Canadian MineralogistVol. 19, pp. 177-194 (1981)

PERALUMINOUS PEGMATITIC GRANITES AND THEIR PEGMATITE AUREOLES

IN THE WINNIPEG RIVER DISTRICT, SOUTHEASTERN MANITOBA*

B.E. GoADt erqo P. denNfDepar tme, t to fEar thsc iences ,Un ivers i ryo fMan i toba,Winn ipeg,Man i tobaR3T2N2

ABSTRACT

Late- to post-tectonic stocks and plugs of Peg-matitic granites were intruded along partlv dilatedfault svstems in the Archean Bird River green-

stone belt of the English River subprovince insoutheastern Manitoba. Each intrusion consists offour phas:s: (1) fine grained leucogranite locallygradine into (2) interlaYered sodic aplite and (3)

potassic pegmatite, and (4) pegmatitic leucograniteihat also contains bands and lenses of potassicpegmatite. This last phass carries occasional Li-,Be-. NbJa-, As-, P-, B-, F- and Zn-bearing min-erals. Textural and compositional rdlationships sug-gest separation of supercritical fluids from volatile-oversaturated melt as the primary cause of internaldiversity. Bulk compositions are silicic, poor in Ca,Fe and Mg and highly fractionated in terms ofK/Rb. K/Cs, K/Ba, CalSr, Rb/Sr, Th/U. Me/Liand AllGa, The rocks are peraluminous. as in-dicated by 0.1 to 5.0Vo of normative corundum(CPW) and by the presence of muscovite' sarnet,tourmaline, gahnite and cordierite. From finegrained leucogranites through sodic aplites and peg-matitic leucogranites to potassic pegmatites. theperaluminous character increases and 2REE con-tents decrease. The genetic evidence available, includ-ing REE abundances and oxygen-isotope ratios. is in-conclusive: juvenile origin of parent melts modi-fied by subsequent fractionation and bv reactionwith greenstone-belt metasediments is possible, aswell as shallow anatexis of greenstone-belt lithol-ogies followed by igneous fractionation and reac-tion with host rocks. Loss of fluids to the countryrocks probably contributed to the enhanced pera-luminous character of pegmatitic fractions and tothe depletion of their >REEs.

Keywords: peraluminous, pegmatitic granites. peg-matites, English River subprovince. Manitoba,trace elements. REE. fractionation.

SoMNaerns

Des massifs de granite pegnatitique tardi- et post-

'lcentre for Precambiian Studies, University ofManitoba, Publication 48.fPresent address: AMAX Minerals Exploration, 535Thurlow Street" Vancouver. British Columbia V6E3L6

tectoniques ont 6t6 mis en place suivant des sysG-

mes de failles partiellement ouvertes dans la cern-

ture de roches vertes arch6ennes Bird River -duli." n"eif.h iiver, dans le Sud-Est du Manitoba'

Ctruqu""massif comporte quatre phases: (1) leuco-

"t*-fi. e. grain fin qui passe ir (2) une aplite sodi-

iu"'-int"i."f"ire et ir (3) une pegmatite potassique'

et (4) leucoeranite pegmatitique i pegmatite potas-

Jiqo"'tonoit" et lenticulaire' Cette derniEre phase

"Jnti"nt accessoirement des mindraux de Li' Be'

N-;j", As,'P, B, i etZn. D'aprds les relations-de

texture'et de composition, ce serait ir la s6paration

dlun" phur" fluide supercritique d'un magm-a satur6

J'eau qu'it faudrait au premier chef attribuer la

diversit6 interne. Les roches sont siliceuses' applll-

;;i; ;" Ca, Fe et Ms, et fortement fractionn6es

O up.et les rapports K/Rb, K/Cs., K/Ba' .CalSr'nuisr, Th/U, Me/Li et AllGa. Elles sont hvpera-

lumineuses, comme en t6moigne la pr6sence -de;;;ffi;; normatif (de 0.1 i 5.0Vo) et de muscovite'

eienat. tourmaline, gahnite et cordi6rite' Dans les

ihases' num6rot6es ci-dessus, le caractBre hvperalu-

mineux augmente et les terres rares diminuent .dansI'ordre (1i, Q\, @), (3)' Les indices p6trog6n6ti-ques, y compris I'abondance de terres rares et le

*ppo.i 180/160, ne sont pas concluants. guant ir

I'orieine des magmas primaires: ceux-ci peuvent

6tre iuv6nites, ult6rieurement modifi6s par cristal-

lisation fractionn6e et par r6action avec les roches

m6tas6dimentaires de la ceinture de roches vertes;

ils peuvent tout aussi bien r6sulter d'une anatexieir faible profondeur de lithologies de ladite cein-ture, suivie de cristallisation fractionnde et de r6ac-tion avec les roches encaissantes. Une dissipationdes fluides dans les roches h6tes a probablement

contribu6 i renforcer le caract0re hyperalumineuxdes fractions pegmatitiques et )r diminuer leurteneur en-terres rares.

