Cleavable olivine in serpentinite mylonites from the Oeyama ophiolite

Olivine that has well-developed parting similar to cleavage, i.e., so-called "cleavable olivine", occurs in peridotites at many localities of orogenic belts and the seafoor. Some conficting hypotheses for the genesis of the parting have been proposed but not yet fully proved or disproved. We present new data of structural, petrological and mineralogical analyses of cleavable olivine and host ultramafc rocks in the Oeyama ophiolitic complexes, SW Japan. The following are our key fndings to understand the genesis of cleavable olivine. 1) Cleavable olivine is distributed in the ultramafc complexes regardless of metamorphic grade of contact aureoles. 2) Cleavable olivine from contact aureoles has variable chemical compositions by the effect of thermal metamorphism. 3) Cleavable olivine commonly occurs in or near serpentinite mylonites. 4) Antigorite blades commonly occur along the parting planes of olivine, and the parting planes along with antigorite blades are locally bent to the direction of foliation. 5) Poles of the parting planes of olivine tend to be distributed around a plane vertical to the foliation of host serpentinite mylonite. From these facts we conclude that cleavable olivine was produced during a sequence of localized plastic deformation and alteration of peridotites at temperatures around 600 °C or lower. The parting is likely to have derived from dislocation arrangement by recovery processes after plastic deformation of hydrous peridotites and have been brought into prominence during syntectonic serpentinization. The preferred orientation of the parting planes suggests that cleavable olivine is a potential indicator of regional tectonics of the upper mantle at supra-subduction zones.