8L7

TTrc C anadian M ine ralo gi s tVol. 33, pp. 817-833 (1995)

THE RARE-ELEMENT.ENRIGHED MONZOGRANITE - PEGMATITE -OUARTZ VEIN SYSTEMS IN THE PREISSAC-LACORNE BAf,HOLITH. OUEBEC.

II. GEOCHEMISTRY AND PETROGENESIS

T}IOMAS MI"ILIA, ANTI{ONY E. WILLIAMS-JONES, SCOTT A. WOOD1 AND MICHEL BOILY2

Deparfinent of Eanh and Planetary Sciences, McGiIl University, 3450 University Street, Montreql, Qucbec H3A 2A7

ABSTRACT

The Archean Preissac-Iacorne batholith in northwestem Qu6bec includes four felsic plutons (Preissac, Moly Hill, Lamotte,Lacorne), which are zoned from biotite to muscovite monzogranite. The l.amotte and Lacorne plutons are associated spatiallywith rare-element pepatites, whereas pegmatites are absent from the Moly Hill pluton and do not contain rare-elementminerals in the Preissac pluton. The mre-element pegmatites are zonally distributed from beryl-bearing in the plutons tospodumene-bearing in the county rocks. Molybenite-bearing quartz veins are associated with all four plutons, and in the caseof the l,amotte and Lacorne plutons, occur beyond fhe spodumene pegmatites. Dikes of molybdenite-bearing albitite occur nort!of the Lacome.pluton. All the plutons are weakly to moderately peraluminous (A/CNK: 1.0-1.3) and exhibit a compositionalcontinuum in major- and trace-element contents from biotite to muscovite nonzogranite. This conpositional continuum extendsto the rare-element pegnatites, indicating that the monzogranites and pegmatites are comagmatic. The chemisty of thepegmatites suggests that the!"utiderwent further evolution from beryl-bearing to spodumene-bearing varieties. The monzo-granites and pegmatites trave 6l8O15prog values (8.6 tl.3%oo). The zonation of the plutons, the geochenical trends, and theoxygen isotopic compositions incficate'that the vaxious types of moMogranite were mainly the products of fractionalcrystallization. Trace element (Rb, Ba, Sr) modeling of the l,amotte and Lacorne plutons suggests that the most fractionatedmonzogranite could have been formed by 8V90Vo crystallization of the magma that formed the biotite monzoglanite. A modelis proposed for the evolution of the Lamotte and Lacome plutons, in which side-wall crystallization produced their observedquasi-concentic zonation, and created volatjle-rich residual melts. These nelts were subsequently injected sequentially into theoverlying parental monzogranite and later the country rocks, producing zonally dishibuted beryl and spodumene pegmatites,respectively. Fluids exsolved from the most evolved pegmatites back-reacted with earlier-crystallized spodumene-bearing apliteto form albitite, or separated from the melts, filling fractures as molybdenite-bearing quartz veins. The smaller Preissac andMoly Hill plutons, which host molybdenite-bearing quartz veins, did not evolve sufftciently to form rare-element pegmatites.Vapor saturation occured during late crystallization of the muscovite monzogranite, and culminated in the formation ofmolybdenite-bearing quartz veins, which filled fracnres in the overlying crust of previously solidified magma.

Keywords: rare-element monzogranite, granitic pegmatite, beryl pegmatite, spodumene pegmatite, molybdenite-bearing albitite,quartz veins, geochemistry, fractional crysrallization, Preissac-Lacome batholith, Archean, Qu6bec.

Sorwtans

Le batholite arch6en de heissac-l,acorne, dans le nord--ouest du Qu6bec, comprend quatre plutons monzogranitiques

@eissac, Moly Hill, Lamotte, Iacorne) pr6sentant une zonation p6rologique depuis un facibs i biotite jusqu'b un facibs imuscovite. Les plutons de l,amot0e et de Lacome renferment de plus des massifs pegoatitiques h 6l6ments rares, tandis que lepluton de Moly Hill n'a pas de cortlge de pematites, et celles qui sont associ6es avec le pluton de Preissac ne montrent pasd'emichissement en 6l6ments rares. Les pegmatites e 6l6ments rares d6finissent une zooation e paxtir d'un facibs b b6ry1 dans lesplutons i un facibs b spodumbne dans les roches encaissantes. Les veines de quartz i molybddnite sont associ6es au( quatreplutons, et dans le cas des plutons de Lamotte et de l,acorne, se trouvent au deli des pegmatites i spodumdne. Des filonsd'albitite I molybd6nite sont disposds le long du flanc nord du pluton de Lacome. Tous les plutons sont faiblement i mod6r6-ment hyTreralumineux (A/CNK: 1.0-1.3), et contiennent un spectre continu de compositions (6l6ments majeurs et traces), demonzogranite i biotite b monzogranite i muscovite. Ce specfie se poursuit jusqu'aux pep.atites a 6l6ments rares, indication queces deux groupes de roches seraient comapatiques. D'aprds la composition des pegmatites, il y aurait eu 6volution progressivedu facibs h b6ryl vers le facibs d spodumbne. les nonzogranites et les pegmatites partagent des valeurs du rapport b'uOlsuowl(8.6 tl.3%o).la zonation des plutons, les tendances g6ochimiques, et la composition isotopique de I'oxyg0ne montrent que lesdivers types de monzogranite ont 6t6 produits par processus de cristallisation fractionn6e surtoul D'apres un moddle de

1 Present address: DE)arbnent of Geology and Geological Engineering, University of ldaho, Moscow, Idaho 83843, U.S.A.2 Present address: G6on, 10785, rue St-Urbaiq Montr6al, Qu6bec H3L 2V4.

818 TrucANADIANMTNERALocTsT

fractionnement fond6 sur la distribution des 6l6ments traces Rb, Ba et Sr daas les plutons de Iamotte et de l,acorne, le magmamonzogranitique le plus 6volu6 aurait bien pu r6sulter par cristallisation d 8G-907a du magna responsable pour le monzograniteI biotite. Une cristallisation le long des parois aurait produit une qistallisation quasi-concentrique, et un effichissement d'unephase aqueuse dans les volumes de magma r6siduel. Ces venues de magmas ont pax la suite 6t6 inject6es dans le monzogranitesusjacent, et ensuite dans les roches encaissantes, pour produire un essaim de filons de pegmuite d b6ryl et i spodumbne,respectivement. Une phase fluide exsolv6e des pegmatites les plus 6volu6es a ensuite r6agi avec une aplite pr&oce i spodumbnepour produire les filons d'albitite, ou bien a rempli des fractures avec quarE + molyM6nite. Les plutons plus petits de heissacet de Moly }Iill, qui renferment des veines de quartz + molybd6nite, n'ont pas 6volu6 suffisamment pour produire des pegmatitesenrichies en 6l6ments rares. Dans ces cas, la saturation en phase aqueuse dans le magma monzogranitique i muscovite 6taittardive, et mena 6ventuellement i la formation de veines de quartz dans les fractures de la coquille sup6rieure de la chambremagmatique solidifi6e.

