Eur. J. Mineral. 2008, 20, 421–438 Published online August 2008 Paper presented at the symposium “Granitic Pagmatites: the State of the Art”, Porto, May 2007

Pegmatite genesis: state of the art

Wm. B. “Skip” SIMMONS* and Karen L. WEBBER

Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148, USA *Corresponding author, e-mail: [email protected]

Abstract: No one universally accepted model of pegmatite genesis has yet emerged that satisfactorily explains all the diverse features of granitic pegmatites. Genesis from residual melts derived from the crystallization of granitic plutons is favoured by most researchers. Incompatible components, fluxes, volatiles and rare elements, are enriched in the residual melts. The presence of fluxes and volatiles, which lower the crystallization temperature, decrease nucleation rates, melt polymerization and viscosity, and increase diffusion rates and solubility, are considered to be critical to the development of large crystals. A number of new concepts have shed light on problems related to pegmatite genesis. Cooling rates calculated from thermal cooling models demonstrate that shallow-level pegmatites cool radically more rapidly than previously believed. Rapid cooling rates for pegmatites represent a quantum shift from the widely held view that the large crystals found in pegmatites are the result of very slow rates of cooling and crystal growth. Experimental and field evidence both suggest that undercooling and disequilibrium crystallization dominate pegmatite crys- tallization. London’s constitutional zone refining model of pegmatite evolution involves disequilibrium crystallization from an undercooled, flux-bearing granitic melt. The melt is not necessarily flux–rich and the model does not require the presence of an aqueous vapor phase. Experimental studies of volatile- and flux-rich melts and fluid inclusion studies suggest that volatile-rich silicate melts may persist to temperatures well below 500 ◦C and even down to 350 ◦C. Studies of melt inclusions and fluid inclusions have led some researchers to suggest that the role of immiscible fluids must be considered in any model regarding pegmatite genesis. Fluid saturation is thought to occur early in the crystallization history of pegmatites. Two types of melt inclusions along with primary fluid inclusions have been found coexisting in pegmatite minerals. Advances by Petr Cernýˇ in pegmatite classification are in wide use and the fractionation trends of Nb, Ta and other HFSE and K, Rb, Cs, Li, Ga and Tl are now well understood. How pegmatitic melts are produced, the types of source rocks involved and how melt generation relates to plate tectonic models are challenging areas for future investigations. Also, the roles of regional zoning, anatexis, and chemical quenching in pegmatite genesis are areas for future pegmatite research. Key-words: pegmatite, pegmatite genesis, thermal modelling, chemical quench, classification, regional zoning.

Introduction Although no universally accepted model of pegmatite genesis has yet emerged that satisfactorily explains all the diverse features of granitic pegmatites, genesis from resid- Pegmatites are intrusive igneous rocks with very coarse-to ual melts derived from the crystallization of granitic plu- gigantic-sized textures. What typifies the world’s most fa- tons is favored by most researchers. Incompatible compo- mous granitic pegmatites is a combination of gigantic crys- nents, fluxes, volatiles and rare elements, concentrate in the tal size and extreme enrichment of rare elements. How- residual melts. The presence of fluxes and volatiles, which ever, pegmatites actually have the greatest range of grain lower the crystallization temperature, decrease nucleation sizes known in any rock type, from sub-millimeter to tens rates, melt polymerization and viscosity, and increase dif- of meters. Pegmatitic textures can develop in any intrusive fusion rates and solubility, are considered to be critical to igneous rock type from ultramafic, to granitic, to syenitic the development of large crystals and pegmatitic textures. in composition. Most commonly, the term is used to refer Most pegmatites appear to form from a single intrusive to granitic pegmatites and is generally understood to re- magmatic event, but undergo a complex internal crystal- fer to rock of overall granitic composition when it is used lization history that can result in a large range in grain without a qualifying adjective (e.g. gabbroic pegmatite). sizes; consistent changes in mineral chemistry from wall Granitic pegmatites are composed predominantly of quartz zone to cores; increasing concentrations of fluxes, volatiles and feldspars with accessory mica. and rare elements and, in rare instances, the formation of

0935-1221/08/0020-1833 $ 8.10 DOI: 10.1127/0935-1221/2008/0020-1833 c 2008 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

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Fig. 2. The relationship between percent crystallization and weight percent of water in the residual rest liquid (after Jahns & Burnham, 1969).

Table 1. Four main categories of Pegmatite classification (after Cerný,ˇ 1991).

Pegmatite Classes based mainly on depth of formation Abyssal -AB high grade, high to low pressure Muscovite -MSC high pressure, lower temperature Fig. 1. Schematic diagram showing the crystallization of granitic Muscovite REL -MSREL transitional between MSC & MSREL magma and withdrawn residual silicate liquid at increasing degrees Rare-Element -REL low temperature and pressure of consolidation of a melt initially containing 0.2 weight percent Miarolitic -MI shallow level water (after Jahns & Burnham, 1969).

large size of crystals in pegmatites. Although more recent pockets containing gem-quality crystals. Additionally, peg- experimental evidence by London (1992, 2005a) suggests matites appear to have crystallized from their margins to- that the presence of a hydrous vapor phase is not essential ward their interiors, based on a variety of factors including to the development of pegmatitic texture, the model pre- geothermometry studies and comb-structure texture with sented by Jahns & Burnham (1969) was a landmark contri- wedge-shaped crystals oriented perpendicular to pegmatite bution to the understanding of pegmatite genesis. margins. Significant advances in understanding pegmatite genesis have been made over the last 40 years. In the 1970’s and Newer developments 80’s the most widely accepted model of pegmatite gene- sis was that of Richard Jahns (Jahns, 1955, 1982; Jahns & Pegmatite classification Burnham, 1969) who promulgated the idea that pegmatites evolve from residual granitic melts comprised of coexist- Modern pegmatite classification schemes are strongly in- ing water vapor and silicate melt. Jahns & Burnham (1969) fluenced by the depth-zone classification of granitic rocks and Jahns & Tuttle (1963) cited experimental evidence that published by Buddington (1959), and the Ginsburg & pegmatites formed by equilibrium crystallization of coex- Rodionov (1960) and Ginsburg et al. (1979) classifica- isting granitic melt and hydrous fluid at or slightly below tion which categorized pegmatites according to their depth the hydrous granite liquidus. Jahns & Burnham (1969) also of emplacement and relationship to metamorphism and firmly established the concept that water acted as an incom- granitic plutons. Cerny’sˇ (1991) revision of that classifica- patible phase that increases in the residual melt as crystal- tion scheme (Table 1–3) is the most widely used classifi- lization progresses until a discrete water-rich vapor sepa- cation of pegmatites today. Cerný’sˇ (1991) pegmatite clas- rated from the silicate-rich melt (Fig. 1 and 2). In the Jahns sification, which is a combination of emplacement depth, model it is the interaction of the melt and vapor phases that metamorphic grade and minor element content, (Fig. 3) has gives rise to pegmatitic textures and the transition of granite provided significant insight into the origin of pegmatitic into pegmatite begins at the point of H2O-fluid saturation. melts and their relative degrees of fractionation. This clas- The presence of a discrete aqueous vapor phase is thus es- sification has been widely accepted and is in general use sential to pegmatite formation and was used to explain the today.

Downloaded from http://pubs.geoscienceworld.org/eurjmin/article-pdf/20/4/421/3127061/421_ejm20_5_421_438_simmons.pdf by guest on 25 September 2021 Pegmatite genesis: state of the art 423 ., et al ˇ Cerný ˇ Cerný, 1991). rict, Colorado ., 1973) Examples ., 1987); Western Keivy, , 1987); Cat Lake- stan, India (Shmakin, rn Baltic Shield (Kalita, et al et al ack Hills, South Dakota y, 1978); Aldan and Anabar et al. mples (after wknife field, NWT (Meintzer, , 1981) e and Hearne Provinces, Sask. 1975); Appalachian Province (Jahns Winnipeg River field, Manitoba ( South Platte dist Kola, USSR (Beus, 1960) (Simmons 1986); Korosten pluton, Ukraine Shields, Siberia (Bushev & Koplus, (Lazarenko et al. 1965) -conformable to cross- Yello oss-cutting veins (Trembla ) to cutting ons of conformable to mobilized Ra xterior cutting 1987); Bl (interior to marginal quasi THE FOUR CLASSES OF GRANITIC PEGMATITES bolite to) low- to none (segregati CCexterior C 1980); Easte ◦ ◦ ◦ ) marginal 1952); Rajah site-sillimanite) (Shearer 5–8 kbar and 1976) 2–4 kbar 650–500 650–580 1–2 kbar cutting dikes Sawtooth batholith, Idaho (Boggs, 4–9 kbar 700–800 ∼ ∼ ∼ ∼ ∼ ∼ ∼ Ta amphibolite facies (kyanite- (anatectic bodies > Elements Environment Granite Features Ta,Sn, Hf, B, P, F upper greeenschist facies to) e Ta, FTa, F cross-cutting exterior bodies Typical Minor Metamorphic Relation to Structural <> > > poor (to moderate)mineralization, micas and sillimanite mineralization, gemstock industrial minerals mineralization, ceramic poor mineralization, U, Th, Nb ceramic minerals Li, Rb, Cs, Be, Ga,Nb low-P, Abukuma amphibolite to Y, REE, Ti, U, Th, Zr,Nb variableminerals Be, Y, REE, Ti, U, Th, Zr, shallow to sub-volcanicgemstock interior to marginal interior to marginal interior pods and cross- interior pods, conformable to Llano Co., Texas (Landes, 1932); Pikes Peak, Colorado (Foord, 1982); REE, Momineralization high-P granulite facies anatectic leucosome) cr poor (to moderate) — U, Th, Zr, Nb, Ti, Y,— (upper amphi Li, Be, Y, REE, Ti, high-P, Barrovian none quasi-conformable to cross- White Sea region, USSR (Gorlov,

