sO/16 0 ratios have been obtained for 134 whole-rocks and minerals from metamorphic and granitic rocks of the Yanai district in the Ryoke belt, Southwest Japan. The 180/,6 0 ratios of pelitic rocks of the marginal metamorphic zone decrease progressively with increasing metamorphic grade. In the gneiss-granite complex (zone of migmatite [1]), the most characteristic feature of the rocks is that oxygen isotopic homogenization proceeds on both local and regional scales in parallel with "granitization" or chemical homogenization. Granitic rocks of various origin are fairly uniform in isotopic composition with 6180 of quartz of 12 to 14~ (SMOW) and 6180 of biotite of 7 to 9~ and are about 3 to 40/00 enriched in 180 compared to other Cretaceous granites of non-metamorphic terranes in Japan. The high 180/160 ratios of granitic rocks of this district were discussed in relation to the a80-depletion in metasediments.
Oxygen isotopic fractionations among coexisting minerals from various rock-types of the gneiss-granite complex indicate that these minerals were formed under near isotopic equilibrium at a temperature of about 600 to 700~ Some abnormal fractionations of quartz-biotite pairs also were obtained for rocks which had undergone a progressive 180-depletion or 180-enrichment. This is due to high resistivity of quartz and contrastive susceptibility of biotite to isotopic exchange during metamorphism and "granitization ".