Clraduit par la R6daction)

Mots'cl6s: monzogranite l dl6ments rares, pegmatite granitique, pegmatite n Mryl, pegmatite i spodumbne, albitib emolybd6nite, veines de quartz, g6ochimie, cristallisation fractionnde, batholite de heissac-Lacorne, arch6en, Qu6bec.

Ivmoouctton

Recent investigations of rare-element-enrichedgranites and granitic pegmatites have been concemedmainly with the very late stages of the evolution of themagma, particuQrly the transition to subsolidusprocesses (e.g., Cem! et al. 1986, London 1986,Trumbull 1993, Linnen & Williams-Jones 1994).Howevero comparatively few studies have focused onthe early_ history of -magmatic crystallization (e.g.,Goad & Cernf 1981, Cem! & Meintzer 1988, Breaks& Moore 1992,Shearcr et al. 1992), which controls thepattern of subsequent enrichment of the residual meltin the rare elements @umham & Ohmoto 1980).Another feature that is poorly documented is the zonaldisribution of bodies of granitic pegmatite with respectto their predominn6 rare-element-bearing minerals,e.g.,the occurrence of beryl-bearing granitic pegmatitewithin and close to the parental pluton, and ofspodumene-bearing granitic pegmatite further away, inthe county rocks.

The Preissac-Lacome batholith in Qu6bec (Frg. 1)contains several excellent examples of zoned morzo-granitic intrusions and associated bodies of rare-element-enriched granitic pegmatite and Mo-bearingquartz veins. The monzogranites vary from biotite- tomuscovite-bearing varieties, and show continuousvariations in the composition of plagioclase, biotite andmuscovite, suggesting that tle various subtypes ofmonzogranite are comagmatic (MuUa et al. I995b).This compositional continuity extends to the bodies ofrare-element pegmatite, thereby linking the evolutionof the monzogranite to the formation of the Be-, Li-,M- and Ta-enriched pegmatites. The distibution ofpegmatite bodies is zoned, from beryl-bearing varietieswithin the monzogranitic plutons to spodumene-bearing varieties in the county rocks @gs. lA-D).hevious investigators (Dawson 1966, Boume & Danis1987, Feng & Kerrich 1992)havepresented some geo-chemical data on the Preissac-Lacome monzograniles,but have not distinguished between types of monzo-

granite (cf. Mulja er al. L995b) nor analyzed the asso-ciated pegmatites. In this study, results of 85 chemicalanalyses of samples representing the various subfypesof monzogranite and pegmatite are used to decipher thegeochemical evolution of the plutons. These data areintegrated with geological and mineralogical data(Mulja at al. 1995b) to develop a model that explainsthe processes responsible for the development of zonedrare-element-endched granitic systerns in and aroundthe heissac-Lacome batholith.

GBot ocrcar, Srrrnqc AND Mn{RALocyOF T}IE MONZOGRANUE

The Preissac-Lacorne batholith crops out over anarea of approximately 600 km2 in the southern Abitibisubprovince of the Superior province, and wasemplaced in mafic to felsic volcnnic rocks of theKinojevis and Malartic groups, and biotite schist ofthe Kewagarna Group @g. 1; Dawson 1966). Infusiveactivity took place in two stages; the first stage at2671-2675 Ma (Feng & Kerrich 1991) was marked bythe emplacement of syntectonic gabbro to granodiorite,and the second stage at 2630-2655 Ma (Gari6py &Alldgre 1985, Feng & Kerrich 1991, Feng et al. L993),by the emplacement of post-tectonic monzogranite andpegmatite. This plutonism is believed to have occurredlate in the development of the Abitibi Greenstone belt,and to have involved collision of continents in aconvergent-plate setting @imroth et al. 1983). Theearly Preissac-Lacorne intrusions are interpreted toresult from partial melting of a mantle wedge above asubduction zone; the later intrusions are considered tobe products of the melting of Pontiac Subprovincesedimentary rocks, which were paxt of a continentalcrust thickened by collision @eng & Kerrich 1992).

The monzogranite forms four plutons, two of which,Lamotte and Lacorne, host and are surrounded bynumerous rare-element-enriched pegmatite bodies. Theother plutons, Preissac and Moly Hill, are associatedwith fewer bodies of pegmatite, most of which do not

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contain rare-element-bearing minerals. Molybdenite-bearing quartz veins and stockworks occur in or nearall of the plutons.

Detailed mapping by Mr;J;1a et aI. (1995b) has shownthat each of the monzogranite plutons comprisesbiotite, two-mica and muscovite subtypes. The mostwesterly pluton, Preissac, is dominated by two-micaand muscovite monzogranites; biotite monzograniteoccurs only as a narow marginal facies adjacent tomuscovite monzogranite in the norfhern part of thepluton. In addition to the principal subtypes of monzo-granite, there are also fine-grained dikes ofmuscovite-garnet monzogranite (Frg. 1A). Contactsbetween the principal subtypes of monzogranite are notexposed, except for that between biotite and muscovitemonzogranite, which is marked by na:row, anasto-mosing veins (< 20 cm wide) containing quartz andK-feldspar. The three subtypes of monzogranite in theMoly Hill pluton form discrete bodies, which areseparated by screens of mafic volcanic rocks andbiotite schists. The contact relationships among themonzogranite subtypes are hidden, except at onelocality, near the edge of the muscovite monzogranite,where there is a gradual transition from fwo-mica tomuscoyite monzogranite (Fig. lB). Both the Lamotteand Lacorne plutons are quasi-concentrically zoned,with the outer unit of biotite monzogranite giving wayinward to two-mica and muscovite monzogranites. Aseparate intusion of muscovite monzogranite forms anL-shaped dike in the southern part of the Lacornepluton. Contacts between subtypes of monzogranite arenot exposedo except in the northern part of the Lacornepluton, where the biotite monzogranite grades imper-ceptibly into two-mica monzogranile.

All subtypes of monzogranite vary from fine tocoarse grained (crystals up to 0.5 cm across) andconsist essentially of quartz (25-35 vol.%o), plagioclase(3O45Vo) and perthitic microcline (2545Eo), approxi-mately 5Vo biotite + muscovite, up to 3Vo garnel andminor epidote. Biotite monzogranite is distinguishedfrom the other types ofmonzogranite by the presenceof biotite and a lack of primary muscovite; the two-mica monzogranite contains muscovite and biotiteroughly in the proportion 2:7, and muscovite monzo-granite contains little or no biotite. Biotite and garnetshow an antithetic relationship in the transition frombiotite to muscovite monzogranite. In the compo-sitional range from biotite to muscovite monzogranite,the plagioclase composition varies from Anrr5 to An5,the Fe/@e+Mg) value of biotite increases from 0.69 to0.85, and the Fe(Fe+Mg) value and NAI of muscoviteincrease from 0.65 to 0.85 and from 1.55 to 1.72 atomsper formula unit, respectively. The garnet is aspessartine-almandine solid solution (Spseo_ooMss_:z) and does not show systematic compositionalvariations with monzogranite subg4les. The accessoryminerals are xenotime, apatite, zircon, monazite,titanite, magnetite and ilmenite. In the muscovile and

muscovite-garnet monzogranite subtytrles, columbite-tantalite and molybdenite also are present as accessoryminerals. With the exceptions of xenotime andilmenite, which are present in similar amounts(< 0.1 vol.Vo) in the main subtypes of monzogranite(xenotime is not present in the muscovite-garnetmonzogranite dike), the abundances of all accessoryminerals decrease systematically from biotite tomuscovite monzogranite.