(Traduit Par la R6daction)

Mots-cl1s: hyperalumineux, granites oegmatitioues.pegmatites, bloc English River, Manitoba' 616'ments traces, terres rares, cristallisation fraction-n6e.

INrnooucrIoN

Since 1975, the Winnipeg River pegmatite

district in southeastern Manitoba has been sub-jected to an intensive examination aimed at

the development of exploratiqn criteria for min-eralized pegmatite deposits (Cerny et al. 198O).

177

178 THE CANADIAN

Deciphering the relationship between the peg-matites and the larger granitoid intrusions of theregion was one of the principal tasks of thisstudy. Stocks and plugs of peraluminous peg-matitic granites proved to be parental to most ofthe mineralized pegmatite groups, as suspectedearlier in some cases (Mulligan 1965). Thepresent paper reviews the petrology and geo-chemistry of the pegmatitic granites and theevidence linking their individual intrusions withspecific pegmatite groups; a forthcoming publi-cation will be devoted to the genetic problems.

RrctoNlr Gnolocy

The area of the Winnipeg River pegmatitedistrict is underlain by three major units thatconstitute the westernmost exposed portions ofthe English River subprovince: the Manigo-tagan - Ear Falls gneiss belt, the Bird River

MINERALOGIST

greenstone belt and the Winnipeg River batholi-thic belt (Fig. l). Rb/Sr isochron dating ofregional metamorphism and igneous activityplaces the evolution of these three units in theKenoran orogeny (2,7-2.5 Ga). Their litho-logical, structural, metamorphic and igneousfeatures are described by McRitchie (1971),Beakhouse (1977), Ermanovics et al. (1979),Trueman (1980) and Cernf et a l . (1981).

The Bird River greenstone belt consists of sixformations of metavolcanic and metasedimentaryrocks of the Rice Lake Group, which form abroad and complex synclinorium (Fig. 1). Twomajor episodes of folding affected the green-stone belt, the second correlatable with thediapiric intrusion of the Maskwa Lake and Mari-jane Lake batholiths and with the peak of theregional metamorphism. The metamorphism at-tained a greenschist-facies level over most ofthe map area but reached amphibolite grade in

95 '30 '

r, m. w" m" !sFtc,. l. rocation map (insert) and geological sketch of the Winnipeg River pegmatite district. (l) unsub-

divided gneisses of the Mnnipeg River batholithic belt and thi illanigotagan * Ear Falls gneiss belt,and the schists of the Bird River greenstone belt: A Eaglenest Formation,

-B Lampry Fals Formation,

C Peterson Creek Formation, D Bernic Lake Formation, Dt subvolcanic tonalite,^E Flanders Laie For-mation, F Booster Lake Formation; (2) composite diapiric batholiths (MWL Maskwa Lake intrusion,MJL Marijane Lake intrusion); (3) early leuiogranite and (a) hte biotite granite of the Lac-du-Bonnet(LdB)

_batholith; (5) pegmatitic granites (ct Crier Lake, ENL Eaglenest t ake, Ax Axial, TL Tin Lake,and OL Osis Lake intrusions); pegmatite groups: LdB Lac-du-BJnnet, SHL Shatford Lake, GL GreerLake, BIS Birse I-ake, RL Rush Lake, ENL Eaglenest Lake, AX Axial, BL Bernic Lake; T the Tancopegmatite; heavy lines represent faults, light lines, lithological boundaries.

PERALUMINOUS PEGMATITIC GRANITES. MANITOBA t79

the eastern and northeastern portions (Fig. l).The Flanders Lake Formation of the green-

stone belt is equivalent to, and transitional into,the paragneisses of the Manigotagan - Ear Fallsbelt to the northeast. The tonalitic to granitoidgneisses of Beakhouse's (1977) early gneissicsuite constitute the margin of the WinnipegRiver batholithic belt adjacent to the greenstonebelt from the south. The mutual relationship ofthese two units is not clear (Ermanovics et a/.1979, Trueman 1980).

