The grain-scale mechanisms of hydration of mafic lower crustal rocks are investigated on chemical maps of continuous rock sections (20 x 2 cm) of partially amphibolitized samples from the Hustad Igneous Complex in the Western Gneiss Region, Norway. The Proterozoic pyroxenite body and crosscutting dolerite dike enclosed in felsic gneiss that underwent (U)-HP metamorphism show different responses to the exposure to hydrous fluids along fractures formed during late Caledonian extension and exhumation. While the dolerite reaches full amphibolitization in a cm-scale reaction halo with a dm-scale transition zone, the pyroxenite has experienced previous metamorphism and is less affected by this event. Dissolution-precipitation reactions and slightly faster grain boundary assisted flow are identified as the main mechanisms of fluid flow through the rock. Limited element mobility is documented by grain-scale compositional gradients in forming amphibole from magnesiohornblende (Si6.8, Al1.6, Mg3.0) to tschermakite (Si6.4, Al2.0, Mg2.6) at boundaries with plagioclase. Phase diagram calculations yield a P/T-window between 650 – 730°C and 0.4 – 0.6 GPa for amphibolite formation. To better understand the mechanism of hydration, a numerical model of Darcy flow coupled to amphibolitization reactions was formulated based on mass conservation and local equilibrium. The simulations suggest the observed difference in front propagation distance is controlled by the main lithologies. A simple 2D model is employed to demonstrate that the gradual transition from dolerite to amphibolite can be achieved by implementing higher permeability along grain boundaries, supported by the observation that flow along boundaries continues before individual grains are fully replaced.