447 Th-eC analinn M ineralo g is t Vol.36, pp. 447462(1998)

THEBERYLLIAN CORDIERITE + BERYL + SPESSARTINEASSEMBLAGE, AND SECONDARYBERYL IN ALTEREDCORDIERITE, GREER LAKE GRANITIC PEGMATITES,SOUTHEASTERN MANITOBA

SCOTTJOBIN-BEVANS I ,qNpPETR EPRNf '

Department oJ Geological Sciences,IJniversity of Manitoba, Winnipeg, Manitoba R3T 2N2

ABSTRACT

Pseudomorphsafter beryllian cordierite (possibly beryllian sekaninaite) occur along bore margins ofArchean peraluminous granitic pegmatites of the beryl - columbite subtype at Greer Lake, southeasternManitoba, and in analogousposition in pegmatitic pods of the parent leucogranite. The pseudomorphs are locally intergrown with primary columnar beryl I, and studded by spessardnethat contains subparallel inclusions of beryl II. The muscovite > biotite > berthierine assemblageof the pseudomorphs after cordierite contains anhedral fine-grained beryl III, locally attaining -20 vol.Voof the breakdown products. This complex mineral association is interpreted as a product of two processes.(i) Magmatic crystallization from a Be-rich and Mn-enriched melt yielded cordierite and beryl I at -550oC and <2.8 kbar. Increasedactivity of Mn triggered intermittent stabilization of spessartine+ beryl tr during the crystallization of cordierite; this assemblage probably represents a Be-rich substitute for spessartine+ aluminosilicate + quartz, the low-pressure equivalent of "manganocordierite", which is unstable above 1 kbar P(HrO). (ii) Subsolidus metasomatic alteration of cordierite by alkali- and F-bearing residual aqueous fluids at about 500-450'C, -2.7-2.6kbar generatedbiotite > muscovite >> beryl III along the basal parting planes, followed by several textural varieties of muscovite > biotite >> bery'l III, which replaced the bulk of the cordierite crystals. Low-temperature alteration of biotite yielded berthierine. The proportion of beryl m (up to -2OVo)in the micaceous matrix of the pseudomorphs indicates up to -2.6 wt.Va BeO in the cordierite precursor, which closely correspondsto the experimentally established maximum value at the given P,T conditions of magmatic crystallization.

Keywords: cordierite, sekaninaite, alteration, beryl, spessanine, granitic pegmatite, Greer Lake, Manitoba.

Sorrlvens

Des pseudomorphesde cordi6rite (ou peut-6tre s6kaninaite) bdryllienne sont parsem6sle long du coeur de massifs archdens de pegmatites granitiques hyperalumineuses de type b6ryl - columbite au lac Greer, dans le sud-est du Manitoba, et en posi- tion analoguedans des lentilles pegmatitiquesdu pluton leucogranitiqueparent. Les pseudomorphesmontrent ici et ld une intercroissance avec le b6ryl I, en prismes trappus, et sont tapiss6sde spessartinequi contiennent des inclusions subparallbles de b6ryl II. L'assemblage d6velopp6 dans les pseudomorphes, muscovite > biotite > berthi6rine, contient en plus des grains x6nomorphesfins de b6ryl III, qui atteignent localementjusqu'd environ 20Vodrt volume des produits de d6stabilisation. Cet assemblagecomplexe de min6raux r6sulterait d'une combinaison de deux processus.D'abord, il y a eu cristallisation magmatique de cordi6rite + b6ryl h partir d'un magma enrichi en Be et Mn, d environ 550oC et (2.8 kbar. Une activit6 accrue du Mn a enclench6 de fagon intenompue la formation de spessartine+ b6ryl II au cours de la cristallisation de la cordi6rite; cet assemblageprendrait la place de l'assemblagespessartine + aluminosilicate + quartz dans un milieu riche en Be, et serait l'6quivalent d faible pression de la "manganocordi6rite", instable au deli d'une pressionP(H,O) 6gale d 1 kbar. (ii) Une m6tasomatosede la cordidrite au-dessousdu solidus suite d son interaction avec une phase fluide r6siduelle enrichie en alcalins et en F tr environ 50H50"C, -2.7-2.6kbar, a produit I'association biotite > muscovite >> beryl Itr Ie long des plans de fissures, et par la suite une sdquenceet une vari€t' de d6veloppements texturaux de muscovite > biotite >> beryl III, en remplacement de la majeure panie des cristaux de cordi6rite. Une alt6ration A faible temp6rature de la biotite est responsablede la berthi6rine. La proportion de b6ryl III (usqu'A environ20Vo) dansla matrice micac6e des pseudomorphesindiquejusqu'd environ 2.6Voen poids de BeO dans la cordi6rite primaire, ce qui correspond 6troitement aux teneurs maximales attendues aux conditions de pression et de temp6rature de cristallisation magmatique, d'aprbs les exp6riences au laboratoire.

(Traduit par la R6daction)

Mots-clds: cordi6rite, s6kaninarle, alt6ration, b6ry1, spessartine,pegmatite granitique, lac Greer, Manitoba.

I Present address: Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7 2 E-mnil address:[email protected]

