Abstract
The Tertiary igenous rocks from the Cape of Ashizuri constitute a small (about 15 kilometers in diameter) ring-like complex occurring in the Shimanto Belt (Mesozoic to Tertiary accretionary terrane), Outer Zone of Southwest Japan. They are composed of five stages of rocks, I (gabbro and dolerite), II (melanocratic syenite to alkali granite), III (coarse-grained syenitic rock and rapakivi granite), IV (coarse- to medium-grained biotite granite) and V (alkali dolerite and syenite porphyry), and characterized by alkalic to midalkalic chemistry. Comparing with the average chemical composition of Japanese granitic rocks (Aramaki et al., 1972), the igneous rocks from the Cape of Ashizuri are highly rich in (Na2O+K2O), while poor in CaO and Al2O3. In addition, F-analysis indicates exceedingly high content of 600 to 4000 ppm in most of the rocks, attaining to 7000 ppm in syenite porphyry. Above-described facts together with some mineralogical data indicating the relatively low H2O fugacity and low to medium oxygen fugacity, suggest the similarity of these igneous rocks with A-type granites (Loiselle and Wones, 1979) which are commonly generated along the rift zone or within stable continental blocks. The reason why the A-type granitic rocks are found within the compressional accretionary terrane has not been made clear and needs further detailed investigation.
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