Dolerite dykes that cut Tertiary baslats near the Skaergaard intrusion, East Greenland, are extensively altered to metasomatic assemblages indicating large scale mobilisation of calcium and alkalis. The alteration is characterised by replacement of the dolerite by prehnite together with lesser amounts of epidote, amphibole, salite, titanite, calcite and chlorite, resulting in a complete loss of primary igneous texture. A related type of alteration in the same dykes consists of albite, epidote, amphibole and chlorite with only partial loss of primary texture. Textural relations indicate that the albitic alteration occurs first and is progressively overprinted by the later CaAl-silicate dominated alteration with a consequent large addition of calcium and removal of sodium. Quartz is absent at all stages of alteration. The metasomatic reactions are confined to the centres of a group 10 to 20 metres wide, compound dykes, that were intruded late relative to the majority of Tertiary dykes in this part of East Greenland. These dykes contain abundant leucocratic segregations, the pegmatitic and drusy nature of which suggest volatile pressure increased at late stages in dyke cooling, possibly leading to an early episode of autometasomatism. The later CaAl-silicate alteration is localised around these segregations, but affects the surrounding dolerite. Hydrogen isotope data indicates that the fluids responsible for precipitation of the CaAl-silicates were meteoric, and derived from the surrounding basalt-hosted hydrothermal system. The physical attributes of the dykes, including their width, compound nature, and high initial porosity, are all factors that favoured the influx of surrounding pore fluids into the dyke centres, where they became isolated and reacted to form the CaAl-silicate assemblages. Fluid flow into the dyke centres may have been characterised by fluid movement orthogonal to isotherms. Changes in the relative concentrations of aqueous species in fluids coexisting with albite and CaAl-silicates as a function of temperature along such flowlines are considered an important factor in driving the observed Ca−Na exchange.