Abstract
A tectonic melange zone containing serpentinites and edo-gite-bearing tectonic blocks (masses) of diverse size and lithology is developed within the high-grade zone of the Sambagawa metamorphic belt in central Shikoku, southwest Japan. Precise geological mapping of the eclogite-bearing tectonic blocks in the melange zone is required to reveal the tectonics of the deeper parts of the Sambagawa accretionary prism. However, such studies have not been published to date because of difficulties in fieldwork. We carried out precise geologic mapping of the Western Iratsu mass as a tectonic block, and of the surrounding metamorphic rocks which comprise the melange zone. The Western Iratsu mass consists of seven lithologies peridotite, clinopyroxene-hornblendite, garnet-epidote-amphibolite, garnet-white mica-amphibolite, marble, epidote-quartz rock and albite-white mica-quartz schist, in ascending structural order. The protoliths of these rock types are considered to be ultramafic rock, gabbro, basic volcaniclastic rock, limestone, siliceous sediment with intercalated limestone, and pelitic rock, respectively. The Western Iratsu mass contains abundant marbles, rather than the metacherts which occur in surrounding Sambagawa schists. Based on reconstruction of the succession of the original lithologies of the Western Iratsu mass, the protolith is considered to have been a topographic high on an oceanic plate such as a seamount, oceanic plateau or island arc with capping reef limestones.
