ALPINE-TYPE GARNET PERIDOTITE IN THE CARIBBEAN

Alpine-type garnet peridotite is known from only two places in the Americas, (1) an occurrence in the Seward Peninsula, Alaska, and (2) a recently discovered occurrence in northern Dominican Republic on the Caribbean island of Hispaniola. Elsewhere around the world, Alpine-type garnet peridotite is known from several of the 15 or so UHP/UHT metamorphic terranes in Europe, Asia, Africa, and Antarctica. The spinel-bearing garnet peridotite was discovered in the Cuaba amphibolite unit of the Cretaceous Rio San Juan complex. The garnet peridotite (forsterite + diopside + enstatite + garnet + spinel + late magnesiohornblende + late serpentine) is associated with hornblende gneiss and schist (hornblende + plagioclase + quartz + rutile +/- garnet +/- biotite +/- epidote), and retrograded eclogite (hornblende + symplectic diopside-plagioclase + garnet + quartz +/- epidote). The occurrence is unusual because the garnet peridotite was educted (delivered to the surface) at an ocean-ocean convergent plate boundary. Elsewhere eduction of garnet peridotite took place at ocean-continent convergent boundaries, where eduction was apparently assisted by the buoying effect of the less dense continental crust. Textural relationships suggest the following sequence of mineral assemblages in the garnet peridotite: (1) forsterite + diopside + enstatite + spinel; (2) forsterite + diopside + enstatite + spinel + garnet; (3) diopside + enstatite + spinel + magnesiohornblende, and garnet + spinel + magnesiohornblende; (4) forsterite + serpentine. The sequence of mineral assemblages, nature of associated rocks, estimated P-T conditions for the first assemblage (>900 °C, >1.8 Gpa), and tectonic setting are consistent with classification as low-P/T garnet peridotite. Very high temperatures for a spinel peridotite precursor (1000-1400 °C, hence the alternative UHT designation) suggest the involvement of upwelling asthenosphere. Existing models for such rocks suggest that upwelling asthenosphere delivers spinel peridotite to the subduction zone, where garnet forms during more-or-less isobaric cooling prior to eduction. Perhaps such diapiric upwelling of asthenosphere is a natural consequence of isostatic disequilibrium accompanying subduction of an ocean ridge or mantle plume.