The bodies of rare-element-enriched pegmatiteoccurs as vertical to subhorizontal dikes (up to 8 meterswide and about 200 meters long), which generallysftike east-west and randomly in the Lacorne andLamotte plutons, respectively. On the basis ofthe predominant rare-element-hosting minslals, thspegmatite is subdivided into beryl-bearing,spodumene-beryl-bearing and spodumene-bearingtypes. In the text that follows, these types of pegmatiteare referred to as beryl, spodumene-beryl andspodumene pegmatites. These bodies of pegmatite arezonally distributed, from beryl pegmatite in themonzogranite through spodumene-beryl pegmatite atand near the margins of the monzo$anite, to spo-dumene pegmatite in the country rocks @igs. lA-D).Beyond the spodumene pegmatiie to the north of theLacorne pluton is a set of east-west-frending dikes ofmolybdenite-bearing albitite, which occupy fracturesin the intercalated schist and basalt. The albitite dikesvary from 20 cm to 1 m wide and dip steeply 75'to thesouth.

The beryl pegmatite and muscovite monzograniteare mineralogically similar, except that the pegmatitecontains beryl, more gamet (up to L0 vol.Vo), consider-ably more columbite-tantalite (up to I vol.Vo), andtraces of galnite. Spodumene-beryl pegmatite in theLamotte pluton is distinguished from beryl pegmatiteonly by the presence of spodumene in the center of thebody of pegmatite. In contrast, spodumene-berylpegmatite associated with the Lacome pluton hasspodumene throughout variable amounts of lepidolite(up to 5 vol.Vo), and taces of schorl and pyrophanite.The spodumene pegmatite differs mineralogically fromthe spodumene-beryl pegmatite in containing muchmore spodumene (up to 30 vol.Vo), much less micro-cline (<10 vol.7o), less garneg and generally no beryl orlepidotte. Both beryl and spodumene-beryl pegmatitesare zoned from a garnet-rich aplite border tlrough analbite - perthite - qlJartz - muscovite zone to a core ofmassive quartz. In contast, bodies of spodumenepegmatite are not zoned, or subtly zoned where largecrystals of perthite occur in the inner part of thepegmatite. The albitite is an almost monomineralicrock, composed mainly of euhedral to subhedral albite,variable amounts of molybdenite, and fraces of zircon,Ta-emiched ilmenite and gamet.

The plagioclase composition in the pegmatite isAnr-5, and the Fe/(Fe+Mg) and rvAl of muscovile aresimilar to those of muscovite monzocranite. The Cs

TIIE PREISSAC-LACORM BATIIOLITII, QIr'EBEC 82r

conte[t of muscovite in the muscoviie monzograniteand rare-element pegmatites correlates positively withthe contents of Ta and Rb, and negatively with those ofSc and K/Rb. The composition of columbite-tantaliteranges from ferrocolumbite in beryl pegmatite tomanganotantalite in spodumene pegmatite (Mul1a et aI.1995a).

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Analytical mzthodx

Concentations of the major and trace elements weredetermined with X-ray fluorescence spectromeuy atMcGill University, the Universit6 de Monfi6al, and theCenfte de Recherches Min6rales du Qu6bec. The

concenftations of rare-earth elements (REE') in selectedsamples were deterrnined by instrumental neutron-activation analyses at the Universit6 de Montr6al, andby inductively coupled plasma - mass spectrometry(ICP-MS) at Memorial University of Newfoundland.Twelve whole-rock compositions from Bourne &Danis (1987) and Feng (1992) were included in thisstudy.

Major and trace elements

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822 THE CANADIAN MINERALOGIST

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g l:11 " '4

.- ll] *"h'F* .0.2{---.---r---,--- 9.2

the data, generally, the SiO2 content increases frombiotite to muscovite monzogranite (to muscovite-gamet monzogranite in the Preissac pluton), whereasthe K2O, CaO, MgO, FerO3 and TiO2 contentsdecrease (Fig. 2); the NarO, MnO and AlrO.contents do not vary. Two exceptions to these general-izations are the trend of decreasing CaO content withmonzogranite evolution in the Lamofte pluton, and thelow SiO, content of the highly evolved muscovitemonzogranite dike in the Lacorne pluton.

lJte concentration ofBa, Sr and Th decreases frombiotite to muscovite monzogranite (Frg. 3). T\e Zrcontent also decreases, but only in the Lamotte andLacorne plutons. ln contrast, the Rb content increasesfrom biotite to muscovite monzogranite, and thecontent of Nb is highest in muscovite monzogranite.The concentration of trace elements other than thosementioned above, and of Li, Be, U, Ga, F and REE.which are discussed below, do not exhibit systematictrends with monzogranite type.

Besides varying with monzogranite type, thecontents of Ba and Sr also vary spatially withinthe Lacorne pluton. In the northern part ofthe pluton,

0.4a

9o, l ro.4

their contents decrease toward the two-mica monzo-granite (Fig. 4). On the orher hand, in the southernbiotite monzogranite, Ba and Sr contents decreaseinward from the margins of the intrusion.

The distribution of bodies of spodumene pegmatite,i.e., thetr restriction to the Lamotte and Lacorneplutons, is reflected in the relative contents of Li in themonzogranites of the four plutons. The content ofthis element in each of the monzogranite types in thePreissac and Moly Hill plutons is low relative to that inthe corresponding monzogranites in the Lamotte andLacorne plutons. Moreover, the content of Li in themuscovite monzogranite of each pluton is markedlylower than that of the other types of monzogranite. Inthe Preissac and Moly Hill plutons, the conients of Liaveraged over biotite and two-mica monzogranites are92 + L2 ppm and 126 + lL ppm, respectively. Incontrast, the corresponding values in the Lamotte andl,acorne plutons arc 265 I tr07 and 202 + 124 ppm.The amount of Li in the muscovite monzogranite of thefour plutons (ppm) are 30 + 2 @eissac), 18 (MolyHiX), 78 x. 32 (Lamotte) and 173 t 70 (Lacorne,including the muscovite monzogranite dike). The

I *-.,1 t*r,

72 74Si02

72 74 76Si02

3" ll ', ,ry*-

o.2l? n , l '>

- " l r - n

o.ol--.--.tBar-

s l:31 .' ,'-{,i, "-. , , l.iO.2t 0.2

o l5 l . -= ^ ^ l r r r - E -

'v . v - f f i u . uo.Offir|lr|n 0.ol---t--f----+rl:A__-Fr_

d 2 l 2 1 r - t r,S :1 '"

r #4: or 11 to 5o4,1|E-* .

