Fluid-induced Dehydration of the Paleoarchean Sand River Biotite-Hornblende Gneiss, Central Zone, Limpopo Complex, South Africa

A clear case study of local-scale, fluid-induced dehydration of the Paleoarchean Sand River biotite^hornblende gneiss from the Central Zone of the Limpopo Complex is presented here. Field and petrographic examination of three adjacent zonesçdarker Sand River orthogneiss with local occurrence of orthopyroxene and clinopyroxene, a lighter intermediate gneissic zone with more ortho-pyroxene than the Sand River orthogneiss, and tonalitic veins containing large orthopyroxene-bearing patchesçindicates the local transformation of a light grey, fine- to medium-grained, horn-blende^biotite gneiss into a greenish brown, medium- to coarse-grained orthopyroxene-bearing dehydration zone. Field evidence indicates that the tonalitic veins were emplaced in discrete ductile shear zones, with development of large orthopyroxene-bearing patches in a sigmoidally transposed foliation bounded by shear planes. Orthopyroxene-forming reaction textures after biotite and amphibole