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IxrnopucnoN the foliation of the host tonalitic and granitic gneisses, which constitutethe basementof the greenstonebelt. Since the publications by Ginzburg & Stavrov Typical dimensions of the pegmatite outcrops are on (1961)and GrifEtts& Cooley(1961) on minor elements the order of 60 x 5 m, rarely up to 400 x 15 m. Most of in cordierite, beryllium was repeatedlyfound in this the pegmatites show a regular pattern of concentric mineral from pegmatites, granites and metamorphic zoning. The thin but continuousborder zone consists rocks in quantities as high as 1.94 wt.VoBeO (Cernf & of medium-grained oligoclase with microcline perthite Povondra 1966,1967,Newton l966,Piyw et al. 1968, and quartz. Large blocks of graphic intergrowths of Povondra & Cech 1978, Schreyer et aI. 1979, microcline perthite+ quartz, imbeddedin coarse-grained Armbruster & Irouschek 1983, Povondra et al. 1984, albite + quartz t biotite * muscovite, constitute the wall Orlandi &Pezzofta 1994). Also, the modes of entry of zone. The blocky microcline perthite + quartz core is Be into the cordierite structure were establishedon commonly segregatedinto a feldspathic core-margin natural minerals as well as experimentally (eernf & zone surrounding a qnartz corc1,platy albite and Povondra 1966, Povondra &Langer l97la,b, Schreyer muscovite are widespread but subordinate.Layers and 1985).However, not much is known about the fate of pods of fine- to medium-grained albite with quartz, Be during alteration, which very commonly affects muscovite and garnet are located at the outer margins cordierite and generatesa great diversity of secondary of the blocky core (or within and around the segregated mineral assemblages.Case histories of beryllium microcline perthite of the core margin), extensively minerals such as milarite, bavenite, epididymite and replacing the potassium feldspar and quartz. eudidymite, associatedwith retrograde breakdown of On the basis of mineralogical and geochemical cordierite, are quite rare (eernj & Povondra 1967, criteria, the Greer Lake pegmatitesbelong to the beryl- Cen! 1967, 1968), and secondary beryl was observed columbite subtype of the peraluminous, rare-alkali- at only two localities of altered beryllian cordierite and tantalum-concentrating LCT family, within the (Yrdna1979, Povondraet aI.1984). rare-elementclass ofeernf (1991). The less evolved Altered cordierite was found in the Archean dikes contain monazite and niobian rutile, the most pegmatites of the Greer Lake group, southeastern fractionated ones carry lithian muscovite and Li, Manitoba in 1969 (Cern! et al. l98l), and secondary Cs-enriched beryl. A few internal pegmatite pods and beryl was identified in the pseudomorphsat a later date offshoots ofthe parent leucogranite contain abundant (Goad 1984). Ourrecent work revealed the presenceof lepidolite, amblygonite and pseudomorphsafter petalite, additional phases in the primary cordierite-bearing characteristicof the more evolved, complex type of assemblage,as well as in the alteration products after pegmatites. However, the most common accessory cordierite. Thus, we present here the description of a minerals in the dominant beryl - columbite dikes are rather exotic primary assemblageof beryllian cordierite beryl, columbite - tantalite and garnet, rarely also + spessartine+ beryl, analysis of the extremely rare gahnite, zircon and xenotime. Apatite, pyrite and beryl-bearing associationof secondary phases after chalcopyrite are extremely rare; the pegmatites are cordierite,and parageneticinterpretation with constraints conspicuously poor in minerals of P, B and S. The on conditions of formation for both assemblases. blocky microcline perthite has a variable K./Rb, from 96 to 13, and contains from 10 to 1790 ppm Cs; Tne PaneNr Pnclrarrre Gnoup core-margin muscovite and late lithian micas range from 22 to 4 in K,/Rb, from 1030 to 320 in K/Cs, The Greer Lake pegmatite group carrying the and from 0.07 to 1.10 wt.Voof Li. eem! et al. (1981, cordierite examined is located at Greer Lake, 135 km 1986) gave a more detailed account of the granite and east-northeast of Winnipeg, Manitoba, at latitude associatedpegmatites. 50"20'39"N and longitude 95"19'w (Fig. 1). The pegmatitesspread south of the parent peraluminous, Seuplwc, E>oenrveNrel- Mgrsoos aNo Tpnvu.tolocv garnetiferous and largely pegmatitic Greer Lake leucogranite,and were emplacedinto metabasaltsof Although many of the pseudomorphscollected at the Bird River GreenstoneBelt (Figs. l, 2). The granite the locations marked in Figure 2 have been examined + pegmatite suite belongs to a broader population of since 1969, the presentdetailed study is confined to these rocks that collectively comprise the Cat Lake - those from the GL-8W pegmatite (marked as 8 in Winnipeg River pegmatite field, in the Archean Bird Fig.2), in which they are particularly abundant and River Subprovince of the western Superior Province typically developed in terms of their internal structure (Cem! et al. 1981,1986, Cernf 1990).The age ofthe and mineral assemblage.References to observationson Greer Lake leucogranite and derived pegmatites is pseudomorphsfrom other locations are identified as 2640 !7 Ma (etched columbite - tantalite; Baadsgaard such. &Cem! 1993). Observations on hand specimens and thin The Greer Lake pegmatites are dominantly sections by optical microscopy were coupled with concordant intrusions, pinching and swelling within X-ray diffraction and microbeam studies. The Cameca

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oooo--{re

Innlpeg r.-d--.

Frc. 1. Location of the Winnipeg River pegmatite district in Manitoba (within the east-west-trending Bird River Subprovince; left), and of the Greer Lake leucogranite (GL) and pegmatite $oup in this district (modified from Cernf et al. 1986; ight). Symbols: MWL: Maskwa Lake tonalite diapir, MJL: Marijane Lake tonalite diapir, LdB: Lac du Bonnet granitic batholith, WRBB: Winnipeg River Batholithic Belt, MEGB: Manigotogan - Ear Falls Gneissic Belt, unpatterned:Bird River Greenstone Belt. Most bodies of pegmatite (each marked by a spot symbol assigned to a particular group) are associated with outcropping pegmatitic granites (in black; GL, ENL, AX, OL, TL).

CAMEBAX SX-50 instrument was used for PRTMARyAND SECoNDARY AssrMsl-acEs ltwolvntc electron-microprobeanalysis (EMPA) of the pseudo- BERYLLIANConlrgnrre morphs and enclosedminerals. Analyses of the micas, berthierine-andberyl were performed under conditions Pseudomorphs after cordierite occur in interior quoted by Cemt et al. (1995); the conditions of garnet potassic pegmatitic pods within the Greer Lake analysis are given in Teertstra et al. (1998). Standard leucogranite, and in associatedexterior pegmatites X-ray powder-diffraction techniques were applied emplacedin the gneissic country-rocks [both marked in to mineral identification; we used a Philips PW 1710 Fig.2;Cem! er al. (1981,1986),Goad (1984),unpubl. diffractometer. data of P.e.]. The pseudomorphsoccur in the same As explained later, the main subject of this paper, position in both the pegmatitic pods of the leucogranite beryllian cordierite, is completely altered in the Greer and the pegmaritedikes. They reside along the transition Lake pegmatites, except for a single microscopic relic from the graphic wall-zone to the blocky core-margin, observed optically in a thin section tlat was subse- commonly beside the blocky crystals of K-feldspar quently lost. However, the former presenceof beryllian that rim the quartz core (Fig. 3). They locally show cordierite was establishedas the primary phase by interference growth-surfaces against quartz and circumstantial evidence. Thus, the term cordierite is K-feldspar, indicative of simultaneous growth of used in description and discussion of the primary these two minerals and the cordierite precursor. cordierite-bearing assemblage,to avoid cumbersome Textural evidence, supported by the chemical repetitive referencesto the precursor of the present-day composition of individual minerals discussedbelow, pseudomorphs, and to clearly distinguish between indicate the presenceoftwo assemblagesgenerated at the primary mineral and its alterationproducts. Also, different stagesof evolution of the parent pegmatites: the term beryllian is applied only sparingly, as it is not (i) primary crystallization of beryllian cordierite, essential for the discussion of all aspects of the associatedinitially with beryl I and subsequentlywith cordierite examined. spessartine+ beryl II, and (ii) alteration of the beryllian

