pegmatites

Granitic Pegmatites as Reflections of Their Sources

Keystone pegmatite, Pennington County, South Dakota, USA. Named in honor ˇ erný. of Petr C SOURCE : M. SCOTT; PHOTO : R OBERT D OWNS

1 ˇ Petr Cerný, David London,2 and Milan Novák3

1811-5209/12/0008-0289$2.50

Cˇ ernýite, Hugo mine,

DOI: 10.2113/gselements.8.4.289

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egmatites accentuate the trace element signatures of their granitic sources. Through that signature, the origin of pegmatites can commonly be ascribed to granites whose own source characteristics are known and distinctive. Interactions with host rocks that might modify the composition of pegmatites are limited by the rapid cooling and low heat content of pegmatite-forming magmas. The trace element signatures of most pegmatites clearly align with those of S-type (sedimentary source, mostly postcollisional tectonic environment) and A-type (anorogenic environment, lower continental crust ± mantle source) granites. Pegmatites are not commonly associated with I-type (igneous source) granites. The distinction between granites that spawn pegmatites and those that do not appears to depend on the presence or absence, respectively, of fluxing components, such as B, P, and F, in addition to H2O, at the source.

their starting compositions or assimilant, granitic magmas evolve toward the bulk composition of the thermal minimum or eutectic in the system NaAlSi3O8 – KAlSi3O8 –SiO2, with slight deviations to the peralkaline (molar Al 2 O3 < [Na 2 O + K 2 O + CaO]) or peraluminous (molar Al 2O3 > [Na 2O + K 2O + CaO]) sides of that compositional system. This liquid line of descent applies to the major and minor components of granitic liquids but generally not to their trace elements (or isotopes).

If the trace elements that are imparted to the melt at its source KEYWORDS : pegmatite, granite, S-type, A-type, I-type, assimilation, contamination behave as perfectly incompatible in all of the ensuing crysINTRODUCTION talline phases, then pegmatites would carry an amplifi ed signature of that trace element Tracing igneous rocks to their ultimate sources represents a recurrent and contemporary theme in petrologic research. pattern. The petrologically important trace elements found To the extent that pegmatites are derived from granites, our in granite–pegmatite systems display variable degrees of incompatibility as functions of the pressure, temperature, ability to recognize the provenance of pegmatites hinges and mineral phases in the system. Depending on their upon our capacity to relate granites to their parental rocks. compatibility in the rock-forming minerals of granites, The sources of granitic magmas are debated as much today the relative and absolute abundances of the initial suite as they were over 60 years ago, when N. L. Bowen and H. of trace elements may be modified by the process of fracH. Read argued over “the room problem” (Young 1998). tional crystallization or via contamination by assimilation They contested, in part, the relative importance of mantle of material from external reservoirs (e.g. Novák et al. 2012). versus crustal sources for granitic magmas. Using today’s However, the idiosyncratic chemical signatures of granitic robust database of trace element and isotope chemistry, pegmatites are manifested by those trace elements that petrologists still cite evidence for entirely crustal origins are highly incompatible in the rock-forming minerals that for granites (e.g. Chappell and White 2001) or sources with crystallize from granitic magmas. Hence, the abundances a large mantle component (e.g. Healy et al. 2004; Smithies of these trace elements increase essentially without moderet al. 2011). The nature and extent of interaction between ation until they form their own distinctive minerals, such granitic magmas and the various rocks they encounter en as beryl, spodumene, tantalite, etc., in the most evolved route toward the Earth’s surface is another area of past and types of pegmatites. present dissonance (compare Roberts and Clemens 1995 with Pignotta and Paterson 2007). CLASSIFICATION SCHEMES Granitic magmas refi ne their compositions by crystal fractionation and by the separation of residual liquids from their crystalline products. Hence, regardless of

1 Department of Geological Sciences University of Manitoba, Winnipeg, MB R3T 2N2, Canada E-mail: [email protected] 2 ConocoPhillips School of Geology & Geophysics University of Oklahoma, 100 East Boyd Street, Room 710 SEC Norman, OK 73019, USA E-mail: [email protected] 3 Department of Geological Sciences, Masaryk University Kotláˇrská 2, 611 37 Brno, Czech Republic E-mail: [email protected]

