Whole-rock chemical composition of the pre-Neogene basement rocks and detritus garnet composition in the Okinawa-jima and neighbor islands

Based on whole-rock chemical compositions of sandstone and greenstone, we estimate the geologic provenance and tectonic setting of pre-Neogene basement rocks of Okinawa Island and neighboring islands. Our results suggest that sandstone clasts in the Iheya Unit were derived from a continental arc and dissected arc, whereas sandstone clasts in the Motobu Unit were formed through the mixing of various rocks derived from different provenances. Sandstone clasts in the Nago and Kayo formations were derived from similar geologic provenances (continental island arc, active continental margin, continental arc, and dissected arc), and sandstones in both formations can be divided into two groups using abundances of Nb. The chemis-try of detritus garnet in sandstones from the Nago Formation indicates the existence of high-grade metamorphic rock in the provenance. The Kerama Formation has traditionally been regarded as a continuation of the Nago Formation, but the whole-rock chemical compositions of sandstones of the two formations are different. Our results, when compared with those for sandstone in the Shimanto belt, present signi fi cant differences in the contents of Sr, Cr, and Ni. These differences were presumably caused by ma fi c rock sources of clastic materials and by the paleoweathering conditions of the provenance.

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