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WINNIPEG NIVER

------OFEER LAKE

Ftc. 2. The Greer Lake leucogranite (open, heary outline) in metabasalt (ruled) and the derived pegmatites (heavy bars) in the gneissic terrane (x) south of an east-trendingfault (-). Numbered sampling stations in the leucogranite and numbers at pegmatite dikes mark the occurrences of pseudomolphs after cordierite. Samples examined in the present study come dominantly from the pegmatite dike marked "8". Modifred after Cemj et aI. (1986).

cordierite into biotite + muscovite + beryl Itr, followed Garnet formed abundant inclusions in cordierite. It by alteration of biotite to berthierine. The two assem- is presentin two modes of distribution. It consfitutesan blages will be described under separateheadings. internal, virtually continuouszone (which circumscribes a ghost surface mimicking the external morphology P rimary cordierite-b earing ass emblag e of cordierite) and scatteredgrains in cordierite outside this zone (Figs. 3,4). Beryl (rype II) forms subparallel The beryllian cordierite formed short-prismatic, inclusions of fibrous crystals in garnet. subhedral crystals up to 20 x 30 cm in size, commonly Beryllian cordierite. Despite a thorough search for slightly cone-shapedand quasi-hexagonalin cross- relics of cordierite in the Greer Lake pseudomorphs, section. Several cordierite blocks enclosedeuhedral none was found, except in a light-microscopic prismatic crystals of green beryl (type I) 0.5 x 3 to observationby Goad (1984), in a pseudomorphfrom 1.5 x l0 cm in size, locally forming slightly radial the pegmatitic leucogranite.The replacementof the clusters. The morphology and color of beryl I are cordierite precursorby the sheetsilicates and associated comparable to those of primary beryl scatteredmainly minor phaseswas evidently complete, at least in the in the graphic wall-zone and core margin of the 15 samples examined to date from six pegmatites cordierite-bearing pegmatites (cf eernf & Turnock and pegmatitic pods in the leucogranite. Nevertheless, 1975). Although locally overgrown by platy albite there is no doubt left about the identity of the primary * muscoviteocordierite does not show any systematic mineral subject to pseudomorphism:besides the afore- relationship with theseminerals, and it is in most cases mentioned optical observation of a cordierite remnant, totally separatedfrom them. the shapeof the columnar pseudomorphs,the orientation

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Ftc. 3. An aggregate of pseudomorphs after cordierite (dark grey), located between quartz core (grey, top) and blocky K-feldspar of the core margin (white, bottom) of the GL-8W pegmatite (marked as 8 in Fig. 2). A continuous ghost zone of medium- grained gamet, surrounded by coarser individual grains of garnet, is well developed in one of the pseudomorphs at the top center of the aggregate.Diameter of the compass is 5 cm.

of parting normal to the elongationof the pseudomorphs, Thus the FeMg of the cordierite phase from Greer which correspondsto the (001) plane of cordierite, the Lake remains uncertain, and the term cordierite is used phasespresent in the pseudomorphs,the early boxwork sensulato for convenience. fabric of the secondary sheet silicates, the highly The close association of cordierite with beryl I peraluminous compositions of the secondary minerals indicatesthat cordieritecrystallized in a Be-rich medium. all correspondto the typical featuresof alteredcordierite Experimental work of Povondra & Langer (197lb) from other localities, where relics of cordierite or indicates that under P-T-X conditions of pegmatite sekaninaitehave been identified in pseudomorphs,and consolidation, cordierite incorporates significant almost fresh crystals were encounteredas well (StanEk quantities of Be via the NaBeAl-' substitution. This 1954,1991, Standk & Mi5kovskf 1975, Povondra & substitution attains maximum values in Be-rich bulk- Cech 1978,Povondra et al. 1984,Schreyeret al.1993, system compositions saturatedwith respectto beryl. and many other cases). These relationships were also observedon a number of The biotite, berthierine and muscovite composing natural occurrences of bery-llian cordierite + beryl in the bulk of the present-day pseudomorphs (described granitic pegmatites (e.9., Cern! & Povondra 1966, below in detail) show considerablepreponderance of 1967,Newton 1966,Piyar et al.1968, Povondra& Fe over Mg. This may suggestthat the primary mineral Cech 1978, Povondra et at. 1984). Thus, the Greer was actually sekaninaite, the Fe-dominant member of Lake cordierite can be safelv assumedto have beenrich the cordierite group: the Fe/1\4gvalues of secondary in Be, as discussedlater. sheet silicates at Gammelmorskiirr (Povondra et al. 1984)and DolnfBory (Scbreyeret al 1993)are close to Spessartine. Grains of garnet of the internal those of their precursors. Sekaninaite also is suggested continuous zone are 0.5 to a few mm in size, whereas by the presenceofberthierine in our pseudomorphs, a the scattered, separate, subhedral to largely anhedral mineral typical of the two above-mentionedoccur- grains attain 5 - 20 mm in diameter (Figs. 3,4), and are rences and of products of sekaninaite breakdown. locally slightly zoned, with a darker brownish red core However, the alteration of sekaninaite at Gammel- and lighter reddish orange outer zones. Flat mm-size morskiirr and Dolnf Bory was much closer to being grains of garnet oriented parallel to the basal parting of isochemical than in the muscovite-dominated cordierite are rare. The large grains of garnet outside pseudomorphs at Greer Lake, in which the starting the ghost-surface zone commonly contain subparallel Fe/IVIgvalue could have been significantly disturbed. bunches of columnar to fibrous bervl II.

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to ferroan spessartine. Among the minor elements, extremely low contents to virtual absenceof Ti, R Sc, Y, Zn, Na, Zr,Y, Cr, Sn and F are prominent, as is the apparent absenceof Fe:*. The compositional range of the garnet enclosed in 0'{;e, cordierite (Spsrr-*o) overlaps in part that of garnet @ associatedwith the relatively late albite + muscovite + quartz assemblagein near-core parts of the pegmatites (Spssauo;Fig 5). However, this latter type of garnet o has much lower content of Ca relative to the Ca-rich margin of garnet included in cordierite. "'\-*t/t, a Beryl I and II. The coarse-columnar beryl I intergrown with cordierite is closely comparable to the beryl scattered in the intermediate zones of the Greer Lake pegmatites, not only in its physical attributes but also in composition (Table 2). Fibrous beryl II is enclosedin spessartinein the form ofloose aggregates of subparallelcrystals (Figs. 7A, B). Its composition is statistically undistinguishablefrom that of beryl I above and its counterpart outside the cordierite assemblage(Table2). Minor constituents. Optical observations revealed exhemely rare microscopic grains of a partly metamict mineral, possibly monazite or allanite. The mode of occunence of other minor phasessuggests a secondary origin; consequently, they are mentioned in the subsequentsection.

