Author name: Petr Cerny

Pegmatites accentuate the trace element signatures of their granitic sources. Through that signature, the origin of pegmatites can commonly be ascribed to granites whose own source characteristics are known and distinctive. Interactions with host rocks that might modify the composition of pegmatites are limited by the rapid cooling and low heat content of pegmatite-forming magmas. The trace element signatures of most pegmatites clearly align with those of S-type (sedimentary source, mostly postcollisional tectonic environment) and A-type (anorogenic environment, lower continental crust ± mantle source) granites. Pegmatites are not commonly associated with I-type (igneous source) granites. The distinction between granites that spawn pegmatites and those that do not appears to depend on the presence or absence, respectively, of fluxing components, such as B, P, and F, in addition to H2O, at the source.

Virtually every conceivable model to explain the internal evolution of granitic pegmatites had been proposed by the 1920s. Two of these hypotheses have prevailed: (1) the fractional crystallization of fluxbearing granitic melt inward from the margins of the pegmatite body to the center, and (2) the buoyant separation of an aqueous fluid from the silicate melt and its effects on the redistribution of components. A recent model combining aspects of both concepts invokes the formation of a flux-enriched boundary layer of silicate liquid in advance of a crystallization front. Though most of the internal chemical and textural features of pegmatites can now be reconciled, the puzzle of pegmatites is far from solved.

Granitic pegmatites have been a focal point of research by petrologists and mineralogists for over a century. Mineralogical interest stems from the diversity of rare minerals that some pegmatites contain. Petrologic efforts are aimed at resolving the processes or agents that produce the complex textures and spatial heterogeneity that distinguish pegmatites from granites. Much of the scientific study of pegmatites has been motivated by exploration for the economic commodities they provide. Pegmatites yield quartz, feldspars, and micas for industrial uses; strategic rare metals for electronic, aerospace, and energy applications; and many of the world’s finest gem and mineral specimens.