Abstract
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.
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