E LEMENTS , V OL . 8,

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FOR PEGMATITES The current system for classifying pegmatites (TABLE 1) begins with a subdivision of pegmatite classes (Ginsburg et al. 1979). The pegmatite classes are distinguished on the basis of the metamorphic environment of their host rocks (the abyssal class), mineralogy (the muscovite class), elemental composition (the rare-element class), and texture (the miarolitic class). Most of the pegmatite classes carry an implied connotation of their environment of emplacement, more or less equivalent to depth of formation. The pressures (depths) at which pegmatites crystallize, however, are poorly constrained by any chemical or textural features of the pegmatites themselves. Most pegmatites are intrusive bodies, and hence postdate their immediately adjacent host

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Class

Subclass

Abyssal

HREE

Type

Subtype

Family

NYF

LREE

NYF LCT

U BBe

Muscovite Muscovite– – rare element

REE

NYF

Li

Rare element

REE

LCT allanite– –monazite euxenite

NYF

gadolinite Li

beryl

beryl–columbite beryl–columbite–phosphate

complex

spodumene petalite lepidolite

LCT

elbaite amblygonite albite– –spodumene albite Miarolitic

REE

topaz–beryl

NYF

gadolinite–fergusonite Li

beryl–topaz spodumene

LCT

petalite lepidolite

ˇ erný and The pegmatite classification scheme of C Ercit (2005), modified to show the correlation between pegmatite classes and families. NYF = niobium–yttrium– fluorine family (green); LCT = lithium–cesium–tantalum family (yellow); HREE = heavy rare earth elements; LREE = light rare earth elements TABLE 1

rocks. The pressure and temperature at which pegmatite crystallize, therefore, may have little or no direct relationship to the conditions of formation and the mineral assemblages of their hosts. For these reasons, the application of the pegmatite classes is fraught with contradiction and ambiguity (Tkachev 2011). ˇ ˇ Cerný (1991) and Cerný and Ercit (2005) expanded the classification of granitic pegmatites to include ten subclasses, four of which are subdivided into thirteen types, and two types are further broken down into seven subtypes. All of these categories are based on the trace element signatures of the pegmatites as reflected in their mineralogy and mineral chemistry. TABLE 1 shows the hierarchy of classifiˇ cation, beginning, as did Cerný and Ercit (2005), with the pegmatite classes. Color bars illustrate how the pegmatite subclasses and their constituents fit into an overarching classification of two pegmatite families.

THE PEGMATITE FAMILIES Large granitic batholiths are probably assembled from multiple plutons, which may arise from different or heterogeneous sources, each contributing its own trace element E LEMENTS

suite. To the extent that pegmatites acquire their trace elements from granitic plutons, therefore, one might expect that the trace element signatures of pegmatites would be hopelessly variable. The fact is, the trace element signatures of most rare-element pegmatites can be grouped into ˇ just two distinctive families (Cerný 1991): one that is enriched in lithium, cesium, and tantalum (LCT) and the other characterized by enrichment in niobium, yttrium, and fluorine (NYF). Most of the pegmatites with the LCT signature have compositional affi nity with S-type granites (Chappell and White 2001). The peraluminous nature of S-type granites is expressed by assemblages that include some combination of muscovite, garnet, cordierite, sillimanite or andalusite, tourmaline, and gahnite (ZnAl2O4). These granites stem from the anatexis of metamorphic schists and aluminous gneisses of sedimentary origin. The original sediments (pelites) consist mostly of clay-rich material produced by extensive chemical weathering of continental rocks. The trace element signatures of the granites, and of LCT pegmatites derived from them, are imparted mainly by the participation of micas and feldspars in the melt-forming reactions. Most of the pegmatites that belong to the NYF family are sourced from A-type granites, where “A” means “anorogenic” (e.g. Eby 1990). The origins of A-type granites are varied and debatable. The source of such granites is generally thought to be gneissic granulites deep in the continental crust, with some contribution from the mantle in

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the form of basaltic melt or low-density carbonic fluid. ˇ Cerný and Ercit (2005) now ascribe a small fraction of the LCT and NYF pegmatites to I-type sources. I-type granites are usually affi liated with subduction-related magmatism, but they can be generated from the metamorphic products of any mafic to intermediate igneous rocks or volcaniclastic sedimentary rocks.

pegmatites to the tectonic environment of subduction. They may be more properly affi liated with a post-tectonic phase in the development of continental-collision belts (Tkachev 2011). The main characteristics of the LCT pegmatites, however, are derived from previously unmelted, micarich metamorphic rocks, irrespective of the tectonic regime in which their initial partial melting occurs.

