The patterns of enrichment in felsic pegmatites ultimately depend on tectonic setting

A classification scheme is proposed for granitic pegmatites focused, not on an offshoot of the depth zone classification, but on petrogenetic processes at work, with an extension to pegmatites in silica-saturated and under-saturated systems. Small-scale pegmatites are encountered also in metamorphic systems undergoing anatexis, as temperature is rising regionally. A newly recognized category of corundum-bearing anatectic felsic pegmatite formed in mantle peridotite earlier subjected to a metasomatic overprint by a mixed H (sub 2) O + CO (sub 2) fluid is included. The bulk of pegmatite bodies encountered are not of anatectic origin, but are derived by fractionation of pluton-scale batches of felsic magma. The LCT (Li-Cs-Ta-enriched) pegmatites are considered to be members of orogenic (calc-alkaline suites) formed in a subduction setting; in contrast, NYF (Nb-Y-F-enriched) pegmatites are affiliated with anorogenic suites, formed in an extensional setting, and not really fundamentally different in petrogenetic lineage from suites in silica-undersaturated systems, including carbonatites. Both LCT and NYF types involve magmas containing crust and mantle components, but in the case of anorogenic magmatism, a metasomatic ground-preparation by a mantle-derived H (sub 2) O + CO (sub 2) fluid phase precedes large-scale melting, to assure a product having alkaline tendencies. A primary NYF-LCT signature may occur in shoshonitic suites. Hybrid pegmatites in which an NYF pegmatitic system takes on an LCT-like overprint owing to contamination from local sources are also discussed. The possibility of an LCT pegmatite also being affected by an LCT-like overprint, to form a 'super-LCT pegmatite' is explored briefly. Proper documentation of the petrogenetic processes at work is essential when assessing the economic potential of pegmatite occurrences.