ABSTRACT

¹⁸O/¹⁶O ratios were analyzed for 14 granitic rocks and their constituent minerals of the Ibaragi granitic complex near Osaka which is composed of the Nosé pluton and the Myoken pluton. The whole-rock δ¹⁸O values of the Nosé pluton are in the range from +8.1 to +9.7‰ relative to SMOW. The data for the coexisting minerals suggest that an oxygen isotope equilibrium has been attained among the minerals. The δ¹⁸O values of the Myoken pluton are remarkably lower than those of the Nosé pluton. This indicates that the magmas of the two plutons are different in their origins and/or differentiation processes. In the Nosé pluton, the ¹⁸O-enrichment of about 1.5‰ is recognized in the differentiation sequence from quartz diorite to adamellite. This isotope variation can be explained as the results of equilibrium fractionation in a crystallizing magma. The δ¹⁸O value of the initial magma of the Nosé pluton is estimated to be around +7.5 ‰. Somewhat ¹⁸O-enriched materials such as metamorphic basic rocks, rather than the fresh upper mantle materials, are likely to be the source of the ¹⁸O-rich dioritic magma. In the Ibaragi granitic complex, no appreciable change in oxygen isotopes is observed in the contact zone as compared to the values of main intrusive body, possibly because the country rocks have isotope ratios similar to those of the granitic rocks. An exception is found in the marginal zone of the Myoken pluton and the dike rocks, where K-feldspar may have exchanged its oxygen with the circulating low-¹⁸O meteoric ground water.