A petrologic assessment of internal zonation in granitic pegmatites

Highlights

• The bulk composition of pegmatite-forming melts

• The magnitude of liquidus undercooling prior to the onset of crystallization

• Subsolidus isothermal fractional crystallization of eutectic or minimum melts

• Constitutional zone refining via the creation of a boundary layer liquid at the crystallization front

• Far-field chemical diffusion, the long-range and coordinated diffusion of ions through melt

Abstract

Cameron et al. (1949) devised the nomenclature and delineated the patterns of internal zonation within granitic pegmatites that are in use today. Zonation in pegmatites is manifested both in mineralogy and in fabric (mineral habits and rock texture). Although internal zonation is a conspicuous and distinctive attribute of pegmatites, there has been no thorough effort to explain that mineralogical and textural evolution in relation to the zoning sequence presented by Cameron et al. (1949), or in terms of the comprehensive petrogenesis of pegmatite bodies (pressure, temperature, and whole-rock composition). This overview of internal zonation within granitic pegmatites consists of four principal parts: (1) a historic review of the subject, (2) a summary of the current understanding of the pegmatite-forming environment, (3) the processes that determine mineralogical and textural zonation in pegmatites, and (4) the applications of those processes to each of the major zones of pegmatites. Based on the concepts presented in London (2008), the fundamental determinates of the internal evolution of pegmatite zones are: (1) the bulk composition of melt, (2) the magnitude of liquidus undercooling prior to the onset of crystallization, (3) subsolidus isothermal fractional crystallization, by which eutectic or minimum melts fractionate by sequential, non-eutectic crystallization, (4) constitutional zone refining via the creation of a boundary layer liquid, chemically distinct from but continuous with the bulk melt at the crystallization front, and (5) far-field chemical diffusion, the long-range and coordinated diffusion of ions, particularly of alkalis and alkaline earths, through melt.

Introduction

Granitic pegmatites (pegmatites for short here) have intrigued earth scientists for centuries. Graphic granite (Fig. 1), the defining texture of pegmatites, was singled out and illustrated by Patrin (1801). Rare minerals from pegmatites attracted some of the earliest practitioners of mineralogy (e.g., Brush and Dana, 1878), and mineralogical studies of pegmatites have dominated the field for the past four decades. Economic geologists placed pegmatite deposits on a par with other granite-associated ores (e.g., Lindgren, 1913), and pegmatites continue to be sole or important sources of rare metals, industrial minerals for glasses and ceramics, and colored gemstones (see reviews by Glover et al., 2012, Linnen et al., 2012, Simmons et al., 2012). Although pegmatites have been studied exhaustively in their many individual parts, they have received surprisingly little investigation as a whole; that is, as rocks, through the methods of petrology.

Most definitions of pegmatites contain genetic interpretations; these rocks are difficult to characterize on a purely descriptive basis. London (2008) defined pegmatites as: “essentially igneous rock, mostly of granitic composition, that is distinguished from other igneous rocks by its extremely coarse but variable grain-size, or by an abundance of crystals with skeletal, graphic, or other strongly directional growth habits.” The modifiers “essentially” and “mostly” introduce purposeful ambiguity to the nouns they modify: pegmatites may not be entirely igneous, and they are not always or entirely granitic in composition. In this definition, the textural attributes are related by the conjunction “or”, meaning that any one of them might, in a particular instance, be diagnostic of pegmatite. Exceedingly coarse crystal size is a hallmark of pegmatites for most geoscientists, but the other mineral textures and rock fabrics cited above are prevalent and may be defining characteristics of some bodies.

The vast majority of pegmatites possess compositions close to the minimum or eutectic assemblage of granite (e.g., Jahns and Tuttle, 1963, London et al., 2012b, Norton, 1966). These common pegmatites (London, 2008) contain biotite and muscovite and accessory garnet, tourmaline, and apatite. Pegmatites that host appreciable beryl, lithium aluminosilicates, phosphates other than apatite, oxides other than magnetite or ilmenite, and other rarer minerals are regarded as rare-element pegmatites. The compositions of even the most chemically complex rare-element pegmatites plot close to the hydrous granite minimum composition at elevated pressure (Stilling et al., 2006). For this reason, most geoscientists have ascribed pegmatites to a fundamentally igneous origin, though exclusively hydrothermal models have persisted since the beginning of modern geology (e.g., Gresens, 1969, Hunt, 1871, Ramberg, 1952, Reitan, 1965, Roedder, 1981).

