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PLAGIOCLASE–QUARTZ ROCKS OF METASOMATIC ORIGIN AT THE EXPENSE OF GRANITIC ROCKS OF THE KOMAKI DISTRICT, SOUTHWESTERN JAPAN

¹ Research Center for Deep Geological Environments, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central-7, 1–1–1 Higashi Tsukuba, Ibaraki, 305–8567, Japan
² Department of Geological Sciences, California State University Northridge, Northridge, California 91330–8266, USA
³ Department of Geoscience, Interdisciplinary Faculty of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue, Shimane, 690–8504, Japan

E-mail address: takagi-t{at}aist.go.jp

In the Komaki district of southwestern Japan, small bodies composed mostly of intermediate plagioclase and quartz occur in a Late Cretaceous granitic batholith. The properties of the plagioclase–quartz rock indicate that the rocks were formed by calcium metasomatism. They are found in a lenticular body 50 m thick and a dome-shaped body 80 m in diameter. The plagioclase–quartz rock formed along fractures and shear zones by hydrothermal replacement of the host granitic rocks. The upper parts of the plagioclase–quartz rock bodies were strongly kaolinized. Greisenized portions are distributed sporadically in surrounding granitic host-rocks. On a ⁸⁷Sr/⁸⁶Sr versus ⁸⁷Rb/⁸⁶Sr diagram, the regression line formed by the plagioclase–quartz rock and a small stock of leucogranite 1.5 km southwest of the plagioclase–quartz occurrences can be regarded as an isochron, whereas other granitic rocks yield an errorchron. The Rb–Sr isochron age agrees well with the oldest K–Ar age of muscovite from a Mo deposit within the leucogranite. The plagioclase–quartz rock shows significantly lower ¹⁸O compositions relative to the leucogranite and the host granitic batholith, suggesting the contribution of meteoric water. The O and H isotope compositions of clay minerals from the plagioclase–quartz rock suggest that the kaolinization was caused by low-temperature (<150°C) hydrothermal alteration. The geological and geochemical features suggest that the Ca metasomatism was caused by hydrothermal fluids that also produced Mo mineralization. Subsequent to the Ca metasomatism, kaolinization of the plagioclase–quartz rock took place by low-temperature hydrothermal alteration in combination with supergene processes.

Keywords: metasomatism, granite, greisen, molybdenum deposit, Sr isotopes, stable isotopes, Komaki, Japan.