Numerous quartz-feldspar pegmatites occur in the gneisses and crystalline schists of West Greenland. Some of these pegmatites are volume-by-volume replacement bodies, others formed by slow growth in dilating fissures. The nondilation and the dilation pegmatites were emplaced by similar mechanism—viz., diffusion through the host rocks and through fissures. Field evidence is against bodily flow of pegmatite magma and crystallization from a stagnant magma. Tension joints and shear zones have commonly localized pegmatites, but other structural heterogeneities such as pre-pegmatitic dikes and inclusions in gneisses were also “sinks” for the diffusion currents of pegmatite-forming matter.
Ptygmatic folding and pinch-and-swell structures are results of tectonic deformation simultaneous with—or later than—the formation of pegmatite.