terrigenious sediment. Ridge subduction beneath paleo-Ryukyu trench at ~100 Ma, proposed by this research, would be a strong constrain for plate tectonic reconstruction of Cretaceous NW pacific.