p l8T_- - f f i . : l - - - - - - - ; l i _F il' , ,€s* , { l : ] . , r rs . f ro 15$ iil :'',', T, ','. , ,,J rl: ftjo,'$ ]i1' ,"rr , 9r.rl . ', ' ,P,., r., , rrl :: ':C+ .73 74 75 76 77 73 74 75 76 Tl

SlO2 SiO2

Fig. 2. Plot of major element oxides (wt.Vo) versw SiO2 for all types of monzogranite in the Preissac, Moly Hill, l,amotte andLacorne plutons. In this a:rd subsequent diagrams, the symbols of the monzogranite, unless otherwise indicated are:r, biotite monzoganite; tr, two-mica monzogranite; o, rnuscovite monzogranite; A, dikes of muscovite-gamet monzogranite(Preissac pluton) and muscovite monzogranite (Lacome pluton). There is only one data point for biotite monzogranite of theheissac pluton owing to the small exposure of the rock (Fig. 1, and Mdjaet aI. 1985b).

tr

Preissac Dluton Moly Hill pluton600

Ba 300

1000800

0n

120

sr 8o40

0

Rb 500

300

40Y 2 0

0

z, 8o400

20Th

0

distribution of Be is much more uniform amongthe four plutons, and does not vary appreciably withmonzogranite fype. However, it may be significantthat the Be content is lowest in the Moly Hill pluton(3 t 0.6 ppm versus 4 t I.4, 4.5 t 1.8 and 5.5 +1.8 ppm in the heissac, Lamotte and Lacorne plutons,respectively); the Moly Hill pluton is the only one thatdoes not contain beryl.

Concentrations of U and Ga were determined onlyfor the Lamotte and Lacome monzogranites, and werefound to increase systematically from biotite tomuscovite monzogranite (Table 1B). The averageTh./U values in the Lamotte and Lacorne plutonsdecrease from 5.4 and 7 in biotite monzogranite,through 2.2 and 3.9 in fwo-mica monzogranite, to0.55 and 0.4 in muscovite monzogmnite, respectively.In the same order of types of monzogranite, the meanvalues of AVGa decrease: 3639 -+ 3150 + 2n2 nthe Lamotte pluton, and 3852 -+ 3196 -+ 2796in the Lacorne pluton.

The fluorine content was established in only threesamples of two-mica monzogranite and two samples ofmuscovite monzogranite, all from the Lacorne pluton.

TIIE PREISSAC_I-A,CORNE BATI{OLITII, QTJEBEC

50

Nb go

1 0

50

Y 3 0

1 0

200

Zr 100

040

Th 20

0

823

800

400

3002W100

CUU

40030040

20

70

301 0

100

100

500

06040200

EN

0140100

I*;!1'h'

20

1 0

!10

20

07 3 7 4 7 5 7 6 7 7 r c 7 4 7 5 7 6 7 7

Siq siozSi02 Si02

Frc. 3. Plot of trace-element contents (pm) versrs SiO2 for all types of monzogranite in the heissac, Moly Hill' Lamotte andl,acorne Dlutotrs.

The average F content is 0.03 wt.7o, and does not varywith rock type.

The sum of the rare-earth elements is 1qrv, lsss rhan100 ppm, in all subtypes of monzogranite, except forthe Moly Hill biotite monzogranite, where it isapprorimately 200 ppm (Tables 1A, B). Chondrite-normalized profiles display light rare-earth enrichmentand negative Eu anomalies, and there is a generaltendency for the profiles to flatten and the Eu anomalyto become increasingly negative from biotite tomuscovite monzogranite (Ftg. 5).

Gsocrm\fisrRv oF THE PEGMATITESaNp Ar:rrrrrs

Major and trace ekments

Only a small number of the occurrences ofspodumene pegmatite in the Lamotte pluton, andof beryl and spodumene pegmatites in the Lacornepluton, could be analyzed (rare-element pegmatites donot occur in the Preissac and Moly Hill plutons), owingto the diffrculty of obtaining representative bulk-

Lamotte pluton Lacome pluton

0

I

t

T

I

I

I

I

I

I

f

II

I I

r1

oE|r

trt r t t

824 TIIE CANADIAN MINERALOGIST

1(m

800

600Ea

400

2NEo.o.

140124100806040

Distance (km)Frc. 4. Bulk-rock contents ofBa and Sr along a north-south

traverse (N-S) in the lacorne pluton (Fig. lD), showingchemical trends from biotite to muscovite monzoganite inthe northem part ofthe pluton, and from the margins to thecenter of the southern biotite monzosanite.

samples of these very coarse-grained rocks. In order tomini6lTs the problem of sample heterogeneity,approximately 4 kg of material were collected fromchannels cut across small bodies of pegmatite (<l mwide).

The beryl pegmatite in the Lacorne pluton contains73.5 to 77 wt.Vo SiO2 (most samples contain between76 and77 ,ut.Vo SiO2), and thus is indistinguishable interms of SiO, content from the muscovite molzo-glanite (fables 18, 2). The spodumene pegmatite inthe Lacome and Lamotte plutons contains 73--74 ttrt,Voand 7 4 wt.To SiOr, respectively.

Harker diagrams of major-element oxides againstSiO, for the muscovite monzogranite and thepegmatites do not show any significant variationsexcept for Na./K, which is slightly higher in beryl

pegmatite. The contents of 84 Y and Th decrease,whereas those of Rb and Ga increase. from muscovitemonzogranite to spodumene pegmatite in both plutons(Frg. 6). The contenls of Zr and Iff decrease in the

TABLE 2. BULK COMPOSMON OF PEGMAME AND A.BTTMLamotte plulon Lasome pluton

Rock Spd Brl Spd Ab1 Ab2

73.6 690.01 0.01

16 19.40.16 0.160.13 0.070.01 0.010.06 0.534,1 10 .1

4 0.090.04 0.03

Sr

& 4 6 7 213.6 4 2.25 2

7 2 4 5 1 1 23 1 4 f f i z

4 5 . 7 4 4 22.55 4 2,25 22 5 2 1 D . 2

4 55.5 125 7 2 6 0 26 . 5 2 6 2

26.5 4853 431.3 4

4 2 49 5 41 5 46 0 49.5 4B 44 3 4

257 4

6s

420N

x n x nWorelanant oxldes (vtL o/o)

siq 74.4 2 7s.9 4Tjo, 0.01 2 0.02 440, 16 2 14.6 4Fs,q 0.97 2 05.| 4MnO o.t6 2 0.3 4Mgo A.24 2 0.og 4CaO 0.n 2 0.9 4Nqo 3.91 2 5.13 4I(?0 2.02 2 3.12 4P,oo 0,01 2 0.02 4

Tracealanetts (ppm)B a 1 0 2Rb 110,. 2S r n 2Z t m 2N b f f i 2Y 14.5 2Ta 4Ath 2.4t t 1 a