Tonalite and trondhjemite components of theMaskwa Lake and Marijane Lake diapirs arerimmed and penetrated by undeformed biotitegranites. The Lac-du-Bonnet batholith consistspredominantly of an analogous late biotite gran-ite, but an early and strained leucogranite makesup its eastern extremity and irregular areas alongits southern margin (Fig. l). The leucogranitephase is alaskitic and highly fractionated, unlikethe more primitive biotite granite (Table 1 ).Trace element and REE contents indicate thatthe two pha$es are not comagmatic, but thatthe leucogranite is closely related to the peg-matitic granites described here (Cerny er a/.1 9 8 1 ) .

Tne Prcrvrerltrc GneNtrrs

Fotrr major pegmatitic granite intrusions havebeen examined during the present study: TinLake, Osis La!e, Greer Lake and EaglenestLake (Fig. t; dernf et al. 1981, Map 1I. ffr"Axial pegmatitic granite was examined in muchless detail than the bodies noted above becauseof its limited exposure and poor internal differ-entiation. To avoid repetition and to emphasizesimilarities and differences among the major in-trusions, we give individual characteristics jointlybelow for all four bodies.

Sampling and analytical methods

The pegmatitic granites were sampled on a30H00 m square grid, modified in swamp anddrift areas, for the different rock types and theirminerals. In total, 53 whole-rock samples, 155K-feldspars, 112 garnets, 64 micas, 16 berylsand 2O columbite-tantalites were analyzed chem-ically, besides a variety of other accessoryminerals. Analytical metho(p, standards and un-certainties are quoted in Cernf et al. (1981)and Goad (1981); all compositional data arelisted in Goad (1981) and are available fromthe authors. The CIPW system was used toderive corundum values comparable with datain the literatureo but a modified MESO I pro-

TABTT 'I.

REPRESEIITAT1YE CHEI'IIC.AL AI{ALYSES OF I1IE I'IAII,I ROC(TYPES OF 'NIE UC.DI'.BONIIET BAIIIOIIfi

Quartz dlorlte Leumgranlte Blotlte granlte

AEc-43 Pc-lsl SR-95 PG-137 AEC-2'| PG-133

sio 68.30 69.60 76.n 77,75 72.65 73.40T l 0 , 0 . 4 2 0 , l l 0 . 1 0 0 , 0 6 O . l 7 0 . l lA lude 15 .14 16 .V, 12 .10 11 .53 14 .46 13 .62Fdo i l .5o 0 .92 o .e6 0 .92 0 .76 o .e7Fe6

- 2 .32 l ,4o 0 .76 0 .52 0 .76 0 .72

l'1n0 0.09 0.07 0.05 0.01 0.03 0.03ilq0 1.84 0.50 0.04 0.05 0.40 0.4s

ilar0 4.85 5.43 3.48 3.66 3.65 3.56Kro 1 .56 1 .66 5 .17 4 .94 s .02 5 .63P;G o .2o o . 13 o .o3 o .o2 o .o9 o .o8lfrr- O.'12 0.06 0.08 0.03 0.09 0.04H20+ 0 .68 0 .39 0 .24 0 .21 0 .51 0 .35

99.96 100.22 99.63 100.04 99.50 100.02

LIRbGBe

BaPb

Ut nLTHfS n t

vRbK/(csxltto)K/BaBa/RbBa/srRb/srl4glLlZrlsnZrlHfAl/GaLaceNdSnEu0yYbLu

4665

'1 .9

l 0

- 8nd*

406

4212

&14,251 4 . 97 . 10.48

1 0 1 . 534.6

30.66 1 . 5

3.821 . 72.451 . 90 .24

49 57224 23'l

2 ,6 4 ,9

2 . 5 2]50 225675 92626 22

33 2A2 6 ?24 16

217 2'i26 . 1 5 6 . 04 5

186 202'160 95

51.8 503 , 0 . 44 . 5 4 , 1 2'1 .5 1 .03

49 47.454.2 42.435.3 35 .3

2318 257441.4 87 .187.5 153

5.48 5 .940.51 0 .692 . 2 1 . 21 . 2 5 1 . 2 50.20 0 .13

'not detected

gram, which includes biotite and garnet but notmuscovite, provided felsic components for thetriangular plots. Standard optical and X-ray-diffraction methods were used to determineplagioclaee compositions and K-feldspar obliqui-ties, respectively.

Intrusive relationshipi

Several features are common to the structuralsetting of all four pegmatitic granites. All wereintruded along early subvertical faults and shearzones that give the greenstone belt its fault-slice configuration (Fig. 1). Intrusion occurre(during a regional dilation event and was com-bined with stoping; stoping is not directly ob-servable in the Greer Lake and Eaglenest Lakebodies but can be assumed from the sharp andangular character of the contacts. Evidence offorcible injection is scarce and is restricted to theeastern part of the Osis Lake body.

