Bulletin of the Geological Society of Malaysia, Volume 9, Nov. 1977, pp. 75 – 89
Institute of Earth Sciences, Faculty of Liberal Arts, Yamaguchi University, Yamaguchi, Japan
Abstract: The late Mesozoic to early Tertiary granitic rocks on the inner side of Southwest Japan are divided into three zones, Ryoke, Hiroshima and Sanin, in the order of emplacement. They are arranged from south to north with E-W extension. The three zones do not show any great difference in whole rock chemistry, except for oxidation ratio. However, there are comparatively marked differences in the chemical composition of the constituent minerals, such as hornblendes, biotites and K-feldspars. The Fe/Mg ratios in both hornblendes and biotites, and the Al-content of hornblendes become lower towards the north zone. The Or-content of the K-feldspars is highest in the granitic rocks of the Ryoke zone and lowest in the Sanin zone.
These data suggest the highest temperature and the shallowest emplacement for the formation of the Sanin zone, and the reverse for the Ryoke zone. This implies that the level of erosion deepens from north to south. The isotopic dating data seem to agree with this conception, because the interval between the mineral age and the whole rock or whole rock-mineral isochron age becomes shorter northward.