Electron-microscope and electron-microprobe observations of amphibolites from the Omachi Seamount in the Izu-Bonin frontal arc show that they contain micrometer-sized inclusions of omphacite ؎ kyanite, indicating eclogite facies metamorphism. The garnetzoisite amphibolite was subjected to metamorphic conditions of 600-700 ؇C and ϳ2 GPa in the early stage of metamorphism. These metabasites were subducted to a depth of ϳ70 km beneath the Izu-Bonin arc and were exhumed together with serpentinite schist. Their intraoceanic unroofing was probably induced by backarc extension of the upper plate, whose axes were intersected and cooled by the newly formed Izu-Bonin subduction system. This intraoceanic exhumation associated with suprasubduction extension can be a modern analogue for ancient on-land eclogite terranes accompanied by ophiolites.

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