P-T PATH FOR UHP GARNET ULTRAMAFIC ROCKS, CUABA GNEISS, RIO SAN JUAN COMPLEX, DOMINICAN REPUBLIC

Ultra high pressure (UHP) rocks in the Cuaba Gneiss include Grt* ultramafites, mafic eclogite, and partially retrograded equivalents. The Grt ultramafites (Spl-bearing Grt peridotite, Spl-bearing Grt clinopyroxenite, Crn-Spl-bearing Grt clinopyroxenite) are of igneous origin, with magmatic conditions of P > 3.5 GPa, T > 1550 °C. The magmatic history took place in the deepest part of the lithosphere or the shallowest part of the asthenosphere. New chemical analyses of minerals give the following subsolidus conditions: Grt peridotite (Ol+Grt+Cpx+Spl), 4.2 GPa, 867 °C; Grt clinopyroxenite (Grt+Cpx+Spl+Crn), 2.75-3.0 GPa, 807-838 °C. The ultramafites are associated with partially retrograded eclogite, interpreted as deep-subducted oceanic crust. New chemical analyses of minerals in the eclogite give conditions that relate to the retrograde decomposition of Grt+Omp+Qtz to Grt+Pl+Di+Qtz, 1.8 GPa, 730 °C.

The P-T path for the Grt ultramafites is modeled in three parts: (1) slow, isobaric (> 4 GPa) cooling in the mantle, >1550 °C down to ~850 °C, (2) relatively rapid, nearly adiabatic (?) decompression, ~4 GPa (~850 °C) down to ~1 GPa (~700 °C), and (3) relatively rapid, non-adiabatic decompression and cooling. The first part of the path (1) took place in the mantle wedge above the subduction zone and relates to delivery of the Grt ultramafites to the subduction zone. The process involved corner-flow in the mantle wedge and erosion of the hanging wall of the subduction zone. Incorporation of Grt ultramafites in the deep-subducted oceanic crust (eclogite) marked the end of this part of the path. The second part of the path (2) was associated with transport up the subduction zone. Retrograde P-T conditions for the eclogite fall on this part of the path, supporting the idea that the Grt ultramafites were transported as blocks in the eclogite. The third part of the path (3) relates to final uplift to the surface. *(Grt = garnet, Ol = olivine, Cpx = clinopyroxene, Omp = omphacite, Di = diopside, Spl = spinel, Crn = corundum, Qtz = quartz, Pl = plagioclase)