A genetic link between granites and pegmatites is beyond reasonable doubt. How pegmatites are derived from, and what makes them different from granites has been debated for more than a century. Whereas granites form large masses of comparatively uniform and mineralogically homogeneous rock, pegmatites are precisely the opposite. Most pegmatite bodies are small, with dimensions on the scale of meters rather than kilometers, and internally heterogeneous in their composition and rock fabrics. They occur as segregations along the margins of the cupolas of granites and as sharply discordant dikes that intrude igneous and metamorphic rocks. Zonation in pegmatites occurs at two scales: (1) regional zonation, manifested as increasing chemical complexity with distance from their granitic or other thermal source, and (2) internal zonation, the mineralogical and textural changes within individual pegmatite bodies. This review mostly concerns the latter topic, because it has been the focal point of most petrologic research and opinion on pegmatites.

Pegmatites share textural features in common with hydrothermal veins, including (1) inward coarsening of crystals, (2) inward-directed crystal growth, (3) mineralogical layering parallel to host contacts, and (4) sharply bounded zonation of mineral assemblages (Fig. 2, Fig. 3). The one texture that is unique to pegmatites, its defining texture, is graphic granite, the skeletal intergrowth of quartz and alkali feldspars (Fig. 1). However, the zonal similarity of pegmatites to hydrothermal veins has, no doubt, contributed to the belief that pegmatites form via a largely hydrothermal process.

Although rare-element pegmatites make up only 1–2% of all pegmatites, most of the schemes to classify pegmatites are based on their rare-element mineralogy (e.g., Černý and Ercit, 2005, Trueman and Černý, 1982). Černý (1991) divided the rare-element pegmatites into two chemical families based on their distinctive elemental (mineral) enrichments. These are the LCT (Li–Cs–Ta) and NYF (Nb–Y–F) families. The LCT family of pegmatites bears the hallmarks of S-type (sedimentary) granite sources: peraluminous, enriched in rare alkalis, beryllium, boron, phosphorus, and tin, with uncommonly low Nb/Ta ratios. The NYF family of pegmatites is most closely associated with the A-type (anorogenic, or within-plate) suite of granites (Černý and Ercit, 2005, Černý et al., 2012, London, 2008); these pegmatites are enriched in heavy rare-earths, fluorine, with high Nb/Ta ratios. Among the 20 types and subtypes of rare-element pegmatites that are recognized by Černý and Ercit (2005), those of the LCT family far outnumber the NYF deposits. Among the LCT family, pegmatites of the beryl and the complex spodumene/petalite types greatly predominate over all others. Consequently, all of the zoning models presented below were developed for and apply to the common pegmatites (lacking distinctive rare-element mineralization) and the Be- and Li-rich rare-element pegmatites of the LCT family.

In a comprehensive survey of the NYF pegmatites, Ercit (2005) notes that the NYF pegmatites lack the regional zonation that is observed in districts composed of the LCT pegmatites, but that the internal zonation (or the lack of it) of individual bodies is similar in all respects to that found in common pegmatites and those of the LCT family. Therefore, the ensuing discussion of pegmatite zoning can be taken to apply to all pegmatites: the abundant common granitic pegmatites, and the uncommon rare-element pegmatites of both rare-element families.

One of the most conspicuous and enigmatic features of pegmatites is their sharply bounded internal zonation. That zonation is both mineralogical and textural. Though Brögger (1890) first proposed that the zonation within pegmatites might involve the simultaneous interactions of silicate melt and aqueous fluid, that concept is now widely attributed to Jahns and Burnham (1969). The Jahns–Burnham model is reviewed in a later section. However, there is no other precedent in petrology for a model that has so completely dominated its field for half a century with virtually no evidence needed by the majority of scientists.