ˇ The pegmatite subclasses, types, and subtypes of Cerný and Ercit (2005) can be assigned with little ambiguity to one of the families (highlighted in yellow or green in TABLE 1); as well, hybrids that arise by mixing between LCT and ˇ NYF sources can also be recognized (Cerný and Ercit 2005; Martin and de Vito 2005; Novák et al. 2012). Pegmatites that carry the LCT signature greatly outnumber those of the NYF family, and within the LCT family, the berylliumand lithium-rich subclasses and types are by far the most common of the rare-element pegmatites.

White mica (muscovite–paragonite–phengite solid solutions) and dark mica (biotite-group solid solutions) carry most of the trace elements that defi ne the signature of the LCT pegmatites (e.g. Dahl et al. 1993). The abundant white mica in schist of marine sedimentary origin reacts extensively at the onset of anatexis (London et al. 2012). The initial extent of melting is small, because it is limited by the low sodium content of the rock. Consequently, a large fraction of the rare-element content of mica schist is transferred to a small volume of partial melt. The melting reactions of white and dark mica also produce K-feldspar + aluminosilicate + spinel as products, especially when the concentration of H 2O in the melt is well below that of saturation (e.g. Acosta-Vigil et al. 2003). Rubidium is slightly incompatible in K-feldspar, whereas Li and Cs are almost perfectly incompatible. Hence, the formation of K-feldspar at the source, and its continued crystallization from the granitic melt, leads the liquid line of descent toward a composition in which the Cs/Rb and Li/Rb ratios become highly elevated. This fractionation trend results in the pattern of rare-alkali enrichment found in the LCT pegmatites.

ˇ The classification scheme proposed by Cerný and Ercit (2005) hinges upon the rare-element signatures of pegmatites, as there is often little else in their composition that serves to distinguish them. In most cases, the rare-element signature is ascertained from the exotic mineralogy of these pegmatites. It is important to bear in mind that the vast majority of pegmatites do not possess exotic minerals (see London and Morgan 2012 this issue). However, the concept of the pegmatite family was meant to apply not to any individual pegmatite, but to a large group of comagmatic pegmatites, of which only a few evolve to develop the diagnostic mineralogy of the family, its subclasses, types, etc. In the common pegmatites that lack more exotic mineralogy, the characteristics of the pegmatite family can be ascertained from and followed through the trace element contents of the common minerals, such as micas, oxides, ˇ mafic silicates, and others (Cerný et al. 1985). ˇ In TABLE 1, Cerný’s (1991) pegmatite families are identified by their diagnostic trace element signatures. If these signatures were substituted for the classes, the classification of granitic pegmatites would be purely on their chemical attributes, without genetic inferences for depth of emplacement or an implied tectonic setting.

The LCT Family Enrichment in the rare element lithium is the most prevalent characteristic of the LCT pegmatites. The predominant lithium minerals include the silicates spodumene, petalite, lepidolite, and elbaite, and the phosphate series amblygonite–montebrasite [LiAlPO 4 (F,OH)] and lithiophilite– triphylite [Li(Mn,Fe)PO4 ]. Cesium can be elevated in beryl and micas, but Cs can achieve concentrations sufficient to precipitate pollucite, CsAlSi 2O6. Although columbite (a Nb-dominant oxide) appears early in the evolutionary sequence of the LCT pegmatites, Ta-rich oxides predominate toward the end (see Linnen et al. 2012 this issue). The important fluxing components B, P, and F are elevated but variably enriched (see London and Morgan 2012). Boron is found in black tourmaline in the margins of pegmatites, but also in gem-forming elbaite in the central zones (Simmons et al. 2012 this issue). Many of the LCT pegmatites contain a plethora of primary and secondary phosphates in addition to apatite, and phosphorus is a significant component of the feldspars (London et al. 1999). S-type granites arise from crustal thickening that is usually associated with subduction and continental collision. In most occurrences, however, pegmatites derived from these granitic sources lack the foliation or pervasive deformation that is expected in a syntectonic environment. Because of the high abundance of granitic pegmatites in orogenic belts, Martin and De Vito (2005) link the LCT family of E LEMENTS