Flc. 4. Distribution of inclusions of spessartinein a crystal of cordierite, in sections norrnal to (A) and parallel with (B) elongation of cordierite; finer-grained garnet coats a ghost growth-surface of cordierite (ghz), whereas larger zoned grains of gamet (g) are scattered outside the ghost zone.

No fundamental difference was found in chemical Frc. 5. Plots of the atomic proportions of Fe : Mn : Mgxl0 (A) (B) garnet from the outer parts composition between the garnet of the ghost zones and Fe : Mn : Cax10 in of the pseudomorphsafter cordierite. Symbols: square: and the individual grains scatteredin the outer parts of rim, circle: core of the gamet grains. Note the prominent cordierite. The first type tends to be slightly more enrichment in Fe and Ca, and slight increasein Mg, in the homogeneous,in contrast to the second variery, which outer part of the garnet grains. The composition of the shows distinct enrichment in Fe and Ca in the outer garnet from the cordierite-free albite + quarz + muscovite zones (Figs. 5, 6, Table 1). Both types correspond assemblageof the parent pegmatite is marked by x.

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2.fi TABLE I. RWRSIIENTATWE COMPOSITIONS OFGARNEf,, GRTERLAKE

GI]CI-E GI-CI-5 Rim Inr erm

SiO, , wtVo 3629 36.M 35.06 fro, bdl 0.m MI SDO2 0.01 bdl bdl AUo3 mJ3 2ns6 m.6E MI MI 0.0r sqor bdl 0.04 0.01 Grot 0.04 MI 0.04 RO. t7.6 12,29 6.86 0.60 MnO 2338 30.3E 35.96 7^O 0.04 bdl bdl CaO t.l0 03t o.46 0.10 SrO o.02 bdt 0.03 Mco o2s 0.15 0.0t 0.10 Vror 0.0t bdl bdl 0.09 bdl bdl 0.01 MI 0.06 0.15 0.08 0.07 0.06 Nubq of iou pq fmula @it sl& 2-994 2985 2.925 0.0s Al3 0.006 0.015 0.m4 P$ 0.04 0.03 Al& 2,W l@a 1.959 o.u2 G$ 0.@3 0.0@ 0.0{B o,w2 0.000 0.01 0.@ 0.@1 0.000 0.000 0.m sc$ RIM INTERMDIATE CORE

NIgt' 0.ml 0.019 0.010 Ftc. 6. Compositional traverse of one of the outer grains Fel l2t8 0.851 0.479 of garnet from the pseudomorphs. From central parts to Mtr2+ t.6u Lt3l 2541 the margin, the proportion of Fe, Ca and Mg increase, coa 0.w7 0.041 o.094 whereas that of Mn decreases. Znx 0.002 0.000 0.000

Almudhe 40.9 8.0 ls.6 Spesitre 54.t 10.2 12.7 P se udo mo rp hs aft er c o rdi erit e Grc$ulor t.l l3

The pseudomorphsafter beryllian cordierite exhibit rtod Fe a rc: bdl - below d@tioD lieib well-developed parting oriented norrnal to their elongation and spaced 2-5 mm apart. The parting planes are covered by dark blackish green biotite oriented parallel to the parting, with minor muscovite (Fig. 8A). Flakes of the micas oriented randomly to of biotite, penetrating on an extremely fine scale along subnormal to the parting compose the bulk of the the cleavage or forming porous, very fine-grained matrix of the pseudomorphs,which is lighter in color. aggregates. Garnet is commonly slightly corroded Muscovite is the dominant mineral in these aggregates, and replaced by the micaceous aggregates of the with subordinate biotite (Figs. 8A, B). Texrurally, pseudomorphs, and locally veined by them along micas along the parting planes predate the intervening fractures. The boxwork texture of the surrounding flaky aggregates, which locally show a distinct pseudomorph after cordierite may extend into the boxwork aggregation (Fig. 8B). Muscovite is variable garnet (Fig. 7D). in shape,size and aggregation.Coarse crystals evidently Dispersed in the micaceous aggregatescomposing predate the finer-flaked, felted or plumose masses the pseudomorphsis beryl of the third type, anhedralto (Fig. 8C), but no uniform pattern can be distinguished subhedraland generally <0.4 mm in grain size. It is in the distribution or temporal sequenceof the latter found interstitial to the micas within the parring planes, (Figs. 8A, B, C, D). Muscovite locally replacesbiotite. and in randomly oriented clusters in the intervening Berthierine is a widespread late product of replacement matrix (Fig. 7C). Beryl III is also encounteredin the

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TABLE2. AVERAGECOMPOSITIONS OF BERYI. micaceousveinlets penetratinggarnet (Fig. 7D). It is GREERI..AKE locally associatedwith accessory quartz or albite, which also are disseminatedthroughout the pseudo- Secondary Primaryl(D lnclusions (m) morphs but in very minor quantities. Beryl Itr locally "%fi" attains up to 20 volume Voof the pseudomorphs. (N--4) (N=2) (N=6) Biotite. The composition of the biotite within the parting surflacesof the pseudomorphs is perceptibly FqOt*,wf/o 0.41 0.35 0.64 different from that intergrown with muscovite between CaO o.0w bdr 0.01 the parting planes.With the exception of a single com- Mso 0.64 0.r5 0.27 position, the Si/Al value of the first type is slightly that of the second variety, but the ranges L\O 0.12 nd nd higher than of FeMg and octahedral-site occupancies are about Na"O 0.45 0.74 0.92 identical (Table 3). Both types correspondto annite &o 0.07 0.01 0.11 with substantialcontent of phlogopite and siderophyllite qAl P$rO 0.013 bdt bdl components (1.05 to 1.72 wMg, 0.25 to 0.75 + vr per24 anions;Fig. g, Cs"O 0.30 0.16 0.25 2.25 to 2.75rvAl, and 0 to 0.45 Table 3). The extremely low Ti content is in agreement ttotal = Fe as FqOr; bdl below detectionlimit; nd with the green color of biotite, which also suggeststhe = not determined;1demf& Tumock(1975)

Frc. 7. Textural and parageneticfeatues of beryl. A. Subparallel crystals of beryl tr (white) in garnet (black). B. Skeletal crystals ofberyl II (grey) in garnet (darker grey, high relief). C. Fine-grained beryl Itr (white, marked by arrows) in the basal-layer biotite (black) and the felted matrix micas (mottled). D. A boxwork of mica flakes (black streaks) penetrating garnet (grey, g) from the surrounding pseudomorph after cordierite, with beryl III (white, marked by arrows) inside the mica aggregates.Photomicrographs berween crossed nicols (A) and in plane-polarized light (B, C, D). The scale bars are 0.5 mm lons.

