Petrogenesis of UHP Eclogite from the Cuaba Gneiss, Rio San Juan Complex, Dominican Republic

Abstract

Ultrahigh-pressure (UHP) conditions were established for the Cuaba Gneiss terrane, northern Dominican Republic, on the basis of phase relationships in garnet-bearing ultramafic rock, a relatively minor constituent of the terrane. Evidence for much more abundant eclogite comes in the form of two types of symplectic intergrowths involving, respectively, Pl + Cpx (Sym-I) and Pl + Ep (Sym-II), interpreted as the products of the decomposition of two types of omphacite, respectively, Omp-I and Omp-II. Sym-II (hence Omp-II) forms mantles on garnet and aggregates of epidote. Otherwise, estimated P-T conditions for the mineral assemblage Grt + Pl + Cpx + Ep + Hbl + Qtz are consistent with upper amphibolite-facies (P ~1.2 GPa, T ~750°C). Thus far, the retrograded eclogite has revealed nothing diagnostic of UHP conditions—e.g., coesite, evidence for coesite (radiating micro cracks in garnet, palisade quartz, etc.), or micro inclusions of diamond. However, by inference from the P-T history of the garnet ultramafic rock, the eclogite must have encountered conditions on the order of P ~4.2 GPa and T ~850°C (within the fields of stability for coesite and diamond), in order for the latter to have incorporated blocks of the former. Compositions for the original omphacite, Omp-I and Omp-II, were reintegrated from Sym-I and Sym-II respectively, using linear algebraic methods. Stoichiometric arguments show that much of the retrograde epidote was derived from kyanite. Omp-II formed as the result of a reaction of the form Omp-II + Coe = Omp-I + Grt + Ky, according to which the maximum estimated pressure for Omp-II is between ~2.8 GPa (~850°C) and ~4.2 GPa (~950°C), depending on the compositions of Omp-I and Omp-II. Therefore, the highest pressure mineral assemblage was Omp-I + Ky + Grt ± Coe. The conditions are consistent with previously estimated conditions for the decompression part of the P-T path for garnet-bearing ultramafic rock. Evidently, deep-subducted ocean-floor basalt (eclogite) was delivered to the surface from depths exceeding 85-125 km.