METAMORPHIC PHASE RELATIONS, DIFFUSIONAL LENGTH SCALES, AND TEMPERATURE-TIME HISTORY IN RETROGRESSED ECLOGITE FROM A LARGE, HOT OROGEN

Small mafic bodies preserving evidence for high pressure (HP) metamorphism are dispersed along the high-strain boundary zone that separates allochthonous nappes from the underlying Laurentian margin within the western Central Gneiss Belt, Grenville Province, Canada. Metamorphic reaction textures and intracrystalline chemical zoning associated with breakdown of the HP assemblage are widespread in most samples, and present an opportunity to place quantitative constraints on the thermal evolution attending exhumation and residence in middle orogenic crust. Reaction coronas surrounding kyanite typically have an inner symplectite domain composed of spinel + sapphirine + anorthite ± corundum and an outer monomineralic domain of fine-grained muscovite, whereas garnet porphyroblasts are locally mantled by thin rims of hornblende + plagioclase symplectite. In settings where the host kyanite and garnet crystals are in close proximity to the other primary reactant phases, corona widths and symplectite lamellae exhibit relatively uniform dimensions, apparently representing reaction fronts along former crystal boundaries. Similarly, increases in Mn concentration within the margins of partially-resorbed garnet porphyroblasts, resulting from syn- and/or post-.resorption uptake and back-diffusion toward the crystal interiors, are typically restricted to the outer ~70-90 microns. The relatively well-defined characteristic length scales associated with each of these metamorphic features suggests a systematic diffusional control on the rates of reaction and chemical equilibration. This contribution explores metamorphic phase relations and approach to equilibrium within local reaction volumes, the variability in the length scales of reaction-related metamorphic textures and compositional zoning, and the application of published models that relate metamorphic length scales with rates of matrix and volume diffusion for various cations and specific temperature-time histories.