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

Abstract

The classification scheme of granitic pegmatites currently in vogue is largely an offshoot of the depth-zone classification of granitic rocks. We propose instead a focus on petrogenetic processes at work, and extend it to pegmatites in silica-saturated and -undersaturated systems. Small-scale bodies of pegmatites are encountered in metamorphic systems undergoing anatexis, as temperature is rising regionally. We include a newly recognized category of corundum-bearing anatectic felsic pegmatite formed in mantle peridotite earlier subjected to a metasomatic overprint by a mixed H2O + CO2 fluid. The bulk of the pegmatite bodies encountered are not obviously of anatectic origin, but are derived by fractionation of pluton-size batches of felsic magma. We consider LCT (Li–Cs–Ta-enriched) pegmatites 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 really not 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 H2O + CO2 fluid phase precedes large-scale melting, to assure a product having alkaline tendencies. A primary NYF–LCT signature may occur in shoshonitic suites. Finally, we discuss hybrid pegmatites in which an NYF pegmatitic system takes on a LCT-like overprint owing to contamination from local sources. The possibility of an LCT pegmatite also being affected by an LCT-type overprint, to form a “super-LCT pegmatite” is explored briefly. Proper documentation of the petrogenetic processes at work is essential in assessing the economic potential of pegmatite occurrences