Takamiyama Acidic Rock　－A S-type Granitic Rock on the Median Tectonic Line, Central Kii Peninsula, Southwest Japan－

Takamiyama Acidic Rock (TAR), located on Median Tectonic Line, central Kii Peninsula, is a small (1km×O.8km) granitic intrusion. Based on XRF chemical analyses, TAR shows lower Al2O3 and CaO, higher K20 and TiO2, similar FeO, MnO and MgO contents, compared with the average composition of Japanese granitic rocks, and shows constant Na2O with the SiO2 increase. These are consistent with the chemical nature of Miocene granitic rocks appearing in the Outer Zone of Southwest Japan on the whole. In addition, the chemical ratio such as Al203/(CaO+Na20+K2O), and microscopic textures indicate that the TAR is a S-tvpe granitic rock, although this is not consistent with the distribution of I- and S-type granitlc rocks in the zone. This inconsistency may be interpreted by the magma assimilation, affected by the locally irregular distribution of the geological structure of Southwest Japan at central Kii Peninsula. Although TAR has been regarded as a possible vent for the Muro Volcanic Rocks (MVRs) located northward from that, the mineralogical, chemical and chronological data show that MVRs were probably erupted from a vent or vents different from TAR