The main body of the Tin Lake pegmatitic

72 tO370 154

2 . 7 2

2 . 2 22 2 8

52 t451 5 1 234 301 4 640 t5

142 2177.75 6.75

l l 1 2116 266158 205v5 289

0,14 0 .9426 5 .2

t @ 4 . 52.9 30

12.9 l8.ti 8 . 3 3 2 . 1

1882 2033? q a q l o o

97 7644 399.59 6 .790 . 1 8 0 . 2 5'12 .05 6 .37 .5 4 .950,92 0 .59

148179

5 . 020894

s2o'134

t077

1470.52

u . &'t3.4

210855.2

105

0.762.95

0 . 1 6

180 THE CANADIAN MINERALOGIST

granite (TL) is a roughly circular plug, sep-arated into two lobes by a cataclastic shearzone that follows the northern boundary of anelliptical subvolcanic intrusion of tonaliie (Fig.1). Smaller irregular bodies of pegmatitic gran-ite extend west-northwest and east-southeastfrom the northern lobe, mainly north of theshear zone; they'appear to be concordant sillsin the clastic volcanic and intrusive rocks of theBernic Lake Formation, which hosts all mem-bers of the intrusion. The northern lobe is vir-tually free of host inclusions. but in the southernpart they are very abundant, giving the peg-matitic granite a local appearance of agmatiticinjection breccia.

The Osis Lake intrusion (OL) is located northof the boundary fault separating the Bernic Lakeand Booster Lake Formations (Trueman 1980).Except for several dyke-like offshoots in itssouthwestern extremity, the intrusion is hostedby the turbidite sequence of meta-argillites andmetagreywackes of the Booster Lake Formation.From east-central parts to the west, the intru-sion grades from a more or less homogeneousinclusion-poor body, through an area withabundant subparallel and subvertical rafts andscreens of host rock, into a fingering-out systemof mutually subparallel dykes that range fromseveral centimetres to several tens of metres inthickness. In its southeastern extremity, the OLintrusion gives the impression of forcible in-trusion; in the western part, dilation and brittlefragmentation of the host rock are evident, butplastic behavior is also documented by localptygmatic structures.

The Greer Lake pegmatitic granite (GL) ishosted largely by metabasalt of the LampreyFalls Formation (Trueman 198O). At the east-ern and western extremities of the Greer Lakebody and at its southwestern offshoot. the con-tacts trend north at a high angle to the layeringand foliation of the metabasalts. These contac$are sharp, truncating both layering and foliationwith no evident deformation. Host-rock in-clusions are rare in the pegmatitic granite, ex-cept for angular blocks of metabasalt in thesouthwestern offshoot. A fault, striking west-nonhwest and transecting the Greer Lake body,appears to display predominantly vertical move-ment and separates the body into two segments(Fig. 1). It seems likely that the pegmatiticgranite is also truncated by a fault marking theboundary between the Bird River greenstonebelt and the Winnipeg River batholithic belt.

The Eaglenest Lake pegmatitic granite (ENL)Iies at the same band of metabasalts as the GLbody. The intrusion is also similar to the GL

body in that it contains only rare inclusions andhas its western extremity sharply truncatedagainst the layering and foliation in the relativelyundeformed host rocks. The eastern end, how-ever, displays a predominantly lit-par-lir finger-ing-out pattern in a host rock that has a pro-nounced foliation.

lnternal composition

The pegmatitic granite intrusions are texturallyand compositionally heterogeneous, containingbodies of pegmatitic leucogranite, patches offine grained leucogranite, transitional into sodicaplite, and lenticular to pod-shaped potassium-rich pegmatite.

Pegmatitic leucogranite is always a substantialif not dominant phase of the pegmatitic gran-ites. It consists of megacrystic K-feldspar (5-100 cm in size) intergrown with graphic quartz,embedded in a medium- to coarse-grained (6-25mm) matrix of albite * quartz * muscovite(-+ garnet, -+ tourmaline). The K-feldsparmegacrysts range from perfectly euhedral tosubhedral, and are moderately corroded by thematrix. Along contacts with country-rock xeno-liths, muscovite is abundant and the graphic K-feldspars tend to grow from the xenolithicsubstrate into the pegmatitic granite mass. Thematrix locally contains patches of plumose mus-covite + quartz intergrowths, mostly evenly dis-tributed but. occasionally concentrated into dif-fuse layers suggestive of a replacement origin.

