TECTONIC IMPLICATIONS OF UHP GARNET PERIDOTITE IN THE CUABA UNIT, RIO SAN JUAN COMPLEX, DOMINICAN REPUBLIC

Spinel-bearing garnet peridotite and a corundum-bearing variant are associated with hornblende gneiss and schist, and retrograded eclogite in the Cuaba amphibolite unit of the Cretaceous Rio San Juan complex. The occurrence is unusual because the dense garnet peridotite was emplaced at an ocean-ocean convergent plate boundary. Geothermobarometry, sequence of mineral assemblages, tectonic setting, and current plate tectonic models suggest the following history, beginning in the Lower Cretaceous, (1) an initially hot protolith at modest depth in lithospheric, ~55 km (>1.8 Gpa, >900 *C), (2) descent to a depth of 80 to 105 km (2.8 Gpa, 810 °C to 3.5 Gpa, 740 °C), (3) incorporation into subducted oceanic crust, (4) cooling during initial ascent, and intrusion by gabbro, (5) Lower Tertiary transtensional tectonics, (6) final ascent to present position as result of Upper Tertiary to recent transpressional tectonics. Elsewhere, Alpine-type garnet peridotite is associated with what had been deep subducted continental rock ascent was presumably driven by the buoyancy of the continental material. In the present case, the buoyant force is not obvious. Indications of initially very high temperature at a modest depth in the mantle (~ 55 km) argue for the involvement of hot upwelling mantle. We offer alternative models wherein the garnet peridotite originates (A) in the mantle wedge above the subduction zone or (B) in the mantle beneath the subduction zone. In the first instance (A), the process may be related to extension in the forearc. In the second instance (B), the process may be related to subduction of an ocean ridge or mantle plume.