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The processes leading to the generation of pegmatite fabrics and rare-element ore deposits are evaluated in terms of pressure, temperature, and compositions of fluids through the combination of recent field, fluid inclusion, and experimental data. The results indicate that primary consolidation of miarolitic and massive rare-element (Li-Be-Ta) pegmatites occurs in the range of 700° to 450°C at 400 to 250 MPa. The model for pegmatite genesis proposed here involves disequilibrium crystallization of quartz and feldspars in a hydrous melt-crystal system that approaches chemical equilibrium but rarely attains it. Fluxing anionic components of boron, phosphorus, and fluorine are not essential to this model, but augment the zoning process by promoting expansion of the liquidus fields of quartz and K-silicates (feldspar and mica) and by driving residual melt toward alkaline, Na-rich compositions enriched in lithophile trace elements. Aqueous vapor, if present, may have a nominal affect on primary zonation and fabric development within pegmatites. Various lines of evidence suggest that rare-element pegmatites may not become vapor saturated until they approach solidus conditions, at which point internal retrograde recrystallization and wall-rock alteration occur. Recent experiments have succeeded in replicating most aspects of pegmatite geology, including fractionation trends, mineral fabrics, and spatial zonation, at vapor-undersaturated conditions.

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