Fine grained leucogranite is an equigranularfacies consisting of perthitic microcline, quartz,albitic plagioclase and muscovite; biotite isabundant only in the TL intrusion, rare in theothers. Accessory minerals are represented main-ly by garneto zircon and apatite. The rock ismostly massive and homogeneous in mineral dis-tribution, but faint banding of micas and garnetis evident locally. Pegmatitic leucogranite hasnever been observed in direct contact with thefine grained phase.

The transition from fine grained leucograniteinto sodic aplite, which is commonly layered, ismarked by an increase in muscovite, albiticplagioclase (which attains a platy habit) andgarnet, and by a gradual disappearance of K-feldspar. The unit typically contains, in order ofdecreasing abundance, albite, quartz, muscoviteand garnet, with accessory apatite, gahnite, mon-azite and possibly Ntr--Ta oxide minerals.

Sodic aplite layers are mostly paired or alter-nate with potassic pegmatite layers. The bound-ary may be gradational but a sharp separationis more common. Textural features indicate a

PERALUMINOUS PEGMATTTIC GRANITES. MANITOBA

directed gfowth of tle pegmatitic assemblage with a classic subsolvus granite texture andnormal to, and away from, the aplite surface. random orientation of all minerals. The contactsThe potassic pegmatite facies also develops as with the surrounding pgmatitic granite arelenticular bands and pods inside pegmatitic leu- eroded and covered by soil and vegetation, butcogranites, by the gradual disappearance of they seem to be gradational and show signsgraphic quartz from K-feldspar megacrysts and of mild shearing.by their accumulation, at the expense of thealbite + quartz + muscovite matrix, around Accessory mineralslenses of quartz. The potassic pegmatite con-sists of blocky (or partly graphic) K-feld- Accessory minerals typical of the four peg-spars and quartz, commonly in concentrically matitic granites are summarized in Table 2.zoned patterns, with or without coarse platy Garnet and muscovite are ubiquitous but themuscovite and granular to cleavelandite-type Greer Lake and Eaglenest Lake bodies lackalbite. Beryl, garnetn tourmaline, cordierite, tourmaline and phosphates, which are character-columbits-tantalite, triphylite, molybdenite and istic, in variable quantities. in the two northernarsenopyrite appear sporadically in central parts intrusions. In terms of the typically pegmatiticof the pegmatite bands. accessory minerals, the Greer Lake intrusion

Pegmatitic assemblages are the only ones that is the most enriched in Be-, Nb-Ta- and Li-locally display cross-cutting relationships with bearing species; the OL body carries abundantthe other facies of the pegmatitic granites, in- Li-, P- and As-bearing minerals, but the Eagle-dicating that they were the last to solidify. nest Lake and Tin Lake intrusions appear bar-Stringers and trains of porphyroblastic K-feld- ren in comparison. The accessory mineral as-spar, transecting textural patterns of fine grained semblages thus suggest significant geochemicalleucogranites and sodic aplites, probably also differenses among the four intrusions.belong to the late potassic phase. Two mineralized pods in the southern part of

The Greer Lake and Eaglenest Lake intm- the Greer Lake intrusion deserve particularsions have a good representation of all four notice because of their high concentration ofphases, with the banded aplitic assemblages 'rare elements, and because of their parageneticoriented roughly parallel to the intrusion margins and geochemical similarity to some of the mostalong the contact but rather chaotic in the in- fractionated pegmatites in the district. These areterior. The Tin Lake intrusion is exclusively the Annie Claim along the southern margin offine grained and leucogranitic in the southern the intrusion and the Silverleaf Claim southwestlobe; in the north, pegnatitic material increases of the main body. The western Annie Claimin abundance towards the west-northwest, but locality is a subellipsoidal pod highly enrichedlarger segments of sodic aplite are unusual in in Li-, Rb-, Cs-, Be-, Nb-Ta- and F-bearingall parts of the body. The Osis Lake intrusion minerals, evolving from the surrounding pegmati-is largely pegmatitic; the fine grained leuco- tic leucogranite through a narrow transitionalgranite is scarce. Ifowever, it contains a cen- zone. The Silverleaf offshoot consists largely oftrally located subcircular plug of biotite granite garnetiferous aplite with a large rounded pod of

TABLE 2. ACCESSoRY I,|INERAIS IN lllE PEGI'IATITIC GRANITES

GREER LAKE EAGLENEST LAKE TTN LArc OSIS LAKE

north rest east tesr easr

bJot l te '

muscovlto

garnet

toumall ne

gahnlte

cordi€r'ite

beryl

colmbl te-tantal l te

a15 enoljn'l te

m'lyMenlte

apatlte

tr lphyl l te l l th lophlI i te

t-

- : - - '

+- - - . .