LCT poor to abundant (andalu NYF poor to abundant NYF Nb

Family

Class Abyssal Muscovite Element - Rare Miarolitic Table 2. The four classes of granitic pegmatites showing minor elements, metamorphic environment, relation to granite, structural features and exa

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Table 3. Relationships between Cerný’sˇ (1991) pegmatite classes, families, types and subtypes, showing the types and subtypes related to LCT and NYF families.

This classification has four main categories or classes. These are Abyssal (high grade, high to low pressure), Mus- covite (high pressure, lower temperature), Rare-Element (low temperature and pressure), and Miarolitic (shallow level). The Rare-Element Classes are subdivided based on composition into LCT and NYF families: LCT for Lithium, Cesium, and Tantalum enrichment and NYF for Niobium, Yttrium, and Fluorine enrichment (Table 3). The Rare-Element Class is further subdivided into types and subtypes according to the mineralogical or geochemical characteristics. Almost all recent pegmatite descriptions classify pegmatites according to LCT- and NYF–families, types and subtypes. The classification has provided insight into the origin of the melts and relative degrees of fraction- ation. Virtually all recent pegmatite descriptions classify pegmatites according to LCT-andNYF-types and sub- types. Another important contribution of the classification is the petrogenetic component of the classification, which shows the association of LCT pegmatites with mainly oro- Fig. 3. Pressure and temperature relationships between the four genic plutons, and NYF pegmatites with mainly anoro- major categories of pegmatite classification, MSC-Muscovite, AB- genic plutons (Table 4). Furthermore, this classification has Abyssal, RE- Rare-Element and MI- Miarolitic (after Cerný,ˇ 1991). proved to be quite successfully, especially in Canada, as an exploration tool for locating rare element pegmatites with economic potential. classifying granitic pegmatites of plutonic derivation (Ta- ble 5). Recently, Cernýˇ & Ercit (2005) published a new revision of the Cernýˇ (1991) classification. They propose a number of changes that address NYF pegmatite classification and a Geochemical fractionation petrogenetic classification of pegmatites derived from plu- tons. New categories of NYF and LCT subclasses are in- In the last two decades, studies mainly by Cernýˇ and col- troduced and the geochemical signatures are expanded. The leagues have documented the geochemical behavior of nu- modified petrogenetic classification is a “family system” of merous minor- and trace-element systems in pegmatitic

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Table 4. Petrogenetic affiliations (after Cerný,ˇ 1991), showing geochemical signature, orogenic affiliation, granite bulk composition, source lithologies and granite type (I, S, A).

Li, Cs, Ta Synorogenic to late Orogenic Orogenic LCT Peraluminous parent granites: undepleted Types upper to middle crust S and I types Nb > Ta,Y,F,REE,Ti,Sc,Zr,U,Th Mainly Anorogenic Anorogenic NYF Types Sub - to - Metaluminous, Peralkaline granites: depleted middle to lower crust A and I types Mixed Cross-bred LCT& NYF Metaluminous to Peraluminous Mixed protoliths or assimilation

Table 5. The family system of petrologic classification of granitic pegmatites of plutonic derivation. For the complete classification refer to Cernýˇ & Ercit (2005).

Family Pegmatite Geochemical Pegmatite Associated Granite bulk Source subclass signature bulk granites composition* lithologies composition LCT REL-Li Li, Rb, Cs, peraluminous synorogenic to peraluminous Undepleted upper- MI-Li Be, Sn, to late-orogenic S, I or mixed to middle-crust Ga, Ta>Nb, subaluminous (to anorogenic) S+I types basement gneisses (B, P, F) largely basement gneisses heterogeneous NYF REL - Nb>Ta, Ti Y, subaluminous syn-, late, post- peraluminous depleted middle to REE MI- Sc, REE, Zr, to to mainly to lower crustal REE U, Th, F metaluminous anorogenic subaluminous granulites, or (to quasi- and juvenile granitoids subaluminous homogeneous metaluminous A and I types Mixed Cross- mixed metaluminous postorogenic to subaluminous mixed protoliths or bred: LCT to moderately anorogenic; to slightly assimilation of & NYF peraluminous heterogeneous peraluminous supracrustals by NYF granites

*Peraluminous, A/CNK > 1; subaluminous, A/CNK ∼ 1; metaluminous, A/CNK < 1atA/NK > 1; subalkaline, A/NK ∼ 1; peralkaline, A/NK < 1, where A = Al2O3,CNK= CaO + Na2O + K2O, and NK = Na2O + K2O (all in molecular values; Cernýˇ 1991).

melts. The fractionation patterns of Nb, Ta and other HFS crease with fractionation in feldspars, micas and other min- elements in numerous pegmatite systems have been well erals. The trends are apparent in minerals between groups documented, especially in LCT pegmatites (Cerný,ˇ 1989a, of pegmatites and within individual pegmatites from mar- 2005; Breaks et al., 2005). The fractionation patterns of K, gins toward cores, with the most fractionated portions of Rb, Cs, Li, Ga and Tl in feldspars, micas, beryl and other pegmatites containing the greatest enrichments. In complex minerals have also been described by Cernýˇ (2005). The pegmatites with replacement units, enrichments generally enrichment trends of alkali elements in feldspars and mi- reach their highest levels in minerals within the replace- cas have proven valuable in understanding pegmatite petro- ment units. Linnen (1998, 2005) has shown experimen- genesis and the internal fractionation of pegmatites. These tally that manganotantalite is more soluble in granitic melts relationships and trends have also proven to be especially than manganocolumbite and that the addition of Li and F helpful in geochemical exploration for pegmatites (Breaks enhances the solubility of manganotantalite in the melts, et al., 2005; Trueman & Cerný,ˇ 1982). Examples of some which helps explain the tendency of Ta-rich phases to oc- cur in the most highly fractionated assemblages (Linnen & of the more significant trends documented by Cernýˇ for the Cuney, 2005). columbite group minerals, feldspars, beryl and micas are shown in Fig. 4–8. In general, in columbite group miner- / / als Mn Fe and Ta Nb increase with fractionation, but pat- Exomorphic reactions terns are strongly related to the composition of the peg- matite and fluorine also plays an important role in the shape Exomorphic haloes of Be (emerald), B (tourmaline) and Li of the trends for Nb and Ta (Fig. 4 and 5). Rb and Cs in- (holmquistite) are well established examples of wallrock

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alteration associated with pegmatites in a number of loca- the few nuclei that do survive and begin to grow. Thus, tions (Simmons, 2007a). In addition, LCT pegmatites have network modifiers act to prevent nuclei formation and at been shown to have exomorphic haloes of alkali enrich- the same time increase the effectiveness of diffusion. These ment in the surrounding wallrocks. Li, Rb, and Cs enrich- two effects combine to facilitate ion migration over greater ments have been shown to be associated with rare element distances and promote the growth of the few nuclei that do pegmatites and soil geochemistry has been successfully manage to form, resulting in fewer, much larger crystals. used to locate buried rare element pegmatites (Galeschuk & Vanstone, 2005). Fluid and melt inclusions