Within individual pegmatites, the Nb–Ta oxides fractionate from Nb-rich at the margins to Ta-rich in the central units. Linnen et al. (2012) explain that trend by the contrasting solubilities of Nb versus Ta oxides in melt as a function of temperature. However, the same general trend of increasing Ta/Nb ratio is present from the start of granite fractionation ˇ (e.g. Cerný et al. 1985), when the distributions of these elements are controlled by major and accessory minerals in which Nb and Ta are nonessential trace constituents. London (2008) reviewed the published data on partitioning of Nb and Ta among rutile, ilmenite, titanite, amphiboles, and biotite. There was no consistent pattern in the partitioning data; that is, the phases in question did not consistently incorporate one element over another. Thus, the factors that fractionate Ta from Nb in granites are not yet fully known. Chappell and White (2001) observed that an elevated phosphorus content is as diagnostic of the S-type granites as is their peraluminous composition. Both chemical attributes are positively correlated with the derivation of melt from metapelite protoliths and with the H 2O content of those melts (London et al. 1999; Acosta-Vigil et al. 2003). Phosphorus- and Cs-rich lithium pegmatites are truly diagnostic of S-type sources for the LCT family of pegmatites (Martin and De Vito 2005).

The NYF Family Pegmatites that fit into the NYF family are notable because they contain chemically complex oxides and silicates that carry heavy rare earth elements (HREEs), Ti, U, Th, and Nb > Ta. These include euxenite/aeschynite [(Y,Ca,Ce,U,Th) (Nb,Ta,Ti)2O6], samarskite/fergusonite [(Y,Fe3+,Fe2+,U,Th,Ca) (Nb,Ta)O4], gadolinite [(Y,Ca) 2Fe3+ Be2Si2O10 ], and allanite(Y) [CaYFe 2+ Al 2 Si3O12 (OH)]. Abundant fluorite or topaz reflects the enrichment in fluorine. The NYF pegmatites are depleted in phosphorus, and tourmaline is uncommon. Their mafic minerals include ferruginous biotite, aegirine, and riebeckite, the latter two denoting peralkaline compositions for these pegmatites.

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HYBRIDIZATION AT THE SOURCE Some members of the NYF family of pegmatites (TABLE 1), like their A-type granitic sources, possess rare-element signatures that are indicative of more than one important source for their elemental and isotopic components. The radiogenic isotope systems of at least some A-type granites possess defi nitive evidence for mixed mantle–crustal materials (e.g. van Breemen et al. 1975). ˇ Cerný and Ercit (2005) suggested that some rare-element pegmatites in both families might be sourced from I-type granites. However, the I-type granites that are clearly associated with subduction zones (mostly Phanerozoic in age) tend to lack significant pegmatitic aureoles at their margins, which is a hallmark of the S-type and A-type granites. As an example, the granitic porphyries of Tertiary age that generated large hydrothermal cells and copper mineralization in the western Cordillera of North America are devoid of pegmatitic textures. These igneous bodies are thought to have exsolved a saline aqueous fluid early in the history of their magmatic consolidation, and hence, according to the Jahns-Burnham model (see London and Morgan 2012), should have been prime candidates for developing a pegmatitic facies (cf Nabelek et al. 2010).

