A re-evaluation of eclogite facies metamorphism in SW Japan: proposal for an eclogite nappe

Known eclogite occurrences in the Sanbagawa metamorphic belt of SW Japan ar e dominantly in meta-gabbro bodies which have complex polyphase metamorphic histories. These bodies are generally described as tectonic blocks and the ir relationship to the Sanbagawa metamorphism is unclear. New findings of f oliated eclogite in the Seba and Kotsu areas show that eclogite facies meta morphism is much more widespread than generally thought. Evidence that the foliated eclogite units originated as lavas or sediments implies that these units can be treated as a high-grade part of the subduction-related Sanbag awa metamorphism. Although separated by an along-strike distance of 80 km, the Seba and Kotsu eclogites have very similar garnet and omphacite composi tions, suggesting that they were formed under similar metamorphic condition s. However, differences in the associated retrograde assemblages (epidote-a mphibolite in the Seba unit and epidote-blueschist in the Kotsu unit) sugge st contrasting P-T paths. In both units, the eclogite rocks occupy the high est structural level of the Sanbagawa belt and overlie rocks metamorphosed at lower pressure. The lower boundary to the eclogite units is therefore a major tectonic discontinuity locally decorated with lenses of exotic materi al. These features can help trace the boundary into other areas. The previo usly known outcrops of eclogite show enough similarities with the newly fou nd areas to suggest that all the eclogite facies rocks in the Sanbagawa bel t constitute a single nappe that lies at the highest structural levels of t he orogen.