G .

f - - -

1 8 1

cmon - rare --- vety r4re.. . . . .

see text for paragenesls of pegmtlte pods in the southem part

increaslng trend +

t82 THE CANADIAN MINERALOCIST

TABLE 3. REPRESEIITATM CHEIICAI- NALYSES 0F THE pEG&lATtTIc CRANTIIS

Flne gralred Pegnatltlc Sodlc potNslcleu@granite leu@grdite aplite pegmfite

TABLE 4. SELEC1ED GEOCHF.IICAT CHARACIERIS1ICS OF THEFINE GRAINED LEUCOGMNI'IE PHASES OF I1IEPEG}NTITIC CRANIIES

TIL-31 Tln osls LakeLakef 13) * Lakef3l

Greer EaglenestLakefs] Lake(3177.50

0.0612.790.760.800.21

0.406.22

0.040. l00 .390.02

75.90 73.500.0 t 0 .03

13.48 14.400.35 0 .470.26 0.880.01 o.ql0 .03 0 .090.31 0 .363.68 3 .985.4 t 5 .630.08 0 .050.03 0.090.34 0 .230.0 t 0 .01

sr0,(u)f l q

ik&FeO

-

t'ln0M9oCa0

t00.1 t-u. 12

99.75 99.85 99.98 100.60- 0 .01 - 0 .03

L t 3 1 . 8 sl r -59

Rb 245'128-338

Cs 7 ,22.9-11.4

Be 2 .01-3

Sr 35.4

Ba 21731-416

Ga 3022-46

u 2 8l3-40

th 10 .30-30

Zr 10361-140

Hf 4 .03 .5-4 .6

5 n d . I

u - t J

K/Rb 't76

69-327(K/cs)xI00 68

34-r54K/Ba 279

75-1258

Ba, /Sr '11 .7

1 . 1 - 8 1 . 3

Rb/Sr 10.93,2-56

mg/Li 491 3 . 5 - t 2 7

Alloa 23511742-?909

77.100.03

12.280 . 3 10.680.010.200.423.235.240.060.02

0.01

75.300.04

I 4.300.860.920.080.03o.?'l5.202 . 1 90.060.08

0.041 5 . 0 1

0.200.010.050 . 1 8

4.650 . 0 t 60 .22

63.347-74I t 7

103- 1 93

8.43 .9-1 7

0 . 80 . 3 - 1 . 2

25.713-36A

3-18

484l-57

238-52

4-12iA

17-74

3 . 0

5829-86

562340- 1050

8 . 95 . r - 1 3 . 7

3 . 8t . 2 - 1 0

1 8 . 2l l-28

1 1 2't2-t50

3a- tJ

t l

8-30

0-25

44il-58

1 . 2

79,727-140

293-8't712.73,8.245 . 60.7- 1 310 .35 -15700 - l r 05856-60

4-870-t8

38-7r2 . 5

0.8-3.4't3'I l-'157547-112

CIPU cor.

Ll (ppn)RbCs

BAPb6u'ItI tHf5n

VRbK/(Csxl00)(/Ba8a/Rbga/SrRb/Sfl,lSlLiZrl5nZtlHlAt/Ga

La(ppn)CeNdSn

DyYbLUY

J . J ' 1 . 4 9

76 45M2 138

2 . 3 1 . 51 . 7 2

31 5I nd*'It

4

8 65 1 2

16 141 . 2 5 1 . 8

41.2 39.879 36

r82000.0020.03

t4 .3 27 .62 .7 4 .40 . 7 l . l

12.8 7.Al l83 1327

7.05 7 .3518.5 23

4.69 5 .30 .05 0 .105.85 4.252.7 2 .750.26 0 .45

lot detected

Li-, Rb-, Mn-, Nb-Ta-, Be-, Sn- and F-enrichedpegmatitic assemblage. Cs-rich beryl, Li-micas,petalite, spodumene, cassiterite, columbite-tan-talite, apatite, triphylite-lithiophilite, amblygo-nite-montebrasite and sphalerite are the char-acteristic accessory minerals.

