Oxygen isotope study of the Ibaragi granitic complex, Osaka, southwest Japan

Abstract

180/160 ratios were analyzed for 14 granitic rocks and their constituent minerals of the Ibaragi granitic complex near Osaka which is composed of the Nose pluton and the Myoken pluton. The whole-rock 5180 values of the Nose pluton are in the range from +8.1 to +9.7%o relative to SMOW. The data for the coexisting minerals suggest that an oxygen isotope equilibrium has been attained among the minerals. The 5180 values of the Myoken pluton are remarkably lower than those of the Nose pluton. This indicates that the magmas of the two plutons are different in their origins and/or differentiation processes. In the Nose pluton, the 180-enrichment of about 1.5 %o 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 5180 value of the initial magma of the Nose pluton is estimated to be around +7.5%o. Somewhat 180-enriched materials such as metamorphic basic rocks, rather than the fresh upper mantle materials, are likely to be the source of the 180-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-180 meteoric ground water. © 1973, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.