Temperatures of formation Melt inclusions (MI), small blebs of melt that are trapped in crystallizing minerals at magmatic temperatures and pres- The proposed temperatures of the final stages of pegmatite sures that become essentially solid at surface temperatures, crystallization have declined over the last few decades. The may record parental melt compositions at the time of host Jahns model suggested that pegmatites crystallize at or near ◦ mineral growth (Roedder, 1979, 1984; Lowenstern, 1995) the hydrous minimum melt temperatures of about 600 C and they may monitor the chemical changes of the residual (Jahns & Burnham, 1969). Based on stable isotopic studies, liquid during magma evolution (Pettke, 2006). Fluid inclu- Taylor et al. (1979) proposed temperatures of emplacement sions (FI) entrap fluid that remains in large part fluid at of pegmatite melts at about 700 ◦C to final temperatures ◦ surface temperatures (Roedder, 1984). Recent studies per- of 525 C in the pockets in San Diego County, California taining to FI and MI are extensive (e.g. Lowenstern, 1995, pegmatites. 2003; Sobolev, 1996; Peretyazhko et al., 2000; Hauri et al., Two-feldspar thermometry using reintegrated perthite 2002; Bodnar & Student, 2006). Many additional refer- compositions from the intermediate zones of pegmatites ences pertaining to igneous systems can be found in Roed- in the South Platte, Colorado NYF district gave tempera- ◦ der (1984, 2003), Samson et al. (2003) and Kontak et al. tures of 550–500 C (Simmons et al., 1987). Two-feldspar (2004). Challenges involved in interpreting MI from fel- thermometry of feldspars showing no evidence of exsolu- sic plutons are discussed in detail by Webster & Thomas tion from the Little Three pegmatite, California (Morgan ◦ (2006). Thomas et al. (2003, 2006a) looks specifically & London, 1999) gave temperatures of ∼ 400–435 C near at MI in felsic aplite and pegmatite systems. Sirbescu & the margins to 350–390 ◦C near the pegmatite pocket zone ◦ Nabelek (2003 a and b) focus on FI in pegmatites. and a sharp decrease to 240–275 C in the pockets where The study of FI and their characterization as either pri- K-feldspar is perthitic (Fig. 9). Nabelek et al. (1992a,b) ◦ mary or secondary is not always straightforward and can reported equilibration temperatures of about 350 Cfor lead to very different interpretations of the same pegmatite. coexisting quartz and K-feldspar in the cores of several For example, London (1985, 1986) describes FI from petal- Black Hills, South Dakota, pegmatites. Sirbescu & Nabelek ite, eucryptite, spodumene, and coexisting quartz from the (2003a,b) suggest that the Li-bearing Tin Mountain peg- Tanco Pegmatite, Bernic Lake, Manitoba. Spodumene oc- matite, Black Hills, South Dakota, crystallized from fluid- ◦ curs both as fine-grained intergrowths with quartz result- rich, compositionally complex melts at ∼ 400–350 C ing from the breakdown of petalite (SQUI) and as coarse- with the low crystallization temperatures resulting from grained crystals embedded in massive quartz. Both types ff the combined fluxing e ects of Li, B, P, H2O and carbon- of spodumene and quartz trapped an abundance of FI that ate anions. Their determinations are based on microther- were uniform within each host mineral phase but very mometric data on primary fluid inclusions cogenetic with different between them. FI hosted by spodumene typi- crystallized-melt inclusions. cally contained a low-salinity aqueous liquid plus vapor (at 25 ◦C) and a consistent assemblage of daughter min- erals including albite, cookeite, pollucite-analcime solid Volatiles and fluxes in pegmatite melts solution, quartz, an unidentified carbonate and lithium tetraborate, Li2B4O7, described as the new mineral species London’s experimental work has shown that water satu- diomignite by London et al. (1987). The minimum pos- ration, as proposed by Jahns & Burnham (1969) is nei- sible entrapment temperature determined from this min- ther necessary nor likely in the early crystallization of peg- eral assemblage was 470 ◦C (London, 1985). The identity matites (London, 1992, 2005a and references therein). His of diomignite, described as an abundant and widely dis- experiments have shown that other fluxes such as B, F, P tributed highly birefringent phase in crystal-rich FI in spo- and Li in addition to H2O play a critical role in the forma- dumene, was established by several analytical techniques tion of rare element pegmatites. (no definitive chemical analysis was performed) and con- The presence of these fluxes alter the melt by appreciably firmed by identical Gandolfi X-ray diffraction patterns for lowering the crystallization temperature, decreasing nucle- diomignite and synthetic Li2B4O7 (London et al., 1987). ation rates, decreasing melt polymerization, decreasing vis- The FI in quartz are described as being quite distinct from cosity, increasing diffusion rates, and increasing solubility those in spodumene in that the inclusions contained only (London, 1992, 2005a; Simmons et al., 2003). The fluxes liquid and vapor phases at 25 ◦C, had few crystalline act as network modifiers that prevent or hinder the forma- solids, were comparatively CO2-rich and the aqueous com- tion of nuclei and increase the diffusion rates of ions to ponent was comparatively saline (London, 1985). Accord-

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Fig. 4. Fractionation trends of Ta/(Ta+Nb) vs. Mn/(Mn+Fe) of columbite group minerals in pegmatites: A-Greer Lake field, Canada; B- Black Hills, South Dakota and Himalaya pegmatite, California; C-Yellowknife field, NWT, Canada; Altai field, China; Cross Lake field, Manitoba (after Cernýˇ et al., 1986).

Fig. 5. Tantalum and Manganese enrichment trends in fluorine-rich Fig. 7. Na/Li vs. Cs weight percent for late beryl from granite peg- and fluorine-poor pegmatites (after Cerný,ˇ 1989b). matites: A- barren and beryl-type pegmatites; B- beryl-columbite and beryl-columbite-phosphate pegmatites; C- albite-spodumene and complex pegmatites; D- Li, Cs, Ta-rich complex pegmatites- spodumene, petalite, amblygonite and lepidolite subtypes (modified from Cerný,ˇ 2002).

tion. The interpretation of the inclusion data by London (1985, 1986) is that the FI in quartz are secondary, whereas the FI in spodumene are representative of the primary liq- uid, a dense, hydrous, borosilicate fluid, which played an important role in the internal evolution of the Tanco peg- matite and the concentration and development of rare-metal ore units. The included solids in spodumene-hosted FI are interpreted as daughter minerals and the daughter miner- als plus the aqueous fluid together represent the products Fig. 6. K/Rb vs. Cs weight percent of blocky K-feldspar in peg- of a single, homogeneous phase (hydrous borosilicate liq- matites of the Winnipeg River district, southeastern Manitoba, uid). Spodumene-hosted FI data was combined with the ex- Canada (after Cerný,ˇ 1989b). perimentally calibrated phase relationships in the lithium aluminosilicate system (London, 1984) to define emplace- ment conditions, cooling histories and fluid evolution in the ing to London (1985) the two very different types of liquids Tanco pegmatite. trapped by spodumene (a dense, hydrous, alkali borosil- Anderson et al. (2001) examined hundreds of crystal- icate fluid) and quartz (an aqueous fluid with low solute rich FI in spodumene from three pegmatite localities in- concentrations) of the intergrowths could not have coex- cluding Bikita and Kamativi, Zimbabwe and Tanco, Bernic isted coincident with the breakdown of petalite and thus Lake, Manitoba by optical examination and laser Ra- were generated at different times during pegmatite evolu- man spectroscopy. They concluded that 96 % of the high