Contact reactions

Reaction of the pegmatite granites with theenclosing rocks varies with the composition ofboth the intrusion and host. Along the meagrecontact exposures of the Greer Lake and Eagle-nest Lake intrusions, reaction with metabasaltsis restricted to very limited recrystallization ofhornblende in the metabasalts and to moderatebiotitization. In the southern lobe at Tin Lake.small inclusions of the tonalite and metavol-canic rock tend to develop porphyroblasticgarnet, and only in the northeastern extremitywhere this intrusion is in contact with BoosterLake metaturbidites is altered cordierite foundin fine grained leucogranites. In contrast, the

t J t J . o

10-t5 5-25

132 8789-156 42-132

28 51 44"t7-46 33-77 16-875402 767 338507-9667 215-2425 325-351

0 . 5 7 . 4 5 . 20 .08- f .4 0 ,4 -12 .4 7 .3 -9 .0

6 32.'t 52.83 .2-7 .9 t7 .0 -61 .8 45 .4-58 .6

1 1 . 5 4 . 7 6 . 06 . 7 - 1 9 . 1 1 . 7 - 9 . 4 1 . 7 - 1 2 . 2

1674 1526 1258'1520-t885 990-2284 1227-1294

ZrlHf 48 .3 1 r o

'nmber of smples averaged; lorer for Hf detemlnations

*arlthnetic man and range, in ppm

tourmaline-bearing Osis Lake intrusion has re-acted extensively with the metaturbidites of theBooster Lake Formation: tourmalinization ofthe latter, coupled with the growth of albiticplagioclase and locally muscovite, is ubiquitous,particularly in thin splinters and screens and onthe surface of larger rafts. Thin screens maybecome completely digested by the pegmatiticgranite.

Exomorphic influence of the pegmatitic gran-ites is probably much more pronounced than theeffects observed along the intrusive contacts.Post-tectonic growth of biotite or muscovite (orboth) is characteristic of most of the green-stone-belt lithologies (A.C. Turnock, pers.comm. 1980), suggestive of widespread alkalimetasomatism. Extensive dispersion haloes of K,Li, Rb and Cs can be expected in the schistshosting the pegmatitic granites, as documented

100.575.48

'107

l 3no5

102

320 . 4

2052.5

:_ : -0 . t5

774

1 . 0 9

0,850 . 60 .09

0.72 1 ,27 1 .4442 57 28

168 340 2754 't3.7

3.21 3 0 . 53 2 0 4 3

244 142 5030 33 2027 45 623 6 9 3t5 25 nd

130 58 54 .35 1 .2 0 .453 I t 3

259 132 t70108 32 145178 316 934

1.45 0 .42 0 .188 r . 3 7 . t 0 l . t 656 17 6 .428,7 3,2 2043.3 7 .2 0 .429.9 48 .3 i l . I

2407 1584 122919.7 5 .5 2 .646 16 .5 85 .5 5 3 .57 .25 2 .95 1 .640. r8 0 .08 0 .088.9 7 .15 2 .55

0.56 0 .32 0 .1361 40 17

30.8 24 .329,3-32 .6

PERALUMINOUS PEGMATITIC GRANITES. MANITOBA 183

PEGTAT ITIC LEUCOORA'{ITESE III{ LAKEA OSts LA(EO SFEER L TE9 EN.EEST L'E

Frc. 2. Mesonormative compositions of the finegrained leucogranite, sodic aplite and pegmatiticleucogranite phases of the pegmatitic granite in-trusions in the Ab-Or-Qtz and Ab-Or-An sys-tems,

for analogous rocks in the Cat Lake and LilypadLakes areas (W.C. Hood, pers. comm. 1980).

Petrochemistry

Results of representative analyses of thefour phases composing the pegmatitic granites,and covering most of the variation of individualoxides and trace elements, are given in Table 3.Fine grained leucogranites are mostly concen-trated around the composition Aba:.rOraz.rQtzrr(Fig. 2). The sodic aplites shift from this com-position towards the Ab apex by a decrease inOr, followed by loss of quartz. The pegmatiticleucogranites are somewhat scattered, at leastpartly because of the difficulties encountered inrepresentative sampling of this coarse grainedrock type. On the average, they appear to beslightly more sodic than the fine grained phase.In the ternary feldspar system, most of the finegrained and pegmatitic leucogranites fall into thelow-temperature trough; compared with theother two intrusions, the Tin Lake and Osis Lakebodies are slightly enriched in An.

Figure 3 shows the normative corundum con-tents (CIPW) plotted against SiOz. No silica-dependent trends can be detected. However, fourobservations qan be made using the data at hand:( 1) among the four intrusions, the Osis Lakeand much of the Eaglenest bodies are generallymore peraluminous than the other two; (Z)among the four rock types in each intrusion,pegmatitic leucogranite is more peraluminousthan the fine grained phase; (3) generally, theperaluminous character increases from the finegrained leucogranites through their gradual trans-ition into sodic aplites (increasing mussoviteand garnet contents); and (4) the excess ofAlrO' is variable in the potassic pegmatites butreaches its highest values, greatly surpassingthose of the other rock types, in muscovite-richbodies (Table 3).