Granite and pegmatite dikes in tonalite along the Piute Pass Trail between Piute Pass and Hutchinson Meadow, central Sierra Nevada, California. The thicker central dike shown here possesses pegmatitic borders, in which feldspar crystals are elongate and branch toward the dike center, followed inwardly by aplitic texture, and then a return to coarse-grained pegmatitic texture in the central zone. Pencil for scale. PHOTO : JAMES T. G UTMANN

FIGURE 1

Most of the NYF pegmatites bear a chemical affi nity to ˇ A-type granites (Eby 1990; Cerný and Ercit 2005; Martin and De Vito 2005). As a general model, A-type granites are sourced from combinations of pyroxene-bearing quartzofeldspathic rocks of the lower continental crust with varying amounts of added mantle components (e.g. King et al. 1997; Christiansen et al. 2007). The magmas are believed to be poor in H2O, but F is imparted by the decomposition of amphiboles and micas to pyroxene (Skjerlie and Johnston 1992). In some instances, these granites may be entirely mantle derived (e.g. Haapala et al. 2007), as indicated by their low initial 87Sr/ 86 Sr ratios (e.g. van Breemen et al. 1975). Where their tectonic setting can be ascertained, A-type granites and NYF pegmatites are usually associated with hot spots or rift zones within continents. Martin and de Vito (2005) state that NYF pegmatites carry much the same trace element enrichment patterns as do peralkaline igneous rocks that fractionate directly from mantle sources. That is true for the high fluorine and niobium signatures of both rocks, but the NYF pegmatites are enriched in the heavy rare earths, whereas alkaline magmas derived from mantle sources mostly show a light rare earth enrichment. In addition, the NYF pegmatites, like their A-type granite sources, are highly depleted in phosphorus and are poor in calcium. The peralkaline magmas of direct mantle lineage culminate in rocks that are not only calcic (carbonatites) but usually also phosphorus rich (as apatite). That does not mean that pegmatites do not arise from alkaline mantle sources. They do, but most do not fit the category, sensu stricto, of the NYF family of pegmatites, which are granitic in their overall composition. The origin of the NYF trace element signature is comparatively obscure. For example, it is not known if the predominance of Nb over Ta reflects their relative abundances in the source rocks, or whether some aspect of the mineralogy or fluid chemistry of their parental alkaline magmas fractionates these two elements. The heavy REEs are associated with rocks rich in fluorine. In turn, the high fluorine content of the NYF pegmatites is believed to come from melting reactions involving F-rich amphiboles and micas.

E LEMENTS

Pegmatites that are locally present in the interiors of I-type plutons of the Sierra Nevada batholith, USA, possess sharp intrusive contacts with their host granites and textures that are indicative of thermal quenching of the pegmatiteforming melts against their hosts (FIG. 1; also see Webber et al. 2001). In this association, the pegmatites are not derived from their immediately adjacent igneous rocks. Tourmaline-rich pegmatites reportedly are common in the Cathedral Peak granodiorite and other Sierra Nevada plutons (Lawford Anderson, pers. comm. 2012). The I-type, tin-rich Mole Granite in Queensland, Australia, possesses along part of its margin a meager pegmatitic facies enriched in beryl, topaz, and lithian dark mica (possibly zinnwaldite). In these cases, however, the probable source of these distinctive and incompatible trace elements (Li, Be, B, and F) is subducted sediment, which was incorporated into the eventual I-type granites (e.g. Bebout et al. 2007). Hence, these dominantly I-type granites appear to spawn pegmatites to the extent that they have incorporated S-type materials (marine sediments), which make these magmas hybrids as well.

HYBRIDIZATION VIA LOCAL CONTAMINATION Pegmatite-forming magmas contain negligible heat to promote melting of rafted inclusions of solid rock. However, the fractionated compositions of pegmatite magmas and their fi nal aqueous fluids are highly reactive with other, less-evolved, common host lithologies (e.g. Morgan and London 1987; Novák et al. 2012). Local contamination of pegmatites occurs principally along dike margins during emplacement, and again at the transition into subsolidus conditions. Alteration of host rocks by pegmatite-derived fluids occurs late in the history of consolidation.

Mafic Components The process of crystallization and separation of mafic minerals from granitic magmas leaves their derivative pegmatites depleted in Fe and Mg. In LCT pegmatites, the crystallization of tourmaline can reduce Fe and Mg in the melt to trace levels (Wolf and London 1997). The A-type sources of NYF pegmatites are also poor in Fe and especially in Mg. It is common, however, for evolved pegmatites of both types to contain spectacular concentrations of biotite or tourmaline along their margins (FIG. 2A, B).