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to be secondary in origin and were trapped within healed fractures and cleavage planes. The solid phases, domi- nantly quartz and zabuyelite, represent reaction products between a host spodumene crystal and trapped low-density, aqueous carbonic fluid during the late stages of pegmatite evolution, and not an equilibrium assemblage that crys- tallized from a hydrous borosilicate melt as proposed by London (1985, 1986). Anderson et al. (2001) suggest the apparent absence of diomignite (even in the type speci- men) and the failure to detect significant concentrations of boron in the inclusions have important implications regard- ing models for the internal evolution of rare-element peg- matites. Additionally, P-T estimates of entrapment for spo- dumene FI (475 ◦C and 2.75 kbar) at Tanco are no longer Fig. 8. K/Rb vs. Cs wt. % in micas across Tanco pegmatite. Dashed valid. boundaries are approximately between muscovite (ms), lithian mus- Fluid inclusions with daughter crystals of orthoboric acid, covite (Li-ms) and lepidolite (lpd) zones (after Cerný,ˇ 2005). sassolite (H3BO3) were found and described for the first time by Smirnov et al. (1999). They suggest that dif- ficulty in identifying sassolite, which must include Ra- man spectroscopic work, prevented previous identification. Peretyazhko et al. (2000) and Smirnov et al. (2005) report that boric acid solutions with daughter sassolite crystals were found in topaz, tourmaline, quartz, beryl, danburite and adularia from 31 miarolitic pegmatites from central and eastern Transbaikalia, the middle Urals, central and southwestern Pamirs, Afghanistan, Pakistan, Nepal, Cali- fornia, Elba and Madagascar. The maximum H3BO3 con- tent found in FI in pegmatites was 27 wt.%. However, boric acid solutions occurred in a wide range of physiochemical conditions and Peretyazhko et al. (2000) suggest that in- clusions of boric acid solutions will be found worldwide in all tourmaline-bearing and some topaz-beryl miarolitic pegmatites. Thomas (2002) found sassolite as a daughter mineral phase in MI in pegmatites of the Ehrenfriedersdorf complex, Germany. DuetotheverysmallsizeofMI,anin-situ analyti- cal technique is necessary to obtain compositional data. The study of MI has advanced tremendously with the ad- vent of sophisticated and accurate analytical techniques en- Fig. 9. Two-feldspar temperatures for the Little Three pegmatite- abling the study of volatile-rich phases in igneous rocks aplite dike, California with respect to height above the base of the (see Thomas et al., 2005 for a discussion of MI analyti- footwall aplite (after Morgan & London, 1999). cal techniques). Thomas (2000, 2002) and Thomas et al. (2006a) utilized confocal laser Raman microprobe spec- troscopy, which made possible the study of very small MI birefringence grains in over 500 FI, including FI in a from a wide variety of pegmatite minerals including quartz, wafer of spodumene from the Tanco pegmatite reported feldspar, topaz, tourmaline, beryl, garnet, and micas. Their to contain the type specimen of diomignite, are in fact work has resulted in the determination of H2O concentra- zabuyelite (Li2CO3). No phase yielding the Raman spec- tions of 0.5 to over 35 wt.% in glassy MI as small as 3 µm trum of Li2B4O7 (diomignite) was observed. with an accuracy of ± 0.2 wt.% H2O. An estimate of the bulk composition of the fluid en- Quartz crystals from the F-, B- and P-rich pegmatites trapped in spodumene at Tanco was obtained by averag- of the Ehrenfriedersdorf tin-tungsten deposit, Erzgebirge, ing the composition of hundreds of coeval fluid inclusions, Germany, continuously trapped two different types of MI additionally, the composition of individual fluid inclusions during cooling and growth, a H2O-poor, P2O5-, F- and was determined using a variety of analytical techniques in- silicate-rich melt (Type A) and a H2O- and B-rich, silicate- cluding laser Raman spectroscopy, synchrotron X-ray flu- poor melt (Type B) (Thomas et al., 2000). The two types orescence and laser ablation-inductively coupled plasma- of MI are interpreted as coexisting melts that were trapped mass spectrometry (LA-ICP-MS). Anderson et al. (2001) simultaneously on both sides of a two-melt solvus. Wa- concluded that FI in both spodumene and quartz from inter- ter concentration in the type A inclusions increases from growths (SQUI) at Tanco trapped a low-salinity, alkali-rich, 2.4 to 16.2 wt.% with increasing homogenization tempera- aqueous carbonic fluid. The FI in spodumene are believed tures from 500 to 700 ◦C whereas water concentration in

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the type B inclusions decreases from 43.8 to 27.8 wt.% uniformly from dike margin to center (Cashman, 1990). over the same temperature range. Complete miscibility is In contrast, most sheet-like pegmatite-aplite dikes display ◦ attained at the critical point of 712 C and 21.5 wt.% H2O changes in crystal size from < 0.1 mm in aplites, to > 10 cm at 1 kbar. Thomas et al. (2000) conclude that at the criti- (or indeed meter size) for crystals in the hanging wall, core cal point conditions detailed above, a P-, B-, and F-bearing zone, and pockets. In addition, the grain size does not al- silicate melt may dissolve unlimited quantities of H2Oand ways increase consistently from the margins to the core. that upon cooling, the silicate melt will separate into two Pegmatite-aplite dikes typically have a fine-grained foot- melts each of which has very different physical properties wall, coarse-grained hanging wall, and a core zone with that will readily separate from one another and fractionate miarolitic cavities. Footwall aplites can be layered or non- incompatible elements differently. layered, layered aplites can alternate with pegmatite, and Veksler & Thomas (2002) conducted experiments with aplites can occur in an irregular distribution throughout the a synthetic peraluminous pegmatite (H2O-saturated, 0.1– footwall. Clearly, changes in grain size of ∼ 5ormoreor- 0.2 GPa) spiked with 5 wt.% each of P2O5,B2O3 and F, ders of magnitude, and an irregular distribution of grain 1 wt.% each Rb2OandCs2O and 0.5 wt.% Li2O, to simu- size with respect to the dike margins and centers, indi- late the most enriched compositions found in natural MI cate that crystallization parameters such as nucleation and in pegmatite quartz (Thomas et al., 2000). Aluminosili- growth rates are not consistent during the crystallization cate melt, low-density hydrous fluid, and hydrosaline melt history of many pegmatites. strongly enriched in Na3AlF6 and H3BO3 components, Another striking feature of pegmatites is the textural het- were observed coexisting in all experimental charges. erogeneity they display with respect to crystal morpholo- Thomas et al. (2006a) and Veksler & Thomas (2002) sug- gies. The experimental studies of Swanson & Fenn (1986, gest that evidence for the stable coexistence of three im- 1992), MacLellan & Trembath (1991) and Fenn (1977, miscible phases – aluminosilicate melt (consisting of SiO2, 1986) on quartz and feldspar crystallization in granitic Al2O3,FeO,K2O, Na2O, Li2O and other, less abundant melts demonstrated that skeletal and graphic morpholo- oxides), hydrosaline melt (borates, phosphates, fluorides gies reflect rapid crystal growth from a highly under- and chlorides of alkaline and alkaline earth elements), and cooled melt. The experimental work of Lofgren (1974, lower salinity aqueous fluid (volatiles, primarily H2Oand 1980) demonstrated that crystal morphology varied from CO2) both experimentally and in MI and FI from pegmatite euhedral to skeletal to radial with increasing degree of minerals should be considered an important factor con- liquidus undercooling. London (1992) found that mineral tributing to the internal differentiation of pegmatite bodies. zoning patterns, sharp changes in grain size, mineral tex- tures, and oriented fabrics that typify pegmatites could all be replicated experimentally from undercooling vapor- Rapid cooling rates and disequilibrium crystallization undersaturated Macusani glass and that at progressively larger liquidus undercooling of a hydrous silicate melt, Pegmatites by definition have crystals that average 2 cm crystal orientations changed from random to increasingly or more in diameter. In actuality, many pegmatites display anisotropic, oriented (comb structure) fabrics. an enormous range in crystal size from mm-size crystals Thus the textural relationships of minerals in pegmatites up to large crystals that are 10’s of meters in length. It reflect the degree of pegmatite undercooling, nucleation has often been assumed (particularly in introductory level rate and growth rate. Strong undercooling is required to ex- geology textbooks) that crystal size in igneous rocks is plain the textural characteristic of many pegmatites, includ- a direct indicator of crystal growth rates and magmatic ing skeletal and dendritic crystal morphologies, elongated, cooling history, i.e., small crystals grow quickly from a and sometimes wedge-shaped crystals, and the develop- rapidly cooling magma producing fine-grained (aphanitic) ment of comb structure along the contacts between peg- textures, whereas large crystals grow more slowly from a matite and country rock. More elongate crystal forms (nee- slowly cooling magma, producing coarse-grained (phaner- dle like, skeletal, branching, wedge-shaped) are favored itic) textures. Clearly this paradigm does not hold in many by rapid rates of cooling, larger degrees of undercooling, pegmatites, particularly when looking at genetically re- higher growth rates and fewer nucleation sites, whereas lated coarse-grained pegmatites and fine-grained aplites in tabular to equant forms are favored by slower cooling rates, pegmatite-aplite dikes, without invoking great variability smaller degrees of undercooling, lower growth rates, and in the cooling history within an individual pegmatite body abundant nucleation sites. (i.e., rapid followed by slow cooling and crystallization). Yet even with textural evidence that seems to clearly in- Pegmatites display striking variability in terms of size dicate rapid crystal growth rates, it wasn’t until the cooling of the pegmatite itself, from thin sheet-like dikes with histories were quantitatively evaluated for the Harding peg- diameters in terms of < 1 m to 20 m or so, to ellip- matite, New Mexico (Chakoumakos & Lumpkin, 1990); soidal or teardrop shaped pegmatites, many of which show the George Ashley, Mission, Stewart and Himalaya dikes, pronounced large-scale zonation and often display well- San Diego County, California (Webber et al., 1997, 1999); developed quartz cores. The country rock into which peg- and the Little Three dike, Ramona, California (Morgan & matites are emplaced also varies widely from brittle, rela- London, 1999) that it became clear that these pegmatites tively cold, country rock to hotter migmatitic terranes. In cooled rapidly (in days to months, not in thousands to most dikes or sills, variations in grain size are small (2 to millions of years as previously believed) thus constrain- 3 orders of magnitude), and grain size generally increases ing crystal growth rates to the cooling history of the peg-

