WHY METASOMATIC FRONTS ARE REALLY METASOMATIC SIDES

Back-scattered electron microscope images of metasomatic calc-silicate rocks demonstrate complex mineral growth patterns; often oscillatory zoning defines a euhedral or subhedral morphology. We infer from this the existence of an appreciable transient porosity (up to a few percen t) during metasomatic calc-silicate growth, and argue that porous, rea cting calc-silicate layers become conduits for irreversible metamorphi c fluid loss. Because the layers are too permeable to sustain a steep gradient in hydraulic head downdip, fluid pressure in the calc-silicat e must generally be lower than in adjacent low-permeability lithologie s, so that steep local gradients in the hydraulic head will drive flui d from surrounding rocks up, down, or sideways into the transiently po rous reacting layer. So-called ''metasomatic fronts'' separate transie ntly permeable layers that have undergone metasomatism from little-alt ered, low-permeability rocks. Thus they develop parallel to the flow p ath at its edge and should be termed metasomatic sides.