G a 4 6 2Li 6735 2

x n x n

4 6 8 24 A.02 24 20,1 24 0.11 24 0 2 24 0.01 24 0.13 24 10.8 24 A 3 24 0.05 2

Rar*oarth elements (ppn)La 0.9 2 5.72c€ 1.75 2 18Pr 2,71Nd 9.5 2 10.5sm 0.65 2 8.68Eu 0,1 2 0.03Gd 5.63Tb 0.15 2 0.6SDy 1,62Ho 0.35 2 0.13Er 0..l9Tm 0.02Yb 0.2 0.12Lu 0,1 2 0.02> REE 7,6 54

Al/Ga 1806 ft20T?VU 1.4 0.58En/Eu- 021 0.004

a.z

487553

u.0672

0.20.520.000.240.340.010.330,ffi0.180.m0.020.010.060.012.W

n n a

Aplite

1.86 0.963.35 3.2.0.37 0.511.63 21.r8 1,810.08 0.051.V2 1.430.19 0.160.08 0.470.06 0.050.t1 0.10.02 0.010.16 0.050,03 0.0210.7 10.8

6180 4.8 8.7-9.5 7.0-io,g 0.8 2Brt beryl pegmatite; Spd: spodumene pegmalite; Ab: albith

TIIE PREISSAC_LACORNE BAfiIOLITH, QIJEBEC

100

10

1

.1

100

MOLYHIttPIUTON

Biodte monzograniteTruo-mica monzogranite

825

.1

100

8060

Ba 40200

60

sr 4o200

120

Nb to

40

0

60

40Y

20

0

100

Zt 60

0

Rb to

Muscovltemonzogranitedike I.ACORM PLIITON

Ia CePr Nd SmEuGdTbDyHo ErTmYbLu I.a CePr Nd SmEuGdTbDyHoErTmYbLu

Flc. 5. Chonclrite-normalize<l REE profiIes for the subtypes of monzogranite in all four plutons. The chondrite values are fhoserecommended by Boynton (1984).

Brl Spd

Spd: spodumsneBrl: beryl

pegmatiteMG: muscovlte

monzogranlts

E Lamotts pluton

I Lacome pluton

MG MGdike

Lacome pluton butincrease in the Lamotte pluton frommuscovite monzogranite to spodumene pegmatite. Thecontent of Sr decreases initially from the monzograniteto the beryl pegmatite, but then increases in thespodumene pegmatite, in the Lacorne pluton. Apossible explanation for the high Sr content of thespodumene pegugtite is the breakdown of radiogenicRb (c/ Clark & Cem! 1987). In the Lamotte pluton,the Sr content is higher in the spodumene pegmatitethan in the muscovite monzogranite. The content of Nbincreases from the monzogranite to the berylpegmatite, and then drops in the spodumene pegmatite.This sharp decrease is reflected by the presence oftantalite as opposed to columbite in the latter pegmatite(Mulja er al. 1995b). In the Lamotte pluton, the Mcontent is higher in the spodumene pegmatite than inthe muscovite monzogranite.

Frc. 6. Selected trace-element (ppm) concentrations inthe muscovite monzogranite and types of pegmatite in theLamotte and Lacome plutons. The elevated Sr contents ofberyl and spodumene pegmatites in the Lacorne plutoncould be due to fhe decay of 87Rb (see text).

MG MGdike

Muscbvite

Brl 8F

826 T}IE CANADIAN MINERALOGIST

100

T\e ZREE content of beryl pegmatite (Lacomepluton) is similar to that of the muscovite monzogranite(54 versus 56 ppm). However, the ZREE content ofspodumene pegmatile in both plutons is sharply lowerat 2.06 ppm (Lacorne) and 7.6 ppm (Lamotte). Bothberyl and spodumene pegmatites display chondrite-lennalizsd gull-wing profiles as a result of depletionsof LREE andHREE (Frg. 7). An aplite dike, which cutsthe L-shaped muscovite monzogranite dike in theLacorne pluton, has a chondrite-normalized REEpatiern similar to that of the pegmatites (Frg. 7).

The albitite dikes, which occur only in associationwith the Lacorne pluton, have a uniform major-element

composition, but form two distinct populations withrespect to their trace-element concentations (Iable 2).Group-l albitite has much higher Rb and Ta contents,and Group-2 albitite has a much higher content of Sr.T\e ZREE content of the albitite (Group 1) is10.7 ppm, intermediate in value between the beryl andspodumene pegmatites in the Lacome pluton. It isworth noting that the REEN profile of such albititeis almost identical to that of aplite (Frg. 7).

Oxycnq Isorops ANat.yses

Oxygen isotope analyses (whole rock) ofrepresen-tative samples of monzogranite, pegmatite, and albititewere performed at the Universit6 de Monft6al and theUniversity of Saskarchewan. Details of the analyticaltechnique are given in Hoy (1993). All 61EO values arereported relative to Standard Mean Ocean Water(sMow).

The whole-rock 6lEO values of the Preissac,Lamotte and Lacorne monzogranites range from 8.1 to9.8%o, with most values lying between 8.1 and 8.8%o(Tables l,2,Fig.8); the Moly Hill monzogranite wasnot analyzed, As a group, the types of monzograniteare isotopically indistinguishable within each of theplutons, or from one pluton to another, i.e., they havenot undergone significant interaction with meteoricwater. A similar langs of 6180 values is also displayedby the four samples of beryl pegmatite, whereas thespodumene pegmatites show a somewhat wider rangeof 6180 values (7.4 to l}.3Voo).Interestingly, the mean6180 values of monzogranite, beryl and spodumenepegmatites are almost identical (Tables l, 2), andsimilar to the 6180 values of the two samples of albititeanalyzed.

Alblfto

SpodumsnsPegmdte

BerylPogmdto

6.0 7.0 8.0 9.0 10.0 11.0

618o2-(suow)

Flc. 8. Results of oxygen isotope analyses of monzogranite,pegmatite (white and black symbols are for Lamotte andLacorne plutons, respectively), and albitite in theheissac-Lacorne batholith. The values are ouoted witlrespectto SMOW.

laCePr Nd SmEuGdTbDyHo ErTmYbLuFIc. 7. REEN profiles of rare-element pegmatites in the

Lamotte and Lacome plutons. Also shown are the REE'*profiles for dikes of aplite and albitite. The stippled areaindicates the range of i?EE* in the monzogranite.

LaCePr Nd SmEuGdTbDyHo ErTmYbLu

oo

AA

oa a

rrTnrl tr tl

TTIE PREISSAC-LACORNE BATIIOLITH, QI'EBEC 827

DIscussIoN

Nature of source rocks

The chemical composition of the heissac-Lacorneplutons, in terms of major and trace elements, andnotably the high level of K, Rb, Th, Ta and M,and the low level of CuZr, Hf, Y and Sr, is typicalof syncollisional granites Qlarris ar al. 1986), and hasbeen noted by previous authors (e.9., Feng & KerrichL992). It has also been shown from Fb isotopicdata (Gari6py & Allbgre 1985) and Nd isotopic data(Ducharrne et al. 1995) that the magmas forming theplutons represent mixtures of juvenile, mantle-derivedand older, crust-derived sources. As previously pro-posed by Gari6py & Alldgre (1985), Feng & Kerrich(1992) and Boily (1995), we believe that the mostplausible source for the magmas is the Pontiac suite ofclastic metasedimentary rocks exposed to the south.This sequence is interpreted to have been thrust underthe Abitibi Southem Volcanic Zone during an arc-continent collision between 2670 and 2630 Ma (Feng& Kenich 1992). According to this model, the mixingcould indicate anatexis of the Pontiac metasedimentarysequence and input of magma from the underlyingmantle, or the melting of sedimentary rocks containingboth crustal and mantle components. Although it isdifficult to decide between the two alternatives. wefavor the latter because of evidence for two populationsof zircon in the metasedimentary rocks @eng er a/.1993), one with an age of up to 3050 Ma, representingan older basement component, and the other with anage of 2700 Ma, possibly representing arc-relatedfelsic igneous rocks.