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Fig. 11. Cooling curve for the George Ashley dike calculated with latent heat of crystallization, emplacement temperature of 650 ◦C into 150 ◦C country rock. Half-width of the dike is plotted with the Fig. 10. Layering in the George Ashley aplite (“line rock”). The dark dike center at 0. The position of the contact between the country layers are enriched in garnet, and the light layers in albite and quartz. rock and the dike is shown. The interval where layered aplite (line K-spar megacrysts are in the center of some of the nested loops. rock) occurs within the dike is between the 2 heavy lines (Webber et al., 1997).

matites. Based in part on textural relationships, it is now thought that many shallow-level pegmatites cooled ex- − tremely rapidly. been on the order of 10 5 cm/s (Fig. 12). The cooling times presented in Table 6 indicate that the dikes cooled and crystallized rapidly, with variable nucleation rates but Thermal model high overall crystal-growth rates. Initial high nucleation rates coincident with emplacement and strong undercool- Many pegmatites can be likened to sheet-like structures. ing can account for the millimeter-size aplite grains. Lower As such, the cooling history of these dikes can be mod- nucleation rates coupled with high growth rates can explain eled with well established conductive cooling models for the decimeter-size minerals in the hanging walls, cores, and thin sheets. In considering cooling models, a number of pa- miarolitic cavities of the pegmatites. The presence of tour- rameters must be evaluated, including the width of the dike maline and/or lepidolite throughout these dikes suggests (which can easily be measured), the emplacement temper- that high melt concentrations of incompatible (or fluxing) ature of the pegmatitic magma (which can be determined components such as B, F, and Li (± H2O), aided in the de- from phase equilibria) and the temperature of the coun- velopment of large pegmatitic crystals that grew rapidly try rock (which can be constrained using estimated depths in the short times suggested by the conductive cooling of emplacement and reasonable geothermal gradients for models. the study area, observations of any reactions or lack of re- In order to evaluate the effect on cooling times with actions between the country rock and magma, etc.). Peg- variations in country rock temperatures and pegmatite matites of the Pala and Mesa Grande Pegmatite Districts, emplacement temperatures, Webber et al. (2005) modified San Diego County (SDC), California are typically thin, the initial cooling parameters used for the SDC pegmatites sheet-like, composite pegmatite-aplite dikes. Aplitic por- including the Himalaya, George Ashley and Stewart, and tions of many SDC dikes display pronounced mineralog- added the Animikie Red Ace (ARA) pegmatite, Wiscon- ical layering referred to as “line rock,” characterized by sin – a thin dike emplaced into hotter country rock (Fal- fine-grained, garnet-rich bands alternating with albite- and ster et al., 1997, 2005). The results presented in Table 6 quartz-rich bands (Fig. 10). Thermal modeling was con- illustrate that even though modifying the initial parameters ducted on four dikes in SDC including the 1 m thick Hi- increases cooling times, the results are still rapid. malaya dike, the 2 m thick Mission dike, the 8 m thick George Ashley dike, and the 25 m thick Stewart dike. Cal- Even the very large, 100 m thick Tanco pegmatite appears culations were based on conductive cooling equations ac- to have cooled quite rapidly. Cernýˇ (2005) recently sug- counting for latent heat of crystallization, a melt emplace- gested that the Tanco pegmatite very likely solidified quite ment temperature of 650 ◦C into 150 ◦C fractured, gabbroic rapidly, “in decades to a maximum of a few hundreds of country rock at a depth of 5 km, and an estimated 3 wt.% years”. In order to quantitatively evaluate this, Webber & ◦ initial H2O content in the melt. Cooling time to < 550 C Simmons (2007), using the cooling model outlined above at the center of each dike was determined (Webber et al., and the parameters reported by Cernýˇ (2005) (thickness of 1997, 1999, results shown in Table 6 and Fig. 11). 100 m; emplacement temperature of 700 ◦C; emplacement Based on these calculations, growth rates for large peg- depth of 10 km; and country rock temperatures of 25 ◦C matitic minerals such as the 10 cm long Himalaya hang- to 300 ◦C calculated that the Tanco pegmatite cooled to ing wall, comb-structure tourmaline crystals may have ∼ 450 ◦Cin∼ 700–1000 years.

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Table 6. Calculated conductive cooling times for three pegmatites in San Diego County, California and the Animikie Red Ace pegmatite, Wisconsin. Pegmatite emplacement temperature and country rock temperature was varied in the conductive cooling equations to observe the effect on cooling times.

Pegmatite Width Emplacement Country Rock Cools To Time Temp ◦CTemp◦CTemp◦C Himalaya, SDC, CA 1 m 650 ◦C 150 ◦C 550 ◦C 5 days Himalaya, SDC, CA 1 m 650 ◦C 150 ◦C 400 ◦C20days Himalaya, SDC, CA 1 m 600 ◦C 300 ◦C 400 ◦C50days George Ashley, SDC, CA 8 m 650 ◦C 150 ◦C 550 ◦C 340 days George Ashley, SDC, CA 8 m 650 ◦C 150 ◦C 400 ◦C3yrs George Ashley, SDC, CA 8 m 600 ◦C 300 ◦C 400 ◦C10yrs Stewart, SDC, CA 25 m 650 ◦C 150 ◦C 550 ◦C9yrs Stewart, SDC, CA 25 m 650 ◦C 150 ◦C 400 ◦C30yrs Stewart, SDC, CA 25 m 600 ◦C 300 ◦C 400 ◦C75yrs Animikie Red Ace, WI 1 m 600 ◦C 400 ◦C 500 ◦C20days Animikie Red Ace, WI 1 m 600 ◦C 400 ◦C 450 ◦C50days

Fig. 12. Photograph of the upper Himalaya dike exposed in the Himalaya Mine. Large wedge-shaped schorl crystals, some over 10 cm in length, can be seen growing out in a comb structure from the pegmatite hanging wall-country rock contact. The dike has a width of ∼ 1m.

Chemical quenching slowly than shallow-level pegmatites. The mechanism for rapid crystallization of deeper seated pegmatitic melts, Metastable undercooled melts can also be produced by the which cool more slowly, may be a chemical quench. This removal of a fluxing component by a crystallizing phase, process involves removal of a fluxing component, such as B instead of by rapid thermal cooling. The remaining melt or F, from a melt by a crystallizing phase which then initi- is then undercooled since it is no longer fluxed by the re- ates rapid crystallization, since the melt is no longer fluxed moved component. This process is referred to as a chemical by that component. For example, crystallization of tourma- quench. line or fluorine-bearing phases can result in melt that is sud- Pegmatites hosted in deeper seated metamorphic terranes denly undercooled even though the surrounding rocks and also occasionally show comb structures and wedge-shaped the melt itself remain at about the same temperature. crystals which are indicative of rapid crystallization, al- The Ipé pegmatite in Minas Gerais, Brazil, (Fig. 13) though it is clear that, thermally, they must cool more which has a well-developed comb structure of extremely

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Fig. 13. The large Ipé pegmatite, Minas Gerais, Brazil shows a spectacular tourmaline comb structure. Some of the wedge-shaped schorl crystals exceed 2 m in length. Large and rapid undercooling initiates heterogeneous nucleation along the hanging wall contact. This produces radially flaring crystals, with their fast-growth axes pointed toward the melt that are able to sustain growth as the crystal- lization front advances. The result is highly oriented comb-structure Fig. 14. Backscattered electron image of experiment PEG-25, a tourmaline. metaluminous haplogranite (200 MPa H2O minimum composition) +3wt.%B2O3. A boundary layer of melt (quenched to glass) is at crystal growth front boundary. Crystal growth direction shown by skeletal and elongated schorl crystals oriented perpendic- arrow. Composition of bulk melt (point 2) and the B2O3 and H2O ular to the pegmatite-country-rock contact, is an exam- enriched boundary layer (point 1) is respectively: SiO2, 68.94 and ple of a large deeper seated pegmatite that is inferred to 50.88; Al2O3, 12.01 and 7.79; Na2O, 5.10 and 6.66; K2O3.37and have crystallized rapidly as a result of chemical quenching 2.90; B2O3 2.93 and 18.30 and H2O 7.61 and 13.42 wt.% (after Lon- (Simmons et al., 2003). don, 2005a).

