The Case for UHP Conditions in the Cuaba Terrane, Río San Juan Metamorphic Complex, Dominican Republic

From the Cuaba terrane in northern Dominican Republic. Ultrahigh pressure (UHP) conditions are indicated for the Cuaba terrane on the basis of phase relationships in garnet-bearing ultramafic rock. Dikes and orthocumulate textures indicate a magmatic origin. Mineral assemblages define a line of descent controlled by fractional crystallization. The original estimate of the magmatic conditions (P>3.4GPa, T>1550°C) was inferred previously from available high-P melting experiments in the CMAS system and high-P experimental determination of the sapphirine-out reaction in the MAS system. Revised estimates of magmatic conditions (P>3.2GPa, T>1500°C) take into account the influence of other components, especially Fe. We propose an origin in the mantle-wedge above a subduction zone. The rock was delivered to the subduction zone by forced convection in the mantle wedge (corner-flow), coupled with erosion of the hanging wall. Thermobarometry indicates >850°C and >3.4GPa when the ultramafic rock was incorporated into eclogite (deep-subducted oceanic crust). Evidence for UHP conditions in the retrograded eclogite is not obvious. Two types of symplectic intergrowths, plagioclase + clinopyroxene (Sym-I) and plagioclase + epidote (Sym-II), are interpreted as the products of the decomposition of two types of omphacite, Omp-I and Omp-II. Theoretically, Omp-II formed as the result of a retrograde reaction of the form, Omp-II + coesite = Omp-I + kyanite + /- garnet, according to which the maximum pressure for Omp-II is between ~2.8GPa (~850°C) and ~4.2GPa (~950°C), consistent with subsolidus conditions for the garnet-bearing ultramafic rocks. For eclogite, the highest-pressure mineral assemblage would have been Omp-I + kyanite + garnet + coesite.