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Ftc. 8. Textural and paragenetic features of the micaceous assemblagesin the pseudomorphs aft.ercordierite. A. Biotite-rich layers parallel to the basal parting of the cordierite precursor (darlq subvertical,marked by arrows) with muscovite-dominated, biotite-bearing matrix of rather chaotic, felted texture. B. Boxwork-like compartmentalization of the muscovite > biotite matrix between two Iayers ofbiotite in basal parting planes (dark, marked by arrows), with felted to plumose aggregation of the micas within the boxes. C. Coarse muscovile, in part deformed, veined and replaced by the felted micas of the matrix. D. A plumose sheaf of biotite + muscovite in a box outlined by nluscovite flakes, adjacent to a biotite layer parallel to the basal parting (anow). Photomicrographs in plane-polarized light. The scale bars are 0.5 mm long.

absenceof Fe3+(or electron transfer between Fez* and Benhierine. A 7 A layer-silicate phase is identified Fe$ in adjacentoctahedra; Rossman 1984). Octahedral- by XRD examination in all samples of the pseudo- site vacanciescould be lower if some Li is present.The morphs. It also is detected by electron-microprobe Rb/Cs value is very low, and F/Cl is high. Most of the analysis of the biotite, which tends to have total Fe as Sr, very high relative to the low Ca content, is probably FeO distinctly higher, and K2O and the sum of wt.Va radiogenic (as demonstratedin previous studies on oxides lower than determined for a pure fresh mica K-feldspar:Clark & eernf I987). phase. Berthierine forms mainly along the basal cleavageof biotite at a scale below the resolution of Biotite locally shows an increased Fe content, the electron beam. It can be analyzed only in relatively which correlates with decreasing K and total of wt.Vo coarset porous granular aggregates(Fig. 10), but even oxides. A fine-scale replacement by berthierine is in these, some overlaps with relics of biotite and indicated in these cases(see below). pore space are unavoidable. However, recalculation Compared to the composition of primary, blackish ofthe resulting data yields surprisingly good compo- brown biotite from the wall zone, the secondarybiotite sitions, matching the crystal-chemical requirements contains more Al, less Mg and Ti, much less Na and of berthierine and suggestive of virtual absence of Ca, and has a lower Rb/Cs and a higher K/Rb. The p"r* lTable 3). The composition of the Greer Lake color ofthe primary biotite suggeststhe presenceof berthierine is very close to tlat of the "septechlorite" appreciable Fer*, although the effect may be related reported from the altered Dolni Bory sekaninaite by to the oresenceof Ti. Schreyer et al. (1993).

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TABLB 3. REPRESENTATTVECOMPOSMONS OF I.AYER SIUCATES. GREERI.AKE

Biodte lvfuscovit€ Bdhi€rine

(etolol B-BP4 AM-5 BM4 GIJeI GIJB M-Ll GL8C2 M-Ml M-Ml GIJM 52 , F?. sio, 3.Ut 33i2 ttl -33- tI.zs 41.st 46.36 47.a 47.9s 44.62 t9.r4 23.s Pp, bdl MI 0.04 0.04 d bdl 0.04 bdl 0.04 trd 0.06 Tio, 0.01 0.04 bdt bdt 0.80 bdl bdl bdl bdl 0.03 bdl Alro, l8.ll 18.02 t9.22 tg.n ls.@ 33.57 32.& 31.37 31.14 35lA 16.44 20.31 FeO* 24.05 22.46 24.15 U.6 I.n 3.51 3.75 5.61 4.93 l.2l 30.14 3722 letlo 0.77 0.74 014 0.71 0.93 0.04 bdl 0.07 0.16 0.16 1.66 2o5 Zao 0.06 0.05 Ml 0.14 nd 0.06 bdl bd 0.01 Dd 0.50 0.61 l,tCO 4.39 5.31 4.ls 4.16 0n 0.76 0.79 1 30 l.l7 0.16 4.42 5.46 Sro bdl 0.03 0.05 0.ll d bdl bdl 0.04 0.03 !d 0.04 CaO Ml bdl 0.03 0.02 0.m 0.01 bdl bdl 0.02 0.05 0.16 LLo ad !d !d trd 0.22 Dd !d nd !d bdl !d N".o Ml 0.0s 0.06 bdl 0.54 0.80 0.55 0.11 0.14 0.62 0.13 &o 8.91 9.t2 812 8.6E 9.n 9.r2 10.28 9.84 8.43 9.94 0.45 Rbro 0.49 0.60 0.66 0.64 I.06 bdl 0.05 d Ml 0.70 bdl CqO 0.20 0.22 0.19 O.n 0.6 0.02 0.01 0.04 0.08 0.02 bdl F 0.E5 1.53 0.81 LV2 !d 0.28 0.31 0.50 0.52 trd 0.24 Cl 0.01 0.01 0.06 O.V2 ad 0.01 O.O2 bdl O.O2 nd Om OEF -0.36 4.4 434 4.43 4.t2 4.13 4.2r 422 {.10 o=Cl 0.00 0.00 -0.01 0.00 0.00 0.00 bdl 0.@ 0.@ HoO' 3.24 3.@ 3.25 3.19 3.il 4.36 4.25 4.23 4.lE 4.36 10.71 L n.ol 73.30 l@.@ Nmbs of im s O- (OtL CL n, fG mi6. O,,(OI{), fq bsthiqire si- s.635 s.1X s.490 5.M 5.869 6.331 6.308 6.396 6.& 6.131 pe Ml bdl 0.@5 0.m5 Dd Ml 0.@5 Ml 0.@5 trd Tr o.@l 0.@6 bdl bdl 0.095 bdl bdl bdl bdl 0.@3

D t.tM Lt70 1.2t8 l.lE6 0.t61 3.603 3.522 3.3@ 3.44 3.E34 t.343 Ff 3.306 3.020 3.319 3.3E5 4.374 0.391 0.427 0.630 0.s57 0.139 3.491

l\ldl 0.107 0.101 0.103 0.09E 0.124 0.@5 Ml 0.008 0.018 0.019 0.195

Ztt* 0.007 0.006 bdl 0.017 od 0.006 bdl bdl 0.@l Dd 0.051 Mf 1.076 Ln3 l.0l? 1.010 0.063 0.151 0.160 0.2@ 0.236 0.033 0.914 LT