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A

B (A) Tourmaline-rich “fringe” along the margins of a thin pegmatite dike hosted by metaconglomerate, from Capoeira 2, Borborema Pegmatitic Province, Brazil. PHOTO : JAN LOUN

(B) Meter-scale crystals of biotite (dark; see arrows) radiate down from the upper contact of the Ipê pegmatite, Governador Valadares, Minas Gerais, Brazil. PHOTO : SKIP SIMMONS

The common hosts for pegmatites, including greenstones, amphibolites, mica schists, and gneisses, are the inferred sources of the mafic constituents (Van Lichtervelde et al. 2006; London 2008). Usually, the biotite- or tourmalinerich “fringe” ends abruptly inward, without any further crystallization of either phase. London (2008) attributed the sharp cessation of these mafic silicates to crystallization along the pegmatite contacts, which effectively seals off chemical communication between the magma and the host rocks. It is notable that although an influx of mafic components into pegmatite appears to be pronounced, there is rarely any counterflow or diffusion of pegmatite-derived components into the host rocks along their margins, except locally and sporadically around the largest rare-element pegmatites at the end stages of their consolidation (Morgan and London 1987).

and granitic melts can occur rapidly and over large distances, such that the composition of a host rock could dominate the resultant alkali ratio in small volumes of intruded melt (London et al. 2012). The extent to which this is true for pegmatites is not yet known. However, the conditions in which pegmatites crystallize (see London and Morgan 2012) are not conducive to an extended period of open-system communication between pegmatites and their hosts.

FIGURE 2

Alkaline Earths Contamination of LCT pegmatites by metacarbonates appears to have modified the sources of some pegmatites in central Madagascar and the Czech Republic (Novák et al. 2012). These pegmatites possess strong enrichment in Li and B (as spodumene, lepidolite, or elbaite) and locally Cs (as londonite, CsBe4 Al4 [B11Be]O28 ), but they contain primary assemblages that include diopside, danburite (CaB2 Si 2O8 ), uvite (Ca–Mg tourmaline), and liddicoatite (Ca–Li tourmaline).

Alkalis Within a given large group of pegmatites (LCT or NYF), a general trend in the fractionation of alkalis begins with chemically primitive K-rich pegmatites closest to their source, followed by Na-rich pegmatites at the distal end of ˇ the most fractionated pegmatite types (Cerný 1991; London 2008). The possible influence of host-rock composition on the alkali ratios of pegmatite-forming magmas, however, has not been adequately considered. In the Middletown district, Connecticut, Stugard (1958) found that the Na/K ratio of feldspars in pegmatites correlates with the lithology of the hosts: pegmatites hosted by metamorphosed granodiorite are dominated by sodic feldspar, whereas pegmatites hosted by muscovite schists are primarily K-feldspar rich. Metasomatic exchange of alkalis between host lithologies E LEMENTS

CONCLUDING REMARKS The compositions of pegmatites reflect an association mostly with two granite types: the S- and the A-types. Pegmatites of the LCT family, especially those enriched in Li, Cs, B, and P, greatly predominate over all others. This indicates that the metamorphosed juvenile sediments from which S-type granites arise are particularly prone to yielding pegmatite-forming melts. Considering what makes S-type and A-type sources distinct from I-type sources, the difference comes down to their abundance of fluxing components, that is, ligands other than silica and alumina that profoundly influence the properties of pegmatite-forming melts. S-type sources are enriched in B and P, but also F, which is derived from the micas. A-types are enriched in F, which is contributed by the eventual melting of amphiboles and biotite. The archetypal I-type granites found in subduction zones, as the sources of arc volcanism and base-metal mineralization, are notably rich in Cl and are hydrous, but they are largely devoid of the fluxing components noted here (see London and Morgan 2012). They generate enormous volumes of quartz veins but lack pegmatites to any significant extent. This distinction points to an essential role for fluxing components like B, P, and F, along with H 2O, in the formation of pegmatites, as has been evident to most petrologists for over a century.

ACKNOWLEDGMENTS Many colleagues and students discussed with us the topics covered here in the field and laboratory, and Karen Ferreira contributed to pulling the diverse contributions together, including the reviewers’s comments.

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