Origin of line rock Constitutional zone refining In the past, numerous theories have been advanced to ex- plain the origin of line rock, fine-grained layered aplite, Rapid crystallization from a strongly undercooled melt pro- in composite pegmatites-aplite dikes. These dikes are typ- duces a boundary layer of excluded phases ahead of the ically thin, low-angle to subhorizontal, sheet-like dikes crystallizing front. A constitutional zone refining model with aplitic portions mainly in the foot wall that dis- of pegmatite formation proposed by Morgan & London play pronounced mineralogical layering characterized by (1999) and London (2005a,b) involves disequilibrium crys- fine-grained, garnet- or tourmaline-rich bands alternating tallization from a flux-bearing granitic melt that is under- ◦ with albite- and quartz-rich bands. The contrast in crystal cooled by about 100–300 C. The melt is not necessarily size between the coarse-grained pegmatite and fine-grained flux rich and the presence or absence of an aqueous vapor aplite has been the subject of numerous investigations. phase is not required (London, 2005a). Jahns & Tuttle (1963) suggested that aplite layering could A lag time between cooling and the initiation of crys- be explained by the periodic loss of water vapor, a viable tallization produces a supersaturated melt. When nucle- mechanism for quenching by relief of pressure (shifts of the ation and crystallization commence, excluded fluxes and alkali feldspar-quartz field boundary with changes of va- volatiles accumulate in a boundary layer ahead of the crys- por pressure related to epizonal degassing). London (1992) tallization front (London, 2005). The solidus of the bound- proposed that significant undercooling could also produce ary layer is lowered by the fluxes and, as crystallization layering and he was able to experimentally produce rhyth- continues, this boundary layer liquid becomes progres- mic layering of quartz and alkali feldspar in experiments sively enriched in fluxes, water and other incompatible el- with no fluctuations in pressure (London, 2005a). Kleck ements relative to the bulk melt composition (Fig. 14). (1996) proposed gravitational crystal settling as an ex- Boundary layers advancing from the wall zones inward planation for the George Ashley line rock. Webber et al. may merge, especially in thin dikes. In the final stages of (1997, 1999) proposed a mechanism of diffusion controlled crystallization an aqueous vapor phase may evolve, giv- oscillatory nucleation and crystallization from a strongly ing rise to the formation of miarolitic cavities and evolved undercooled melt for the formation of line rock in San suites of pegmatitic minerals (London, 2005a). Diego Co., CA pegmatite-aplite dikes. They suggested that

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the initiation of line rock formation could be caused by tons formed in continental or oceanic rift zones. His classi- a number of “triggers”, including a thermal, chemical or fication has three main categories based on aluminum satu- pressure quench. Morgan & London (1999) proposed con- ration of the parent granite, peralkaline, metaluminous, and stitutional zone refining for the Little Three pegmatite, Cal- peraluminous. Within each group, pegmatite types are dis- ifornia. Frindt & Haapala (2004) proposed that diffusion- tinguished by mineralogical and geochemical characteris- controlled oscillatory nucleation and rapid growth from tics. This classification is comprised of 6 types and 9 sub- undercooled melt, coupled with changes in vapor pressure types. His classification relates NYF pegmatite mineralogy could explain the layered aplites and undercooling textures to variations in the alkalinity of the parental granitic melts. from the Gross Spitzkoppe stock, Namibia. Cernyˇ & Ercit (2005) introduced a revised petrogenetic We believe that aplite line rock forms by oscillatory nu- classification in which three families are distinguished: “an cleation and crystallization that can be initiated by a high NYF family with progressive accumulation of Nb, Y and degrees of undercooling alone or by an external forcing fac- F (besides Be, REE, Sc, Ti, Zr, Th and U), fractionated tor such as pressure reduction produced by dike dilatancy from subaluminous to metaluminous A- and I-granites that (fracture propogation). Any event that results in strong un- are generated by a variety of processes involving depleted dercooling has the potential to initiate line-rock formation. crust and/or mantle contribution; a peraluminous LCT fam- Emplacement of melt into relatively cool country rocks ily marked by prominent accumulation of Li, Cs and Ta (thermal quench), or a “chemical quench” resulting for ex- (besides Rb, Be, Sn, B, P and F) derived mainly from S- ample from crystallization of tourmaline that effectively granites, less commonly from I-granites; and a mixed NYF removes boron from the melt (Rockhold et al., 1987), or + LCT family of diverse origins (e.g., NYF plutons con- dike rupture or dike dilatancy (pressure quench), can all in- taminated by digestion of undepleted supracrustals)”. crease the degree of melt undercooling and act as a trigger Ercit’s (2005) report on REE-enriched granitic peg- to destabilize the crystallization dynamics of the pegmatite matites elaborates on the Cernyˇ & Ercit (2005) classifi- system. Such events can initiate rapid heterogeneous nucle- cation. He reports that Cernýˇ rejects the use of the term ation and oscillatory crystal growth, the development of a “LCT” and “NYF” in a purely geochemical sense for in- layer of excluded components in front of the crystallization dividual pegmatites as opposed to granite-pegmatite suites front, and the formation of line rock. (families). LCT, NYF and Mixed are families of granitic pegmatite suites. In examining more than 500 pegmatite descriptions, Ercit Areas for future research (2005) found a low degree of correlation between acces- sory mineralogy and depth of emplacement for NYF peg- Classification and origin of pegmatitic melts matites. He proposed that NYF pegmatites belong to the Abyssal, Muscovite-Rare-Element class, as well as the A number of new modification to Cerný’sˇ (1991) classi- Rare-Element and Miarolitic classes. He also subdivided fication, which was based on a combination of depth of the abyssal class into two subdivisions: the LREE-enriched emplacement, metamorphic grade and minor element con- and the HREE-enriched types. The Rare Element Class tent have been proposed over the last few years (Zagorsky, consists of the Rare Earth type and the Allanite-Monazite, et al., 1999; Wise, 1999; Pezzotta, 2001; see Simmons, Euxenite and Gadolinite subtypes. This paper discusses 2005 for an overview). Many pegmatites fall nicely into numerous details and problems of the systems of REE- Cerný’sˇ categories, but during the last decade various in- enriched pegmatites. Clearly much future research will be vestigations have revealed pegmatites that don’t fit well devoted to REE-enriched pegmatites. into these categories. Most notably pegmatites of the NYF Martin & De Vito (2005) contend that the depth zone affiliation have required a more detailed classification as classification cannot account for the two main geochemi- more studies revealed a greater diversity of NYF peg- cal categories of pegmatites: LCT and NYF. They propose matites. One problem is the classification of some peg- that the tectonic setting determines the nature of the par- matites as NYF that contain little or no yttrium or others ent magma and the derivative rare-element enriched mag- that contain little or no niobium. Many pegmatites from mas. Thus, LCT pegmatites are generated in compressional Madagascar described by Pezzotta (2001) that are hyper- tectonic settings (orogenic suites) and NYF from exten- enriched in Cs don’t fall into these categories. Addition- sional tectonic settings (anorogenic suites). Mixed NYF ally, a number of pegmatites show “mixed” NYF and LCT and LCT are proposed to be the result of contamination, characteristics and the origin of these are not addressed in either at the magmatic or postmagmatic stage, in which the the classification. Cerný’sˇ classification also fails to address evolved NYF rocks get “soaked” with a fluid bringing in the possibility of pegmatites forming by direct anatexis. not only Li and B, but also Ca and Mg from the host rock, Moreover, attempts to relate pegmatite types or subtypes to such that part of the pegmatite body may contain dravitic, magma genesis or tectonic regimes, as has been attempted elbaitic and liddicoatitic tourmaline, danburite, and other in granite classifications, are not satisfactory (Simmons, exotic species such as microlite, fersmite, londonite and 2005, 2007a,b). pezzottaite. They propose that some of the exotic Mada- Wise (1999) introduced a new expanded classification gascar pegmatites with a hybrid or “mixed” NYF / LCT of NYF pegmatites. His classification relates pegmatites character may be caused by remelting of just-formed NYF with NYF geochemistry to A-type granite plutons. He re- pegmatites by such metasomatic fluids. They also propose lated these pegmatites to post-tectonic to anorogenic plu- that pegmatites may form by anatexis from both crustal and

