Abstract
A serpentinized cataclasite characterized by many angular harzburgite clasts of various size in serpentine matrix was found as an alluvial boulder in the Horoman peridotite complex, Hokkaido, northern Japan. A computer-aided image analysis of the serpentinized cataclasite indicates that clast shapes do not change with size variations, and that clast-size distributions obey a modified power-law distribution of Nagahama ct al. (1994). The serpentine matrix of the serpentinized cataclasite shows no distinct structures such as foliation and lineation, and has few fine-grained peridotite clasts (>10μm) and their pseudomorphs, whereas cataclasite matrix at an outcrop of the Horoman complex is almost all occupied by pseudomorphs of fine-grained peridotite clasts. The size distribution of mesoscopic clasts (visible to the naked eye) in the serpentinized cataclasite is similar to that of microscopic peridotite clasts (visible under microscope) in the serpentine matrix, but is different from that of microscopic fine grains bounded by serpentine mesh in clasts of the serpentinized cataclasite, These lines of evidence suggest that the serpentine matrix of the serpentinized cataclasite (more than 50% of the rock) had been once occupied by very fine-grained peridotite clasts (<10μm) formed by fragmentation of the peridotite during faulting before the serpentinization. Deficiency of smaller clasts on clast-size distributions is, therefore, interpreted as resulting from selective serpentinization of very fine-grained peridotite clasts, but not from normal serpentinization of the peridotite under static conditions. Fluid must have been locally supplied in the fault zone for a short period during or after the brittle deformation to cause the serpentinization.
