Zonal distribution of oxygen isotope ratios in the Hiroshima granite complex, Southwest Japan

18O/16O ratios have been obtained for 99 minerals from rocks of the Hiroshima granite complex and adjacent Ryoke granites. Zonal distribution of oxygen isotopes is observed on a regional scale almost parallel to the extension of the Ryoke plutono-metamorphic belt, granites in or around the metamorphic belt being 2-3 promille richer in 18O than those farther away from the belt. Isotopic fractionations among coexisting minerals indicate that isotopic zonation existed at a magmatic stage. The zonal enrichment of 18O in the granite magma in the Ryoke belt and its periphery is a result of isotopic interaction between country rocks and the magma through fluid media. Genetic relationships between granites of the Ryoke and Chukoku belts are discussed with regard to the geological situation of the former belt. (Auth.)