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mantle rocks, which may have been previously metasomat- studies of rhyolitic ash-flow tuffs such as the Bishop Tuff ically altered. (Hildreth, 1979). The more fractionated melts at the top of It is clear that a trend toward a petrogenetic classification the zoned magma chamber could then move farther into is emerging. We feel that a petrogenetic classification that the surrounding country rocks than melts derived from the can relate pegmatites to tectonic regimes and the related lower portions of the zoned magma chamber. However, this magma generating processes is ultimately essential in order would seem to require rather highly zoned plutons which to advance our understanding of pegmatite genesis within have not been documented. The zoning proposed in plu- the larger-scale earth processes. This is an area for much tons as evidenced by ash-flow tuffs is not nearly as extreme needed future research. as would seem necessary to produce the most fractionated LCT melts. Furthermore, the best examples of highly frac- tionated rhyolites such as the topaz and red beryl rhyolites Chemical quenching in deeper seated pegmatites in Utah, are from anorogenic settings and not orogenic. Also, given the evidence for rapid crystallization of peg- Some pegmatites that formed in deeper seated metamor- matitic melts, how is it possible for even the most fluxed phic terranes may show skeletal and comb textures indica- melts to travel large distances from the pluton into progres- tive of rapid crystallization even though it is clear that they sively cooler country rock? High over pressures and very must cool more slowly than shallow level pegmatites. As rapid injection velocities may be the answer, but there is discussed above, the mechanism for rapid crystallization little data supporting either concept. of deeper seated pegmatitic melts, which cool more slowly, We suggest that perhaps in some cases the pegmatitic may be the removal of a fluxing component, such as boron melts farthest from the pluton are not derived from the plu- or fluorine, from the melt by a crystallizing phase. Chem- ton by protracted fractional crystallization but are very low ical quenching can initiate rapid crystallization since the degree partial melts generated in the thermal halo of the melt is no longer fluxed by that component. Crystallization pluton. In other cases the pegmatitic melts may form in of tourmaline or fluorine-bearing phases can result in melt migmatitic terranes with no associated pluton. Roda et al. that is suddenly undercooled even though the surrounding (1999) in a study of pegmatites in the Fregeneda area, Sala- rocks and the melt itself remain at about the same tempera- manca Spain, proposed that pegmatites surrounding the ture. Comb structures and tapered crystals, seen in numer- Lumbrales granite, showing a typical regional zoning pat- ous pegmatites, may have resulted from rapid crystalliza- tern from proximal barren pegmatites to distal enriched tion initiated by a chemical quench. Evaluating the role of pegmatites, were formed by three different paths of frac- this process is another area for future research. tional crystallization of melts generated by partial melting of quartzo-feldspathic rocks and were not derived by frac- tional crystallization of the granite. More research needs to Regional zoning be done on the possible causes of regional zoning.

The mechanism of regional zonation described by Cernyˇ (1991) for orogenic LCT pegmatites is a problematic pro- Anatexis cess that needs more research. Regional zoning does not appear to occur in NYF pegmatite fields. The typical LCT pegmatite zoning pattern is concentric around an inferred Pegmatites are generally acknowledged to form by a pro- parental pluton with the least fractionated pegmatites ad- cess of fractional crystallization of a granitic composition jacent to the parental pluton, and the most fractionated melt. In many cases the connection of pegmatites with a pegmatites distal to the parental pluton. How pegmatites parent pluton is obvious from spatial relationships, age de- can be derived from a source pluton by periodic escape of terminations and chemical characteristics. However, it also pegmatitic melt such that the earlier proximal pegmatites has been proposed that it may be possible to form a peg- are the simpler less fractionated pegmatites, and the later, matitic melt of the same composition as that formed by pro- more distal pegmatites are the most fractionated and com- tracted fractional crystallization by direct anatexis of rocks plex, remains to be satisfactorily explained. We see sev- with the appropriate composition. eral problems. If less fractionated pegmatitic melts escape Simmons et al. (1995, 1996) proposed that in general early from a fractionating pluton as proposed by Cernyˇ low-degree partial melts produced around plutons in oro- (1991), it would seem that those melts would remove con- genic environments could form pegmatitic melts and that stituents that should remain in the pegmatitic melt in order some pegmatites in western Maine could form by direct to achieve the high levels of concentration necessary for the anatexis. Metasedimentary rocks containing evaporite se- formation of the more highly “evolved” pegmatites. How quences may provide a source of fluxing components such do Li, Cs, Ta, etc. remain in the pluton to be released later? as boron and lithium and other incompatible elements. Ba- Further, how can they travel outward from the pluton and sically the argument is that in collisional tectonic environ- leave no trace of their passage? ments if a sedimentary sequence (including volcanics) con- London (2005a) has suggested that a zoned magma tains fluxing components and incompatible elements that chamber might give rise to different batches of pegmatitic ultimately end up in a granitic melt (pluton) which can sub- melts, with the more evolved melts related to the upper sequently fractionally crystallize to form a pegmatitic melt, parts of the magma chamber, as has been described from then those same fluxing components and incompatible el-

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ements will be preferentially partitioned into a low-degree model for pegmatite genesis must reconcile the fact that at partial melt that can directly form a pegmatite. the outcrop scale the majority of pegmatites are granitic in Other researchers have also proposed anatexis as a mech- composition and mineralogically simple. anism for forming pegmatitic melts. Based on oxygen iso- tope systematics, Nabelek et al. (1992a, b) proposed that the granite-pegmatite couplets in the Black Hills were de- Acknowledgements: We thank Alexandre Lima, Encar- rived from partial melting. Falster et al. (1997, 2005) pro- nación Roda-Robles, François Fontan, Frédéric Hatert, Tâ- posed that the Animikie Red Ace and related pegmatites nia Martins and Romeu Vieira for organizing and conduct- in Florence County, Wisconsin could have formed directly ing the International Symposium on Granitic Pegmatites: the State of the Art. It was an excellent and well or- by partial melting of the surrounding metasedimentary ff and metavolcanic country rocks, possibly aided by evap- ganized meeting that o ered an opportunity for pegma- orites within the metasedimentary package. Roda et al. tologists from around the world to gather and exchange (1999) in a study of pegmatites in the Fregeneda area, Sala- ideas and visit pegmatites in the field. Thanks to Alexan- manca Spain proposed that the regionally zoned pegmatites der Falster for discussions and reviewing early versions of around the Lumbrales granite were anatectic in origin. Ex- this manuscript. Thanks also to Alexandre Lima, Jacques perimental research on forming pegmatites by direct ana- Touret and Tânia Martins for their careful reviews and texis is another area of possible future research. valuable comments and suggestions which improved the manuscript.

Isotopic ages versus conductive cooling models References There are some glaring discrepancies between the cool- ing times implied by 40Ar/39Ar zoned mica ages (Snee & Foord, 1991; Snee et al., 2006) versus the conduc- Anderson, A.J., Clark, A.H., Gray, S. (2001): The occurrence and tive cooling models. Snee et al. (2006) report that mus- origin of zabuyelite (Li2CO3) in spodumene-hosted fluid inclu- covite cores of several zoned mica samples from the Lit- sions: implications for the internal evolution of rare-element tle Three Pegmatite, San Diego County, California, were granitic pegmatites. Can. Mineral., 39, 1513-1527. up to 1.3 million years older than their corresponding rims Beus, A.A. (1960): Geochemistry of beryllium and the genetic types 40 39 of similar composition based on Ar/ Ar ages. This is of beryllium deposits. Academy of Sciences of USSR, Moscow, clearly at odds with conductive cooling models for the Lit- 329 p. (in Russian). tle Three Pegmatite (Morgan & London, 1999) as well as Bodnar, R.J. & Student, J.J. (2006): Melt inclusions in plutonic other San Diego County pegmatites (Webber et al., 1997, rocks: petrography and microthermometry. in “Melt inclusions 1999). Snee et al. (2006) suggest that the discrepancies in plutonic rocks”, J.D. Webster, eds. Mineral. Assoc. Can. ff may be explained by either di erential cooling rates, or Short Course, 36, Montreal, Quebec, 1-25. ff ff di erent argon closure temperatures, or di erent times of Boggs, R.C. (1986): Miarolitic cavity and pegmatite mineralogy of crystallization resulting from a complex multi-event peg- Eocene anorogenic granite plutons in the northwestern USA. matite emplacement process. Smith et al. (2005) showed Abstr. 14th Mtng. Internat. Mineral. Assoc. Stanford, USA, 58. that incorporation of Li, F, Rb and Cs in the mica structure resulted in lower argon closure temperatures in lepidolite. Breaks, F.W., Selway, J.B., Tindle, A.C. (2005): Fertile peralu- Research to help clarify this age discrepancy would be very minous granites and related rare-element pegmatites, Superior useful. Province of Ontario. in “Rare-Element Geochemistry and Mineral Deposits”, R.L. Linnen & I.M. Sampson, eds. Geol. Soc. Can. Short Course Notes, St. Catharines, 17, 87-125. Role of immiscibility in pegmatite genesis Buddington, A.F. (1959): Granite emplacement with special refer- ence to North America. Geol. Soc. Am., Bull., 70, 671-747. The MI and FI studies of minerals in pegmatites detailed Bushev, A.G. & Koplus, A.V. (1980): Rare-earth pegmatites of the by Thomas et al. (2000, 2005, 2006b) and Webster et al. granulite facies of metamorphism: Internat. Geol. Rev., 22, 221- (1997) suggest that pegmatite-forming melts are low vis- 232. cosity, H2O-rich, high diffusivity, alkali-rich aluminosili- Cashman, K.V. (1990): Textural constraints on the kinetics of crys- cate melts, with the ability to easily transport trace and tallization of igneous rocks. in “Modern Methods of Igneous other elements leading to the extreme enrichment of peg- Petrology: Understanding Magmatic Processes”, J. Nicholls & matites in elements such as P, F and B. Additionally, melt- J.K. Russell, eds. Reviews in Mineralogy, 24, Mineralogical melt immiscibility occurs early in the crystallization his- Society of America, 259-314. tory of pegmatite magmas. Cerný,ˇ P. (1989a): Exploration strategy and methods for pegmatite The relative roles of immiscibility, fractional crystalliza- deposits of tantalum. in “Lanthanides, Tantalum and Niobium”, tion and boundary layer crystallization with respect to peg- P. Möller, P. Cerný,ˇ F. Saupe, eds. Springer-Verlag, Berlin, 274- matite genesis, as well as an understanding of the percent- 302. age and composition of pegmatitic fluids, are topics for — (1989b): Characteristics of pegmatite deposits of tantalum. in further research. As pegmatologists we enjoy finding the “Lanthanides, Tantalum and Niobium”, P. Möller, P. Cerný,ˇ F. mineralogical and chemical oddities, but in the end, any Saupe, eds. Springer-Verlag, Berlin, 195-239.