0 20 ,lO 60 80 100120 0 50 1(X) 150 200

0 50 1(x) 150 200 0 S0 100 iso 200

Sr (PPm)

FIc. 9. Plots illustrating Ba versus Sr concentrations in thefour monzogranite plutons of the heissac-Lacomebatholith. All values are in ppm.

Geochemical evolution

The systematic chemical trends described above andthe geological relationships discussed earlier, and inMulja er al. (1995b), provide strong evidence of theevolution of the Preissac-Lacorne plutons fromearly biotite through two-mica and muscovitemonzogranite to beryl and spodumene pegmatites. Themost plausible explanation for this evolution isfractional crystallization of a parental monzoganiticmagma.

Depletion of Ti, Fe, Ca, Ba, Sr, Zr and Th, andconcomitant enrichment of Rb, M and U from biotiteto muscovite monzogranite (Figs,2,3,5) are consistentwith early fractionation of plagioclase, biotiten zircon,monazite and apatite, and late crystallization of albite,muscovite, garnet, and columbite-tantalite. Similarly,the systematic increase in the negative Eu anomalywith magma evolution is consistent with fractionationof early, more calcic plagioclase (cl Cullers & Graf1984); the corresponding decrease in total REE,particularly b the IREE, can be explained by earlyfractionation of monazite and apatite (Fig. 5). Finally,the linear relationship between Ba and Sr, and themarked decrease in Ba content from biotite tomuscovite monzogranite (Ftg. 9), are strong evidencefor fractional crystallization of plagioclase andK-feldspar.

Other processes that have been proposed to explaincompositional evolution in monzogranitic plutonssimilar in extent of zonation to that observed in thePreissac-Lacorne plutons are restite unmixing(Chappell et al. 1987), source-rock heterogeneity(Nabelek et al. 1992), and sequential partial melting(Holtz 1989). We can rule out restite unmixing as thecause of the monzogranite zonation, because ofthe conspicuous absence of xenoliths or xenocrysts thatcould represent restite, and the requirement that thecontents of all elements vary linearly with SiO2contents (ChappeTl et al. 1987). Although theconcentrations of many of the major and tTace elementsdo show such linear variations (Tigs. 2, 3), a significantnumber shows trends that are clearly nonlinear,e.g., Nb, Y, and Rb. Source-rock heterogeneity alsocan be ru1ed out because of the remarkable consistencyof 6180 values from biotite (8.6 t o.l%o) throughtwo-mica (8.5 + O.lVoo) to muscovite (8.6 t 0.4Vao)monzognnite. Finally, sequential partial melting canbe eliminated because this process tends to producerelatively constant concentrations of compatibleelements (Hanson 1978). In the heissac-Laccrneplutons, several of the compatible elements displaywide variations in concentrations, e.8., Ba and Sr'The relatively high contents of Ba and Sr in thebiotite monzogranite and their extreme depletion intfie muscovite monzogranite simply cannot beexplained by partial melting (c/. Mittlefehldt & Miller1983).

EELCL

ctto

m0

0

828 TIIE CANADIAN MINERALOGIST

PlsdelasoK-tsld8pafBiotts

MrEcovile

C: LamotbCLaome

TASLEs. MINEMUMELT (1o) usEDIN TRACE.B$TETT MODS.JNG, ANDVOIUME % OF PHASESFRACNOMTED FFOM EOTITETO MUSCOVIIE MOMOGRINITEMhe|al Rb Vol.%

Trac e - elemznt rutdelin g

In order to test the hypothesis that fractional crystal-lization was the principal process responsible for thecompositional diversity of the Preissac-Lacornemonzogranites, we applied Rayleigh fractionationcalculations to simulate the behavior of Ba Sr and Rb,which are generally considered to be petogeneticindicators in granitic rocks (e.9., McCarthy & Hasty1976, Tindle & Pearce 1981). This modeling wasrestricted to the Lamotte and Lacorne plutons becausethe data for the Preissac and Moly Hill plutons areconsidered inadequate. The following equation wasused to describe fractional crvstallization (after Hanson1978):

CYCo = F@-1).

1.3 2.8 0,096.12 3.6 t.66.4 0.12 4.312 0.4 1.5

635 r3l 26562. \U 3@

a l

32

I

Source ot lg dsh:tlenoon ('1978), Nash & Cmraft 0S85), Mahood &Hfiedh (1sB]d: odgtulcofistadonof olEnenl h tErgnf neff (ee text)

104

103

1eBa

10

1

1d

1&Sr

1 0

Lamotte pluton2O o/"

6 0 %80o/o

90 o/"

95o/o

20 o/o

608Oo/"

90 o/o

95 o/oI

to3 exto3 to2Rb

Frc. 10. Comparisons of the Rayleigh fractionation model (solid line) and observed dataof all subtypes of monzogranite and pegmatites in the Lamotte and l,acorne plutons.The agreement between the theoretically calculated and measured ssmpssitionsstrongly supports the conclusion that ftactional crystelliTation was the main processresponsible for the compositional diversity of the monzogranite. Deviation of somemeasured values for some of the pegmatites suggest that the distibution coefficientsused for the monzogranite are not applicable to volatile-rich batches of psgmatite-forming liquid. Symbols: I biotiG monzogtanite, D two-mica monzogranite,. muscovite monzogranite (main body), O muscovite monzogranite (dike), A berylpegmatite, + spodumene pegrnatite.

