PETROFABRIC ANALYSES OF A FOLDED LAYER OF BANDED CHROMITE-DUNITE IN THE HIGASHI-AKAISHI-YAMA PERIDOTITE MASS, SHIKOKU, JAPAN

Abstract

Fabric of a folded dunite layer of banded chromite-dunite from the Higashi-akaishi-yama peridotite mass (Shikoku, Southwest Japan) is described. The banded chromite-dunite has alternating chromite-rich and olivine-rich layers. A thick layer of the olivine-rich layers consists essentially of olivine with a small amount of chromite and serpentine. The fold of this dunite layer is isoclinal and concentric. Any megascopic planar structures cannot be found within the dunite layer. The olivine grains are equidimensional, ranging from 0.04 to 1.2mm in diameter. Undulatory extinction is rare in the olivine grains. In each portion within the folded dunite layer, the preferred lattice orientation of the large grains of olivine does not coincide in direction with that of the small grains of olivine. In the folded dunite layer, the small grains of olivine show the tendency for (010) to be oriented parallel to the axial surface and for [001] to lie parallel to the fold axis. It seems that the fabric of the small grains of olivine was developed through recrystallization within the layer which was undergoing folding. The preferred orientation for (010) of the large grains of olivine in the limb area tends to be oriented in direction oblique to the local attitude of the folded surface. It is highly probable that a prefolding pattern of preferred orientation of olivine in the dunite layer was inherited in the observed state of the orientation pattern of the large olivine grains preserved from recrystallization. It is likely that the older olivine fabric was rotated by grain rotation within the dunite layer which was undergoing folding. During the phase subsequent to such deformation and recrystallization the Higashi-akaishi-yama peridotite mass has been brought into contact with the upper member of the Minawa formation involved in the large recumbent fold in the Sambagawa belt of Shikoku.