L CsA Ml Ml 0.00s 0.003 0.034 0.001 bdl Ml 0.@3 0.@7 BaF Ml bdl 0.006 0.004 !d 0.@6 0.@2 Ml bdl Ed st} bdl 0.003 0.005 0.010 trd Ml bdl 0.@3 0.@2 nd Ns' Mt 0.016 0.019 Ml 0.165 0.207 0.148 0.029 0.037 0.165 1.869 l.t7l 1.828 1.803 1.849 1.533 1.785 1.685 1.454 1.742 Rb' 0.0s2 0.62 0.070 0.067 0.107 Ml 0.@4 bdl bdl o.Mz Ca' onr4 0nr5 onr? nnro oms oml onol 0002 0.005 0.@l

L 0A42 0.778 0.421 0.525 dd 0.118 0.133 O2l2 0.222 nd 0.003 0.003 0.017 0.006 Dd 0.@2 0.m5 bdl 0.@s nd oH' ?sss ?rrs 7*) ?6s 4rn0 ?flto 3 2 3?88 3.773 4.000

bd-belwd4dd liniE !d-mtddqEh€d; td@EiErdbyeichioEftysh@F- @d cl-dde,tqEirc4 l,2 - bidiE inb@l patingplme;3.a-biotibhBalia5-prituEybidiEfi@bqdq@-ch@i@ldslFirbyRClr@m,l98l;6'7-@ flak6ofd|MviF,8,9-ftltytI]]lsitehEqtix,l0-prin4ytlllls@itetoEthtAb+@+I\,l5as@blag6-^hnidldtlFis byR. Chaped, 1981; ll -p@NbsrhiqirehlaglrMwithrclieofbidire; 12-bsthiqi@acssubh@tioof thebiotib @p@qt (bssed @ tolrt Koo in mpositimof #l I rclatiwtoEalto€d biotitepwwith826 wPlo &O) @d adjotuslb lWo.

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Siderophyllite "Eastonib' Siderophyllite KrFenAlrlsloAlnorl(oH,Do K2MgaAl2[SlaAlaoal(oH,Da .,. nvr=0--t-

(B)

Id=1-

Phengite atomic7o Me -K2MO6[sl6Alzo6i(oH,% K2F66[sleAl2oa](oH,% I I Annite Phlogopite sit sils sik sijz

Flc. 9. Compositional characteristicsof the pseudomorph-forming micas. A. Quadrilateral diagrarn for the ferromagnesianmicas (after Denr et al. I 992). Biotite compositions plot in the annite quadranglebut with substantial phlogopite and siderophyllite components, outside the shaded field representing the variability of most natural micas in this system. Dots: biotite in the basal parting planes, x: biotite flakes in matrix, *: primary annite from wall zone of the GL-8W pegmatite. B. The K-Fe- AI-Si plane of the vector diagram for micas (dern! & Burt 1984). The biotite compositions (symbols as in A) plot close to the annite end member, with subordinate percentage ofsiderophyllite and considerable octahedral vacancies. Muscovite shows a substantial proportion of the phengite component and excess of octahedral cations. Symbols: triangles: coarse crystals, squares: flakes in matrix; diamonds: primary muscovite from the core margin of the GL-8W pegmatite.

mineral (Table 3). In general, all varieties of muscovite in the pseudomorphs are distinctly phengitic, with considerably elevated Mg and pafticularly Fe contents (6.15to 6.55 Si, 0.15 to 0.30Mg, 0.36to 0.68Fe, and 0.10 to 0.35 vln per 24 anions;Table 3, Fig.9). In contrast, its contents of Rb, Cs and Ca are negligible, and Na also is distinctly low. As in the case of biotite, the octahedral-site vacancies may be lower if some Li is present. The F content is highly variable, as are the mi nor amounts of Mq Zn and Ti. The sum of interl ayer cations tends to be low, associated with low wt.Vo totals (Table 3), suggestingan iltitic character of some of the matrix muscovite. All of the above features become particularly conspicuous when compared to the compo- FIc. 10.Biotite (grey)in part replacedby porousaggregates of sition of primary muscovite from the wall zone and berthierine(white); back-scatteredelectron image; the the late albitic assemblage,which closely conforms to scalebar is 20 pm long. the ideal composition of muscovite (Table 3, Fig. 9). Beryl III. Comparison of the partial chemical compositions shows that the primary coarse-columnar A 14 A diffraction peak was observed only in three beryl I, the inclusions of beryl II in garnet, and the samples,two of them from locations other than the secondaryberyl III dispersed in micaceous aggregates GL-8W pegmatite. This indicates, in conjunction with of the pseudomorphs do not show any systematic optical observations of Goad (1984), the rare presence differences that could be genetically meaningful, of a true chlorite phase. However, the relationship of except a progressive increase in Na (Table 2). The chlorite to biotite and berthierine could not be observed. secondary beryl III does not show any particular Muscovite. The coarse tabular muscovite (Fig. 8C) enrichment in Fe or Mg relative to the other types. The has perceptibly higher (K Na) andAl, and lower Si, Fe, contents of all elements summarized in Table 2 are Mg and Mn, relative to the other textural types of this moderate, considering their ranges established for beryl

