Abstract:
Ultrahigh-pressure (UHP) rocks in the Cuaba Gneiss include Grt ultramafic rocks, mafic eclogite, and partially retrograded equivalents. The Grt ultramafic rocks (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 asthenosphere. New chemical analyses of minerals give the following subsolidus conditions: GrL peridotite, 3.0-4.2 GPa, 838-867°C; Grt clinopyroxenite (Grt + Cpx + Spl + Crn), 2.75 GPa, 807°C. The ultramafic rocks 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 ultramafic rocks 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 above the subduction zone and relates to delivery of the Grt ultramafic rocks to the subduction zone. Incorporation of Grt ultramafic rocks 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 ultramafic rocks were transported as blocks in the eclogite. The third part of the path (3) relates to final uplift to the surface. Copyright © 2006 by V. H. Winston & Son, Inc. All rights reserved.
