THE CLASSIFICATION OF GRANITIC PEGMATITES REVISITED

Abstract

The classifi cation of granitic pegmatites was frequently attempted during the past century, with variable degrees of success and applicability. Internal structure, paragenetic relationships, bulk chemical composition, petrogenetic aspects, nature of parent medium, and geochemical features were applied. However, all schemes were marked by contemporary degrees of understanding of these parameters, and most attempts were hindered by ignoring differences in geological environment. Substantial progress was achieved only since the late 1970s. The classifi cation is approached here from two directions, based on but broadened and refi ned from earlier works by Ginsburg and Čern´y. The fi rst concept deals with geological location, leading to division of granitic pegma-tites into fi ve classes (abyssal, muscovite, muscovite – rare-element, rare-element, and miarolitic), most of which are subdivided into subclasses with fundamentally different geochemical (and in part geological) characteristics. Further subdivision of most subclasses into types and subtypes follows more subtle differences in geochemical signatures or P–T conditions of solidifi cation, expressed in variable assemblages of accessory minerals. The second approach is petrogenetic, developed for pegmatites derived by igneous differentiation from plutonic parents. Three families are distinguished: an NYF family with progressive accumulation of Nb, Y and F (besides Be, REE, Sc, Ti, Zr, Th and U), fractionated from subaluminous to metaluminous A- and I-type granites that can be generated by a variety of processes involving depleted crust or mantle contributions; a peraluminous LCT family marked by prominent accumulation of Li, Cs and Ta (besides Rb, Be, Sn, B, P and F), derived mainly from S-type granites, less