Rb

TIIE PREISSAC_LACORNE BATI{OLITH, QT]EBFT 829

where Cr and Co arc weight concentrations of a traceelement in the derived melg and in the parental melt,respectively; F is the fraction of melt, and D is the bulkdisfibution-coefficient. The concentation of a ftaceelement in the residual mineral phases, Cs, relative tothe parent, Coo was calculated from the relationshipCs = Cl * D. Barium. Sr. and Rb were used in themodeling; among the trace elements whose concentra-tion was established" thsse are the elements for whichthe mineraUmelt distribution coefficients (Kr) are bestknown (Table 3). The values of Ko used in thecalculations were selected from the lilerature becausethe rocks from which they were estimated havecompositions similar to those of the heissac-Lacomemonzogranites. Initial values of Ba, Sr and Rbconcentrations were those of the least evolved biotitemonzogranite, i.e., that with the smallest negative Euanomaly. The results of the modeling suggest that thevarious subtypes of monzogranite in the Lamotte andLacorne plutons can be produced by moderate to highdegrees of fractional crystallization (6G-957o) fromsuch a biotile monzogranite parent (Frg. 10). Such highdegrees of fractionation are consistent with tlerelatively small volumes of the more evolved monzo-granites relative to those of biotite moMogranite.Although the calculations also produce goodcorrespondence between the model and observedcompositions of most of the occurrences of berylpegmatite in the Lacome pluton (over 90Vo fuacnonalcrystallization, Fig. 10), spodumene pegmatites havesubstantially higher Rb, Ba and Sr contents thanpredicted. The reason for this is unclear. However, onepossible explanation (at least for Rb) is that themagmas that produced the spodumene pegmatites hadhigher contents of volatile constituents such as F and B,which are known to increase the solubility of Group-lelements in the melt (London 1987, Manning &Pichavant 1988). This explanation is consistent withthe presence of tourrnaline and a higher content offluorine in the muscovite (Mulja et aI. 1995b) n thespodumene pegmatite.

These arguments, the coherent relationships amongtrace elements between the monzogranite andpegmatites (Figs. 6, 7), and the evidence from thefrace-element data for muscovite (Fig. 8; Mtila et al.1995b) illustrating a linear relationsbip of Sc, Rb andTa versus Cs, demonstrate the genetic link of themonzogranite to the rare-element pegmatites.

Behavior of REE durtng pegmatite evolation

The concentrations ofEu, and ofthe light and heavyREd decrease progressively from beryl to spodumenepegmatite in the Lacome pluton @g. 7). The largenegative Eu anomaly is most reasonably interpreted byfractionation of plagioclase, as discussed previously.Given the relative abundance of xenotime in themuscovite monzogranite (Mulja er al. I995b) and

the absence of this mineral in the beryl pegmatite, weinterpretthe strong depletion of t/REEinthe latterrockto be a result of xenotime fractionation (in view of itswell-established tendency to concentate tJlre HREE).The depletion n HREE may also be due in part tofractionation of gamet in the muscovite-monzogranite-forming magmas. The depletion of IREE in the berylpegmatite can be interpreted to reflect fractionation ofmonazitq' monazite is considerably more abundant inthe muscovite monzogranite than in the berylpegmatite (Mulja er al. 1,995b). The extreme depletionof both LKEE and HREE n the spodumene pegmatitecan be explained by the continued fractionation ofmoruEite and garnet in the beryllegmatite-formingrnagma, and the absence of these minerals in thespodumene pegmatite. Other explanations that havebeen proposed for extreme depletion of REE inpegmatites are F-REE complexing @ynn & Bumham1978) and subsolidus mobilization of REE byhydrothermal processes (e.9., Sverjensky 1984).Although we do not have a test of the former hypo-thesis, replacement of amphibolite by biotite near thespodumene pegmatite and replacement of hornblendeby holmquistite indi-cate there was sispodumene pegmatite to the country rocks. Moreover,altered amphibolite within a centimeter of the contactcommonly contains abundant apatite. It is thus possiblethat some REE may have been hydrothermallytransported out of the pegmatite and concentrated inapatite in the county rocks.

Formation of albitite dikes

The almost monomineralic nature of the albititedtkes (97Vo albite and minor molybdenite) rules outtheir formation by equilibrium magmatic crystal-lization. A purely metasomatic replacement of the hostbasalt or biotite schist is unlikely because of the sharpcontact between the atbitite and the country rocks.Moreover, the REE{ pattem of the albitite does notresemble that of the biotite schist (cf Feng L992). Onthe other hand, the bodies of albitite could representfelsic dikes that have been metasomatically altered.This interpretation is supported by the similarity of theREE{ profile of the albitite to that of aplite (Fig. 7). Wetherefore propose that the dikes are the products of acomplete replacement of highly evolved granitic rocksby residual aqueous fluids, which evolved duringthe late-stage crystallization of the magm4 i.e., theprecursor magmatic rock stewed in its own residualfluids. A question that needs to be addressed, howevelis why quartz was removed completely from the rochgiven that the residual fluids would have been quatu'saturated. The precursor may have been a spodumene-rich aplite, in which spodumene and quartz reactedwith the fluid phase to form albite. This process hasbeen predicted thermodynamically by Wood &

830 THE CANADIAN MINERAI'GIST

\ \ \ \ - - :Muscovitd iotite monzogranitemonzogranite

Re.sidual melt Tbo-mica monzogranite

Williams-Jones (1993) to be an inevitable consequenc€of the cooling of a fluid initially in equilibrium withspodumene-bearing pegmatite.

THEMAGMA CHettanm

The (quasi-) concentric zonation displayed by theLamotte and Lacome plutons, i.e., less evolved biotitemonzogranite along the margn of the innusions andthe most evolved muscovite monzogranite in thecenter, suggests tlat tle magna underwent side-wallcrystallization, in which the eartest solidificationoccurred at the contact between the liquid and the

FIc. 11. Skerches depicting the proposed history of crystal-lization of the rare-element-enriched granitic magma ofthe l,amotte and Lacome plutons. Note that if drawnto scale, the roof would appear several times thicker torepresent its pre-intrusion thickness of about l0 km.A) Early side-wall crystallization lnoduces marginalbiotite monzogranite and less dense boundary-layer melts,which ascend to the roof of the magma chamber. Thesemels displace downward the denser crystal-mushes thatformed earlier. B) Continued fractional erystallizationforms successive two-mica and muscovite monzogranitelayers and more differentiated melts. Late-stage crystal-lization moves toward the center of the chamber. wherethe residual magma continues to differentiate. A smallportion of the more differentiated muscovite-bearingmonzogranitic melt breaches the chamber and intrudes thecountry rocks as a dike (MG) in the Lacorne pluton.C) Expulsion of pegmatite-forming volatile-rich magmafrom the chamber owing fluid ove4nessure results in theemplacement of the beryl @rl) pegmatite in the overlyingmonzogranite. D) Later contraction of the pluton, partlydue to its advanced stage of crystallization, and to thegooling of magma, reactivates fractures in the countyrocks and produces new fractures, into which the moreevolved melts are intruded. This gives rise to thespodumene-beryl (Spd-Brl) and spodumene (Spd)pegmatites. Aqueous fluids exsolve from the spodumenepegmatites; some back-react with earlier-formed apliteand pegmatite to form Mo-bearing albitite (Mo-Ab), andsome precipitate quartz and molybdenite as veins(Meqtz), in the country rocks.

