BLUESCHIST METAMORPHISM IN AN ACTIVE SUBDUCTION ZONE

THE high-pressure, low-temperature metamorphic rocks known as blueschi sts have long been considered to form in subduction zones, where the d escent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures. Until now, however, the link bet ween blueschist-facies rocks and subduction zones has been indirect, r elying on a spatial association of blueschists with old subduction com plexes, and estimates of the geothermal gradients likely to exist in s ubduction zones. Here we strengthen this link, by reporting the discov ery of blueschist-facies minerals (lawsonite, aragonite, sodic pyroxen e and blue amphibole) in clasts from a serpentinite seamount in the fo rearc of the active Mariana subduction zone. The metamorphic condition s estimated from the mineral compositions are 150-250-degrees-C and 5- 6 kbar (16-20 km depth). The rocks must have been entrained in rising serpentine mud diapirs, and extruded from mud volcanoes onto the sea f loor. Further study of these rocks may provide new insight into the te ctonics of trench-forearc systems, and in particular, the processes by which blueschist-facies clasts come to be associated with forearc sed iments in ancient subduction complexes.