Ultrabasic Mass in the Higashiakaishiyama District, Shikoku, Southwest Japan

The ultrabasic mass of Higashiakaishiyama occurs within a zone of recumbent fold developed in the Sambagawa crystalline schist zone in Shikoku, Southwest Japan. The ultrabasic body consists largely of dunite with small amounts of clinopyroxene-bearing peridotite, clino-pyroxenite, and eclogite, and it also contains several large masses of hornblendic and clino-pyroxenic rocks. This body is interpreted as an ultrabasic complex that has been brought up from below through an orogenic process. Within the ultrabasic body individual minerals, such as olivine in the massive dunite and in the massive peridotite, black clinopyroxene in the massive peridotite, and light-coloured clinopyroxene in the massive peridotite, in the clinopyroxenite, and in the eclogite, show marked preferred orientation. Hornblende, clinopyroxene, and epidote in the hornblendic and clinopyroxenic rocks also show preferred orientation. It is highly prob-able that the tectonite minerals contained in the ultrabasic body acquired their observed state of preferred orientation at an earlier stage of orogeny involving the formation of the Sambagawa metamorphic belt, and in a deeper part of the metamorphic belt. Multiform patterns of pre-ferred orientation of olivine suggest that the mechanical conditions under which the olivine was oriented were not homogeneous throughout the ultrabasic mass. It seems that the ultrabasic body has been intruded into the folded zone through a stage subsequent to development of the preferred orientation of the tectonite minerals in the ultrabasic body. The megascopic foliation and lineation of foliated dunite must have originated during the intrusion of the ultrabasic body.