A genetic model is proposed for pegmatitic Al2SiO5-bearing segregations in regionally metamorphosed rocks of the Lepontine Alps. Components that form the segregations were derived from the host rocks by dissolution of minerals in alkali-bearing aqueous fluids produced by metamorphic dehydration reactions. The segregations formed syntectonically within extensional (dilatant) fractures; infiltrations of fluids into the fractures was driven by fluid-pressure gradients ("tectonic pumping"). Precipitation of aluminum silicates from alkali aluminum complexes (rather than alkali silicates) was favored by reduced fH2O within the segregations. The transformation of kyanite to andalusite within the segregations resulted from localized pressure reduction (via dilatancy) during the later stages of decompression of the Pennine nappes subsequent to the main Alpine metamorphism.