Garnet zoning and metamorphism of the Barrovian type area,Scotland

A microprobe investigation of the high grade metamorphic zones from the Barrovian type area in Angus, Scotland, shows the importance of local zones of retrograde cation exchange between garnet, staurolite and biotite. The interpretation of this zoning, established during a slow cooling history, is critical to any study of metamorphic reactions or conditions. The extent and intensity of these diffusion effects are dependent on a number of parameters including grainsize, fabric orientation, heating and cooling history, and the modal abundance of the phases. Increasing diffusion within garnets with metamorphic grade, and the subsequent retrograde effects are modelled using Temperature-Time-Transformation diagrams and provide information on the activation energy for Fe-Mg diffusion in garnet.