intuded country-rocks. In the Lacorne pluton, thegeochemical (Frg. 4) and mineral-chemical (Fig. 6,Mda er al. 1995b) trends corroborate strongly theprocess of side-wall crystallization. The Lamottepluton, which lacks observable systematic chemicalvariations across the monzogranites, could have beenformed in a regime of slow solidification, as is the casefor homogeneous plutonic rocks (Sawka et al. 1990).Furthermore, the comagmatic beryl and spodumenepegmatites, which represent the more and mostevolved felsic liquids, respectively, cut all types ofmonzogranite, indicating that the liquids must haveevolved toward the interior of the body of magma.On the basis of these geological and geochemicalobservations, we envisage an initial stage of side-wallcrystallization that produced a crystal mush (biotitemonzogranite) and a less dense boundary-layer liquid(Fig. 11A) [see McBirney et aI. (1985) for a discussionof the dynamics of magma chambersl. According tothis model, boundary-layer liquidrises buoyantly to thetop of the chamber, displacing earlier-formed, densercrystal-mush layers from the roof. Qely6sliea 6d highdensity eventually drive these layers into the lower partof the chamber, where they undergo further upward

. ) ) ) ) i - :- : : : : t , . , . , . ' , , ,

\ \ \ sq&wi5.\ \ \ \ \ \ v

\ \ \ \ \ \ v\ \ \ \ \ \ v

: - - - - - J / / ' /

\ \ \ \ \ \ _ :

TIIE PREISSAC_I.ACORNE BATHOLITII, QUEBEC 831

crystallization. This stage of continued crystallizationalong the walls produced the more evolved two-micamonzogranite, which led to the ascent of an even moredifferentiated boundary-layer (Fig. 1lB). In theLacorne pluton, a portion of the biotite monzograniticcrystal-mush layer (southern biotite monzogranite)must have separated from the other layers prior to itscomplete consolidation, and must have begun its ownpattem of side-wall crystallization. The separation andindependent crystallization were necessary to producethe observed geochemical and mineral-+hemical trendsmentioned above. One cause for the separation ofthe southem biotite monzogranite was perhaps theemplacement of the magma along reactivated pre-existing fractures or along new ones. This tectonicallyinduced emplacement of magma seems applicable alsoto the emplacement of the L-shaped muscovite monzo-granite dike (labeled MG-dike, Fig. lD), whichsuggests that a batch of somewhat evolved felsic liquidbreached the walls of the magma chamber throughftactures.

A consequence of the inward crystallization was theaccumulation of more differentiated and volatile-richbatches ofresidual liqui{ which were below an alreadylithified carapace. Fractures in the roofrocks, producedtectonically or by overpressures, provided the conduitsfor the early, beryl-pegmatite-producing liquid(Fig. 11C). At an advanced stage of crystallization,corresponding to the onset of withdrawal of the aboveliquid from the chamber, the pluton underwentcontraction, producing conical fractures, analogous tothose associated with the main events of calderaformation, and reactivated pre-existing east-west-trending fractures in the country rocks. These fracturestapped deep pockets ofmore evolved fiquid, which wasdrawn upward and outward, solidifying asspodumene-beryl pegmatite along the margins of theplutons, and eventually as spodumene pegmatite inthe county rocks @ig. llD). Crystallization of thespodumene pegmatite terminated with the exsolutionof aqueous fluids from the melt, which formed systemsof quartz veins or, in some cases, caused the albitiza-tion of the associated apttes. Molybdenum, which hasa fluid/melt distribution coefficient greater than I(Candela & Holland 1984), partitioned into the residualfluid and concentrated as molybdenite in the veins andalbitite. The proposed evolution of granite-relatedmineralization is consistent with the spatial disributionof pegmatites, and the proximity of the most evolvedspoftrmene pegmatites to the dikes of albitite and theveins of quartz.

OnrcN or Mor-yaosrnre-BEARTNG QUARTZ VEtr{sIN TTIE PREISSAC AND MOLY IfuT huToNS

In contrast to the Lamotte and Lacome plutons,where molybdenite is concentrated in albitite dikes andquartz veins that are spatially associated with

spodumene pegmatite in the country rocks, molyb-denite in the heissac and Moly Hill plutons occursalmost exclusively in quartz veins in the muscovitemonzognnite. Pegmatite bodies are relativelyuncommon in these two plutons; they rarely containberyl, never contain spodumeneo and show no obviousspatial association with molybdenite-bearing quartzveins. The small size of the plutons and the fact thatthey did not undergo differentiation to the same extentas the Lamotte and Lacome plutons are consideredresponsible for these characteristics. Consequently,vapor saturation occurred during crystallization of themuscovite monzogranite under a relatively thin crust ofsolidified magma, which was readily fractured. Theexsolved fluid phase escaped through these fracturesand formed molybdenite-bearing quartz veins in themuscovite monzogranite. This exsolution is reflectedin the finer-gra:ined nature of the monzogranite moundthe veins (Mulja er aI. 1995b), which developed as aresult of compositional quenching of the melt. Supportfor the above interpretation is provided by the fine-grained muscovite-gamet monzogranite dike, whichcontains small molybdenite-bearing vugs that probablyrepresetrt tle site of vapor-phase exsolution.Significantly, this dike crystallized late in the history ofthe pluton, and in fact cuts molybdenite-bearing quartzveins.

CoNclusIoNs

Field, petrochemical and oxygen isotope data forfour peraluminous monzogranitic plutons that make upthe Preissac-Lacorne batholith, taken in conjunctionwith geological and mineralogical indicators (Mulja eral. 1995b), have distinguished suites of comagmaticmonzogranite and pegmatite subtypes, and shown thattheir compositional diversity was mainly the result offractional crystallization. Magma in the Lamotte andLacorne bodies underwent the most extensivedifferentiation, and evolved to the stage ofproducingrare-element-bearing pegmatites. The mechanism offractionation in these plutons was side-wall crystal-lization, which developed quasi-concenfically zonedintrusive bodies. The rare-element-enriched pegmatiteswere derived from batches of residual melt trapped inthe interior of the cooling plutons. Beryl andspodumene pegmatites were emplaced sequentially,the former filling prlsssurg-induced fractures in theoverlying parental monzogranites, and the latter fillingexiension fractures in the county rocks, reactivatedand produced by contraction of the adjacent solidifiedplutons. The terminal stage of pegmatile evolution wasmarked by exsolution of an aqueous flui4 which insome places back-reacted with earlier-formedspodumene-bearing aplite to form albitite, and in otherplaces separated from the parental bodies of pegmatiteto precipitate quartz, muscovite and molybdenite inveins.

832

The Preissac and Moly Hill plutons underwent muchless extensive differentiation than the Lamotte andLacorne plutons, probably owing to their small size.Consequently, they did not evolve magmas capable ofcrystallizing rarc-element-endched pegmatite. Vaporsaturation occurred during crystallization of themuscovite monzogranite under a relatively thin crust ofsoliffied magna. The latter was readily fractured,allowing the exsolved fluid to escape into the overlyingmuscovite monzogranite, and there to form molyb-denite-bearing quartz veins.

ACKNOVLEDGEMENTS

This study was supported financially by NSERCoperating grants to AEW-J and SAW, a FCAR teamgrantto AEW-J, SAWandMB, and anMERQ contractto MB. Reviews by D. Kontak, R.F. Mafiin, D.B.Clarke, P. Cemf and an anonyrnous referee helpedimprove the presentation of this manuscript.

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Received. June 13, 1994, revised manuscript acceptedJanuary 11, 1995.