Structural-Petrological Studies of Peridotite and Associated Rocks of the Higashi-akaishi-yama District, Shikoku, Japan

The Higashi-akaishi-yama peridotite mass lies along the axis of the root-zone of the Besshi recumbent anticline, nearly concordantly with adjacent Sambagawa crystalline schist beds, extending E.-W. for a distance of about 4.5 km or more. It is a large lenticular mass dip-ping to the north, bounded by the Iratsu amphibole schist on the north and by spotted black schist on the south. The mass is mainly compossed of massive and foliated dunite, accompanied by a small amount of clinopyroxene-bearing peridotite. Clinopyroxenite and eclogite occur as fine streaks, bands and lenticular masses in the upper (northern) part of the peridotite mass. Large masses of amphibolic and pyroxenic rocks also occur, being enclosed in the peridotite mass. No contact effects of higher temperature have been found in crystalline schists around the peridotite mass. Small irregular-shaped masses of eclogite have been locally developed in the Iratsu amphibole schist.
Throughout the massive dunite is developed a continuous s-surface clearly defined by the statistical lattice orientation of olivine. The preferred orientation of olivine in the massive dunite must have been developed through the process of recrystallization or neocrystallization of the mineral during the Besshi recumbent folding. Preferred orientation of olivine is less remarkable in the foliated dunite than in the massive one. Foliation of the foliated dunite is defined by closely spaced alternation of serpentine-rich layers and olivine-rich ones. The foliated dunite is believed to have been intensely folded during or after the formation of preferred orientation of olivine.
Clinopyroxenes in both the clinopyroxenite and the eclogite tend to show marked preferred orientation. The fabric axes defined by the orientation of clinopyroxene in both the clinopyro-xenite and the eclogite roughly coincide in direction with those defined by the orientation of olivine in the adjacent massive dunite. The preferred orientation of clinopyroxene as well as olivine is believed to have been developed under the same tectonic setting.