ABSTRACT

The Tonaru epidote-amphibolite is one of the large metagabbro dominated bodies occurring in schistose lithologies of the Sanbagawa metamorphic belt, central Shikoku. Major and trace elements, including rare earth elements (REEs), in the Tonaru epidote-amphibolite and surrounding basic and pelitic schists along the Kokuryo River in the Besshi region of central Shikoku have been systematically investigated, and material interactions at their lithologic boundaries are discussed. Most parts of the Tonaru epidote-amphibolite have petrographic and chemical features similar to other epidote-amphibolites and eclogites in the Besshi region, which are dominantly metagabbro. In contrast, the epidote-amphibolites collected from the southern margin of the Tonaru body trend toward SiO₂-rich compositions. Some of them show intermediate compositions between the basic igneous and pelitic lithologies in the SiO₂-FeO*/MgO, Al₂O₃-CaO/(CaO + Na₂O + K₂O), Ti/100-Zr-Sr/2, and V-Ba systems, and have light REE-enriched patterns similar to sedimentary lithologies. Mafic and Al₂O₃-poorer samples are sometimes richer in Cr and Ni. These data suggest that the protoliths of the Tonaru epidote-amphibolites can be grouped into: (1) a gabbroic lithology; and (2) sedimentary mixtures of mafic, ultramafic and/or pelitic materials, likely derived from an oceanic island arc. There is no apparent compositional modification detected at a shear zone of 1.5–2.5 m width that developed at the northern boundary between the Tonaru epidote-amphibolite and pelitic schists. This feature implies that lithological mixing at this boundary was relatively insignificant during metamorphism and might not have effectively altered compositions of the metamorphic rocks without element transport promoted by circulating metamorphic fluids.