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from granitic pegmatites in general (cl Aurisicchio el in the Greer Lake pseudomorphs corresponds to -2.6 al. 1988), and for beryl of the Greer Lake pegmatites in wt.VoBeO in the cordierite precursor, provided all Be particular (Cernf & Turnock 1975). was preserved during the breakdown of cordierite in the pseudomorphs, and incorporated into beryl alone. Miscell.aneous mirnr phases. Fine-grained, anhedral Unless someBe was totally dispersed,the first condition apatite and ilmenite are dispersed among the silicates seemsto be fulfilled, as no other secondaryminerals of constituting the pseudomorphs,and they most probably Be are known in the pseudomorph-bearingpegmatites. are of secondary origin. Ilmenite is in part altered The secondcondition is not necessarilymet, as some to "leucoxene". A single cluster of microscopic fibrous Be could have been incorporated into the secondary crystals was observed, with the composition of an sheetsilicates. Consequently,tle maximum Be content aluminosilicate of Mg and Fe containing 3.5 to 6.5 of the cordierite could have been slightly higher. rttt.VoZnO, but yielding a low total of only 80-81 wt.7o. This phase also seems to belong to the alteration Garnet + beryl IL The composition of the garnet assemblage, rather than to represent an inclusion inclusions in cordierite shows this spessartine to originally contained in the cordierite precursor. be distinctly different from that dispersed in the intermediate zones ofthe host pegmatites. The garnet DtscusstoN inclusions evidently are compositionally specific to the cordierite-bearing assemblage.The high Mn content of Primary crystallization garnet in the internal zone and in the core of garnet grains surrounding this zone suggeststhat an increase Cordierite + beryl /. Cordierite crystallized in the in the activiry of Mn, which does not substantially enter waning stagesof magmatic consolidationin many of the cordierite, could have been the primary cause of the Greer Lake pegmatites and in pegmatitic pockets garnet nucleation. The gradual decreasein the Mn of the Greer Lake leucogranite,simultaneously with content of garnet toward the surface of its grains K-feldspar and quartz in the core-margin zone. indicates that decreasingactivity of Mn eventually According to London's model of pegmatite crystalliza- intemrpted crystallization of garnet. The ghost-surface tion, the parent medium was a volatile-enriched but zone of spessartine+ beryl II precipitated after initial homogeneoushydrous melt (London et al. 1989,London crystallization of cordierite alone, and spessartine + 1992). In the given category of beryl - columbire beryl II continued to crystallize intermittently in the subtype of rare-element pegmatites in general, and in outer parts of cordierite crystals. The bulk of this those of the P-, B- and F-depleted Greer Lake group in particular, the liquidus and solidus temperatures can 4 kbor Pt O be estimated at -680 and -520oC (by interpolation betweenthe data for the haplogranite+ I{2O system,and for the Macusani glass: London et al. 1989). Conse- quently, the temperature of cordierite crystallization can be roughly estima[edat -550oC and, in view of the aforementioned occurrences of petalite in cogenetic pegmatites of the Greer Lake group, the maximum pressurecould not have exceeded-2.8 kbar (Fig. I 1; cl London 1986, dern! 1991). These conditi,ons are marginally within the stability field of the cordierire + K-feldspar + quartz assemblage(Seifert 1976). Crystallization of the Greer Lake cordierite proceeded in a Be-rich melt sarurated with respect to beryl; this phase is abundant in the cordierite-bearing zones, It- and it locally coexists with cordierite in mutual 3@ 4@ 500 600 700 intergrowth (coarse-columnar beryl I). Experimental "c work of Povondra & Langer (l97la) shows solid FIG. 11. Crystallization of the beryllian cordierite and its solution of cordierite and beryl to be negligible. alteration in terms of the P-T diagram. Symbols: Ip and However, the entry of Be into cordierite by the substi- sp: approximate liquidus and solidus of the Greer Lake tution NaBeAl_,, first observed on natural material pegmatites,respectively; arrow: cooling and decompression pathway of the pegmatites; solid square: approximate by Cernf & Povondra (1966), is extensive @ovondra & temperature of crystallization of the beryllian cordierite; Langer 197lb). Under the P-T conditions estimated double line: internal subsolidus range of the pegmatites above, the Greer Lake cordierite could have contained and bulk of the cordierite alteration; vertical ruling: as much as 0.60 Be apfu (atoms per formula unit), approximate maximum temperature of thermal equilib- corresponding to -2.5 wt.Vo BeO. The maximum ration of pegmatites with the host rocks. See text for concentration of -20 volume 7o of secondarv bervl III sources of data.

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assemblagerepresents cocrystallization of cordierite crystals; during subsequentcrystallization of cordierite, with a highly manganoanperaluminous silicate, and excess of Mn and Si were accommodated in the with beryl II enclosed in the garnet. intermittently crystallizing isolated garnet + beryl II Synthetic manganoanend-member cordierite was aggregates. prepared by Snow (1943), Eberhard (1962) and Dasgupta et al. (1974), but according to the last authors Subsolidusprocesses its stability field does not exceeda P(H2O) of I kbar. Natural cordierite does not contain more than 15 mol.Vo Breakdown of cordierite. As shown in experimental of the Mn end-member,"manganocordierite"; even this work (Seifert & Schreyer 1970, Seifert 1976) andin content is rather exceptional, attained only in numerous examples of natural occurrences, cordierite sekaninaitefrom Japanesepegmatites (Leake 1960). becomes unstable at low temperafuresin the presence Sekaninaitefrom its type locality also is somewhat ofH2O in general, and ofalkali-bearing aqueoussolu- enriched in manganese,but its content does not exceed tions in particular. The assemblagemuscovite + biotite 1l mol.Vo of the Mn end-member (Schreyer 1965, qualitatively corresponds to many cases of cordierite Schreyer et al. 1993, (.ern! et at. 1997).If the Mn breakdownexamined at other localities (e,g.,Layman content and Fe/Mn value of green biotite (Iable 3) ftom 1963), although the dominance of muscovite is the pseudomorphs examined can be taken as at somewhat unusual. Besides incorporation of K(>> Na) least a very approximate indicator of the Mn content and OH (> F), the alteration of cordierite must have ofthe cordierite precursor, as suggestedby the data of been accompanied by considerable removal of Mg Povondra et al. (1984) and Schreyer et al. (1993),then and Fe and their total dispersal in an open system; no the Greer Lake cordierite would conform to the above hydrothermal ferromagnesian minerals were found in limits. the pseudomorph-bearingpegmatites. Dasgupta et al. (1974) found the "manganocor- The alteration to muscovite + biotite probably dierite" MnrAl*Si5O,r.-rH2Obreaking down to a denser proceededat -50G450'C (Fig. ll), as approximated assemblageof spessartine+ aluminosilicate + quafiz at from experimental data of Michel-L6vy (1960), a P(HrO) above 1 kbar. It is conceivablethat t}re assem- Schreyer (1965), Seifert & Schreyer (1970) and Seifert blage of ferroan spessartine+ beryl II associatedwith (1976). For this temperaturerange, the pressureshould the Greer Lake cordierite represents an analog of the have been -2.7-2.6 kbar P(HrO), given the P-T course above synthetic product in a Be-rich environment,in of consolidation of the nearby (and broadly related) which the aluminosilicate + quartz pair is substituted Tanco pegmatiteestablished by London (1986).What by beryl, a phaseless aluminousbut more silicic than could have been the nature of fluids responsiblefor this an aluminosilicate. However, no relevant experimental alteration? data are available for the BeO-MnO-AlrOr-SiO2-H2O Low-pressure conditions transitional from upper- system. greenschist to Iower-amphibolite subfacies were Except for minor biotite restricted to the wall zones anained in the greenstone-belt lithologies adjacent to of the Greer Lake pegmatitesand the cordierite dis- Greer Lake (-500'C; Trueman 1980, A.C. Turnock, cussed,ferroan spessartineis the only ferromanganoan pers. commun. 199'1). At that stage, the gneissic phase precipitated during the main stages of the basement that hosts the Greer Lake pegmatite group pegmatite consolidation. Besides the spessartine must have been uplifted to the same P-T conditions, enclosed in the sparse cordierite, abundant ferroan despite the general lack of retrograde reactions. spessartineis dispersedin the intermediateand near-core As discussedin Cernf et al. (1981) and Cernf (1991), parts of the pegmatites, and represents the only the peraluminous Greer Lake leucogranite and its Fe,Mn,Mg-bearing phase ip most of the volume of the pegmatite aureole were emplaced after the peak of pegmatitedikes. The slightly Mn-dominant composition regional dynamothermal metamorphism, incipient of this spessartine(Fig. 5) indicates that the pegmatite- retrogression and subsequent faulting of the host forming melts attained an appreciable degree of greenstonebelt, i.e., after cooling of the country rocks Mn fractionation from Fe. Under the P-T conditions to temperatures well below the metamorphic peak, to estimated above, the entry of Mn into the cordierite approximately 450 to 400"C (Trueman 1980). Thus the structure was severely limited; consequently,precipita- alteration of cordierite must have proceededbefore tion of spessartine(distinctly enriched in Mn relative thermal equilibration of the cooling pegmatite dikes to its counterparts in the albitic units) was triggered with the enclosing mehmorphic assemblages(Fig. 1l). along with stabilization of beryl II. Spessartine+ Consequently, the breakdown of cordierite charac- beryl II apparently substitute for the beryllian teized above must have been promoted by residual "manganocordierite" component of the system. After postmagmatic fluids released from the solidifying an initial period of Mn build-up in the cordierite- pegmatite-forming melt. The relatively high proportion crystallizing melt, the ghost zone of spessartine+ beryl of F in the biotite and muscovite supports this II crystallized on the surface of growing cordierite conclusion. In contrast, the late replacement ofbiotite

