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 pegmatites 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 commonly from I-type granites, and a mixed NYF + LCT family of diverse origins, such as contamination of NYF plutons by digestion of undepleted supracrustal rocks.Keywords: classifi cation, granitic pegmatites, geochemistry, mineral assemblage, petrogenesis. SOMMAIREIl y a eu plusieurs tentatives de classifi cation de pegmatites granitiques au cours du siècle dernier, avec un taux de réussite et une applicabilité variables. La structure interne, les relations paragénétiques, la composition chimique globale, les aspects pétrogénétiques, la nature du milieu de croissance, et les caractéristiques géochimiques ont tous été utilisés comme bases de classifi cation. Toutefois, ces schémas ont été limités par le niveau de compréhension de ces paramètres lors de leur application, et par négligeance des différences du milieu géologique. Des progrès substantiels ont seulement été atteints depuis la fi n des années 1970. La classifi cation est abordée ici de deux directions, fondées sur les travaux antérieurs de Ginsburg et Černý, mais affi nés et considérés dans un contexte élargi. Le premier concept porte sur la situation géologique, et mène à cinq classes de pegmatites granitiques: abyssale, à muscovite, à muscovite -éléments rares, à éléments rares et miaro...

Rare Earth Element Deposits of Alkaline Igneous Rocks

“…In fact, these rocks, particularly of nepheline syenitic and granitic composition, can host REE deposits, which are commonly enriched in yttrium and HREE ( Figure 3). The Nb-Y-F type of pegmatites [11], which may host Be, REE, Th, U and Zr mineralization, has also been correlated with the peralkaline granites. Alkaline rocks including peralkaline ones, are commonly found in anorogenic and within-plate tectonic settings, mostly in continental rift and/or crustal extension zones [3].…”

Chemical Composition of Mn- and Cl-Rich Apatites from the Szklary Pegmatite, Central Sudetes, SW Poland: Taxonomic and Genetic Implications

“…The Szklary granitic pegmatite has an affinity for the beryl-columbite-phosphate subtype of the REL-Li class and LCT (Li-Cs-Ta) geochemical family sensu [31]. The enrichment in P and the LCT affinity of the pegmatite point to metasedimentary source rocks for the pegmatite-forming melt (e.g., [32]).…”

“…The pegmatite consists mostly of sodic plagioclase (An ≤ 14%), microcline, quartz, and biotite, with minor Fe 3+ -bearing schorl-dravite tourmaline and muscovite. It is poorly zoned with, ideally: (1) a few centimetres thick marginal quartz-feldspar graphic unit with biotite and black tourmaline; (2) a coarser-grained intermediate graphic unit (microcline > plagioclase; individual feldspar crystals rarely exceed 15 cm across) enriched in muscovite and tourmaline (schorl-foitite) and containing accessory chrysoberyl; and (3) a biotite-free central graphic zone containing locally developed blocky feldspar segregations (microcline >> plagioclase; individual crystals in blocky feldspar segregations The Szklary granitic pegmatite has an affinity for the beryl-columbite-phosphate subtype of the REL-Li class and LCT (Li-Cs-Ta) geochemical family sensu [31]. The enrichment in P and the LCT affinity of the pegmatite point to metasedimentary source rocks for the pegmatite-forming melt (e.g., [32]).…”

Corrigendum to: Phosphate minerals in granitic pegmatites from the Mount Wills District, north-eastern Victoria

“…Using the classification system of Cerny and Ercit (2005), both the Blue Jacket and Knocker pegmatites are highly evolved rocks within the LCT (Li-Cs-Ta) family. This is unsurprising, given that LCT granitic pegmatites are typically associated with late tectonic peraluminous granites formed during anatexis of primarily sedimentary source rocks, a setting represented by the Wagga-Omeo Zone.…”