Cretaceous ridge subduction and transition in sediment provenance reconstructed by geology of Kerama Islands, Ryukyu arc

Abstract:

Accretionary prisms and metamorphic rocks in east Asian continental margin record plate tectonic configurations since Paleozoic. Geological investigations suggested that Kula-Pacific ridge subduction at 60–70 Ma (Isozaki et al., 2010), however, recent numerical model simulated that Izanagi-Pacific ridge subduction occurred 50–60 Ma (Seton et al., 2015). Wu et al. (2016) suggested that East Asian Sea plate, relatively stationary to Eurasian plate, covering large area of western paleo-Pacific Ocean at 52 Ma. To constrain Cretaceous plate tectonic setting of NW Pacific, we reexamined geological records of the Ryukyu arc. In this study we focus on Kerama Islands, Okinawa, located at the southwestern edge of the Shimanto accretionary complex. The bedrock of Kerama Islands is made up of greenstone, metasandstone, and pelitic schist. Greenstone has N-MORB geochemistry, and intruded into terrigenous sediments. Raman spectra of carbonaceous materials in pelitic schist showed 470-525°C of maximum temperature, which is higher than any reported temperatures in the Shimanto complex. Mineral assemblage of greenstone shows epidote-amphibolite facies regional metamorphism (0.3-1 GPa). This P-T condition was only achieved by subduction of mid-oceanic ridge (Iwamori, 2000), and therefore ridge subduction beneath the Cretaceous Ryukyu arc is suggested. The youngest U-Pb age of detrital zircon grains ranges 104–110 Ma. Three populations (100–140, 150–190, and 200–260 Ma) are recognized in the U-Pb age distribution of detrital zircons. This characteristic detrital zircon age distribution is common among early Cretaceous accretionary complexes and forearc basins throughout NE Japan to Taiwan. However, at late Cretaceous, zircon grains of early Cretaceous age became dominant in SW Japan, while age distribution pattern in Ryukyu and Taiwan is similar to those in early Cretaceous. This transition in sediment provenance suggest that sediments were constantly supplied from source regions including 100–140, 150–190, and 200–260 Ma igneous rocks to trench throughout Cretaceous, while in SW Japan such sources were replaced by abruptly exhumed Cretaceous granites.