Gradients in chemical potential are the driving force for chemical diffusion and their spatial distribution is thus essential to investigate equilibrium in metamorphic rocks. At high temperatures (>650 degrees C), where chemical diffusion is relatively fast, the development and preservation of compositional zoning in minerals can be controlled by mechanically maintained pressure variations. Therefore, the dependence of chemical potentials on pressure plays an important role in correct interpretations of rock microstructures. Distinguishing between pressure-controlled chemical zoning and zoning reflecting chemical diffusion is a challenge. To tackle this challenge, we investigate a symplectitic microstructure around kyanite in an amphibolitized eclogite from the Rhodope Metamorphic Complex (Greece). The sample recrystallized at high temperatures (similar to 720 degrees C) and low pressures (<1GPa) during which kyanite was replaced by fine-grained symplectites of sapphirine, spinel, plagioclase and corundum. The plagioclase rim around kyanite shows a cusp-shaped chemical zoning of calcium along the grain boundaries. The results from combined phase equilibria and diffusion modelling show that the cusp-like shapes were developed by fast grain boundary diffusion at constant pressure after the relaxation of grain-scale pressure variations across the plagioclase rim. The results illustrate an example where petrographic observations help to distinguish between mechanically- and diffusion-controlled chemical zoning.