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Black Hills, South Dakota – evidence from fluid inclusions. Thomas, R., Webster, J.D., Rhede, D., Siefert, W., Förster, H.-J., Geochim. Cosmochim. Acta, 67, 2443-2465. Heinrich, W., Davidson, P. (2006b): The transition from peralu- Smirnov, S.Z., Peretyazhko, I.S., Prokof’ev, V.Y., Zagorsky, V.Ye. minous to peralkaline granitic melts: Evidence from melt inclu- (1999): Daughter sassolite (H3BO3) in fluid inclusions: the first sions and accessory minerals. Lithos, 91, 137-149. evidence of natural fluids with high boron acid content. Terra Tremblay, L.P. (1978): Uranium subprovinces and types of ura- Nostra 99/6 ECROFI XV, Abstracts and Program, Potsdam, nium deposits in the Precambrian rocks of Saskatchewan. Geol. 283-285. Survey Can., Paper 78-1A, 427-435. Smirnov, S.Z., Thomas, V.G., Demin, S.P., Drebushchak, V.A. Trueman, D.L. & Cerný,ˇ P. (1982): Exploration for rare-element (2005): Experimental study of boron solubility and speciation granitic pegmatites. in “Granitic Pegmatites in Science and in the Na2O-B2O3-SiO2-H2Osystem.Chem. Geol., 223, 16-34. Industry”, P. Cerný,ˇ ed. Mineral. Assoc. Can. Short Course Smith, S.R., Kelley, S.P., Tindle, A.G., Breaks, F.W. (2005): Handbook, 8, 463-494. Compositional controls on 40Ar/39Ar ages of zoned micas from Veksler, I.V. & Thomas, R. (2002): An experimental study of B-, a rare-element pegmatite. Contrib. Mineral. Petrol., 149, 613- P- and F-rich synthetic granite pegmatite at 0.1 and 0.2 GPa. 626. Contrib. Mineral. Petrol., 143, 673-683. Snee, L.W. & Foord, E.E. (1991): 40Ar/39Ar thermochronology Webber, K.L. & Simmons, Wm.B. (2007): Crystallization dynam- of granitic pegmatites and host rocks, San Diego County, ics. in “Granitic Pegmatites: The State of the Art, Book of California. Geol. Soc. Amer. Abstracts with Programs, 23, Abstracts, Memórias N. 8”, T. Martins & R. Vieira, eds. Porto, A479. Portugal, 17-19. Snee, L.W., Foord, E.E., Morton, D.M., Landis, G.P., Rye, R.O., Webber, K.L., Falster, A.U., Simmons, W.B., Foord, E.E. (1997): Sheppard, J.B. (2006): Pegmatite genesis-complex or simple Theroleofdiffusion-controlled oscillatory nucleation in the emplacement? Revisiting Southern California. Proceedings 4th formation of line rock in pegmatite-aplite dikes. J. Petrol., 38, Internatl. Gemological Symp. & GIA Gemological Research 1777-1791. Conf. San Diego, CA, Gems & Gemology, XLII, 151-152. Webber, K.L., Simmons, Wm.B., Falster, A.U., Foord, E.E. (1999): Sobolev, A.V. (1996): Melt inclusions in minerals as a source of Cooling rates and crystallization dynamics of shallow level principle petrological information. Petrology, 4, 209-220. pegmatite-aplite dikes, San Diego County, California. Am. Swanson, S.E. & Fenn, P.M. (1986): Quartz crystallization in ig- Mineral., 84, 708-717. neous rocks. Am. Mineral., 71, 331-342. Webber, K.L., Simmons, Wm.B., Falster, A.U. (2005): —, — (1992): The effect of F and Cl on albite crystallization: a Rapid Conductive Cooling of Sheet-Like Pegmatites. model for granitic pegmatites? Can. Mineral., 30, 549-559. Crystallization Processes in Granitic Pegmatites, Taylor, B.E., Foord, E.E., Friedrichsen, H. (1979): Stable isotope Intl Meeting, Elba Island, Italy. MSA web site, and fluid inclusion studies of gem-bearing granitic pegmatite- http://www.minsocam.org/MSA/Special/Pig/PIG_articles/ aplite dikes, San Diego Co., California. Contrib. Mineral. Elba%20Abstracts%2022%20Webber.pdf Petrol., 68, 187-205. Webster, J.D. & Thomas, R. (2006): Silicate melt inclusions in felsic Thomas, R. (2000): Determination of water contents of granite melt plutons: a synthesis and review. in “Melt inclusions in plutonic inclusions by confocal laser Raman microprobe spectroscopy. rocks”, J.D. Webster, ed. Mineral. Assoc. Can. Short Course, Am. Mineral., 85, 868-872. 36, 165-188.

— (2002): Determination of the H3BO3 concentration in fluid and Webster, J.D., Thomas, R., Rhede, D., Förster, H-J., Seltmann, R. melt inclusions in granite pegmatites by laser Raman micro- (1997): Melt inclusions in quartz from an evolved peraluminous probe spectroscopy. Am. Mineral., 87, 56-68. pegmatite: geochemical evidence for strong tin enrichment in Thomas, R., Webster, J.D., Heinrich, W. (2000): Melt inclusions in fluorine-rich and phosphorous-rich residual liquids. Geochim. pegmatite quartz: complete miscibility between silicate melts Cosmochim. Acta. 61, 2589-2604. and hydrous fluids at low pressure. Contrib. Mineral. Petrol., Wise, M.A. (1999): Characterization and classification of NYF-type 139, 394401. pegmatites. Can. Mineral., 37, 802-803. Thomas, R., Förster, H.-J., Heinrich, W. (2003): The behavior of Zagorsky, V.Ye., Makagon, V.M., Shmakin, B.M. (1999): The sys- boron in a peraluminous granite-pegmatite system and associ- tematics of granitic pegmatites. Can. Mineral., 37, 800-802. ated hydrothermal solutions: a melt and fluid inclusion study. Contrib. Mineral. Petrol., 144, 457-472. Thomas, R., Forster, H., Rickers, K., Webster, J.D. (2005): Formation of extremely F-rich hydrous melt fractions and hy- drothermal fluids during differentiation of highly evolved tin- granite magmas: a melt/fluid-inclusion study. Contrib. Mineral. Petrol., 148, 582-601. Thomas, R., Webster, J.D., Davidson, P. (2006a): Understanding pegmatite formation: the melt and fluid inclusion approach. in Received 13 January 2008 “Melt inclusions in plutonic rocks”, J.D. Webster, ed. Mineral. Modified version received 28 March 2008 Assoc. Can. Short Course, 36, 189-210. Accepted 22 April 2008

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