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by berthierine was most probably a low-temperature Hawthorne. Field work was also supported by the process that could have been triggered by late meta- Canada - Manitoba Subsidiary Agreement on Mineral morphic fluids, permeating the pegmatites a.fterthermal Exploration and Development, 1975-1979. The authors equilibration with country rocks. are indebted to B.E. Goad and M. Egan for initial Precipitation of beryI III. To the best of our reconnaissancework on the pseudomorphs, to R. krowledge, no experimental work is available on the Chapman for assistanceduring the electron-microprobe behavior of beryllian cordierite in low-temperature analysis,and to N. Ball for help in the X-ray-diffraction hydrous, alkali-bearing systems. Consequently, no work. Special thanks are due to Iva Cernf, who P-T-X constraints can be derived from the mere persuadedthe second author in 1969 to have a second presence ofberyl in the alteration products. It should look at the first fragments of the pseudomorphs be noted, however, that the stability of beryl may be collected in the field, which he initially dismissedas relatively extensive in this environment, as the mineral xenoliths of a garnetiferousschist. We much appreciate assemblagescontaining secondaryberyl are diversified: the constructive reviews by A.J. Anderson, D.B. Clarke, muscovite + chlorite (Vr6na 1979), muscovite + E.S. Grew, R.F. Martin and W. Schreyer, which chlorite > paragonite + qtraftz (Povondra et al. 19841, straightened out two consecutive versions of the muscovite > biotite > berthierine (this study). In any manuscript. case, the stabiliry field of beryl whose composition is near-ideal to slightly Na,Fe,Mg-bearing evidently RSFEnnNcES extends to much lower temperatures than that of beryllian cordierite - sekaninaite lcf. Burt (1978), ARMBRUsTER,T. & Inouscrer,A. (1983):Cordierites from --> Barton (1986), and experimental work quoted thereinl. the lrpontine Alps: Na + Be Al substitution,gas content,cell parameters,and optics. Contrib. Mineral. CoNcr-uolNc CoNaMsNTs Petrol.82,389-396. AuRrsrccHro,C., Fronavaln,G., GRUBEssr,O. &ZtNezzt, The three occurrences of secondary beryl docu- P.F.( 1988):Reappraisal of thecrystal chemistry of beryl. mented to date indicate that under certain but so far Am. M in e ral, 73, 826-83'7. unconstrained conditions, beryl may be a stable phase in alteration products after beryllian cordierite. BAADscAARD,H. & dEnNY,P. (1993):Geochronological However, natural mineral assemblagesgenerated by studiesin the WinnipegRiver pegmatitepopulations, hydrothermal breakdown of this phase show that southeasternManitoba. Geol. Assoc. Can. - Mineral. they may be highly sensitive to rather minor changes Assoc.Can., Program Abstn 18, A-5. in P-T conditions, composition of the aqueousfluid, and open yers&s restricted or closed character of the BARroN,M.D. (1986):Phase equilibria and thermodynamic - - - reacting system. Differences are evident even among propertiesof mineralsin the BeO Al2O3 SiO2 H2O (BASH) system, with petrologic applications.Am. the three assemblagescontaining secondary beryl, as Mineral.7l,277-3O0. quoted above. Further diversity, promoted by increased activity of additional components,can be illustrated by Bunr, D.M. (1978):Multisystems analysis of beryllium -r the example of Ca, which generates the celadonite mineralstabilities: the system BeO - AlrO3- SiOz- HzO. milarile >> bavenite assemblagesurrounding the so-far Am. Mineral. 63, 664-676 u nexplored p_seudomorphs after beryllian cordierite from VEZn6(Cernf 1968), and milarite associatedwith dBnNf, P. (1967): Notes on the mineralogyof some biotite-rich pseudomorphs from Biskupice (eernf West-Moravianpegmatites. eas. Mineral. GeoL 12, 1967). 461-463(in Czech). We intend to expand our study to pseudomorphs (1968):Berylliuniminerale in Pegmatitenvon after beryllian cordierite from other localities, and to VdZn6und ihre Umwandlungen.Ben deutsch.Ges. geol. expand the analytical approach by ion-microprobe Wiss.,Reihe B, Mineral. l,agestiittenk.13(5), 565-578. determinations of Li and Be in cordierite as well as its breakdown products. The need for experimental work (1990):Distribution, affiliation and derivation of on the breakdown of beryllian cordierite is of course rare-elementgranitic pegmatites in the CanadianShield. clearly indicated. Ge ol. Rundschau 79. 183-226.

(1991): granitic pegmatites. I. ACKNOWLEDGEMENTS Rare-element Anatomy and internal evolution of pegmatite deposits. Geosci.Can. 18,49-67. This study is based on a laboratory assignment to S.J.-B. for an undergraduate course in genetic & BuRr, D.M. (1984): Paragenesis, mineralogy. The researchwas supportedby NSERC crystallochemical characteristics, and geochemical Researchand Major Installation Grants to P.e. and evolution of rnicas in granitic pegmatites. 1z Micas (S.W. Equipment plus Infrastructure Grants to F.C. Bailey, ed.). Rev.Mineral. 13,257-291.

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