The highly fractionated character of the peg-matitic granites, shown by high SiOz and alkalisand by low Ca, Fe and Mg, is also expressedin their trace element contents (Tables 3 and4, Fig. 4). The extremely low values of theK/Rb (42) , K/Cs (16O0), Mg/Li (1.7) andAl/Ga (990) ratios and the high values of K/Ba(>> 20000), shown by the pegmatitic granites,are rarely reached in igneous sequences; graniticaverages for the above ratios are in the orderof 23O, 1Om0, 50, 3000 and 5O, respectively.The individual intrusions have different levelsof fractionation (at the present erosion surface),and the fine grained leucogranites are relatively

Q T Z

'1:t!'

FNE-OMIIED LEUCOCRANITESt Tlt{ LA(E. Ogl9 LAI(E. ONEER UUE

" EALEiESI LIIE

ABr lPtlflc LEtco@Ar{tTEs

IOLEUCMA|{|TIC APUTE8o soDtc APutEs

O R

AB/

AB

A N

o o o t o

OR

OR

184 THE CANADIAN MINERALOGIST

S i O a , w t % 80

Frc. 3. Normative corundum and diopside (CIPW) in the four phases ofthe pegmatitic granite intrusions and in the Lac-du-Bonnet leucogranite.

200qt

o ^ b e l a I

t r -o

I a

5 n

F IN E- GR.LEI.JCOGRAI.TITEPEGMATITI C LEUCOGRANITESODIC APLITEPOTASSIC PEGMATITE

70

i t 'o t l t

aI

a-'

Flc. 4. Selected major and trace element relation-ships in the fine grain€d (solid symbols) andpegmatitic (open symbols) leucogranites of thepegmatitic granite intrusions. Symbols as in Fig-ure 3.

Li / to

Ftc. 5. Li versus F and Sn-(Lil10)-(F/10O) rela-tionships in the fine grained leucogranite, peg-matitic leucogranite, sodic aplite and potassicpegmatite phases of the pegmatitic granite in-trusions. Dashed lines join adjacent pegmatiteand aplite samples.

L i , P P m

Rb, ppm

t uFE,oz.or l o()

LrJJo-=o

PERALUMINOUS PEGMATITIC GRANITES. MANITOBA r85

too

Ls C. Nd Lo Ce Nd Sm'Eu

Lo Co Nd SnEu Dy Yb Lu Lo Co Nd Sm Eu Dy Yb Lu

Fro. 6. REE abundances in the fine grained (solid lines) and pegmatitic (dashed lines) leucogranites ofthe pegmatitic granite infusions. A field of REE abundances in the plug of biotite granite cenbal tothe Osis Lake intrusion is also shown.

UJFE,(fzo5rolrlJ(L=o

more primitive than their pegmatitic counter-parts within the same intrusion.

The F content of the granites examined ap-p€ars to vary with the Li concentration (Fig.5). Both elements are low in the Tin Lake andOsis Lake intrusions, whereas most phases ofthe Greer Lake and Eiglenest Lake bodies areenriched in them. In the triangular projectionSn-(Lil10)-(F/100), the points shift along the

Li-Sn sideline from Li-enriched, fine grainedleucogranites through aplitic phases to Sn-richpegmatitic leucogranites. Among the four rocktypes within each intrusion, the pegmatiticphases tend to be enriched in F and Sn.

Chondrite-normalized REE abundances areshown in Figure 6 for the two predominant rocktypes, the pegmatitic and fine grained leuco-granites. Low total REE contents and extreme

Eoglenesl Loke

cLE - 6( b)

186 THE CANADIAN MINERALOGIST

negative Eu anomalies, flanked by rather flatLREE and HREE distributions with low Cer/Ybrv ratios, are predominant. The only exceptionis the Osis Lake intrusion (including its centralplug of biotite granite) which has a moresteeply sloping distribution and a moderate Euanomaly. Within a single intrusion, the REEconcentration tends to be slightly to distinctlylower in the pegmatitic leucogranites than in thefine grained phase, but the type of pattern isidentical for both. The limited data availablefor the other two phases indicate that the sodicaplites have REE contents within the limits ofthe associated fine grained leucogranites, butthe REE abundances decrease drastically in thepotassic pegmatites (Table 3). The total REEcontents tend to decrease with increasing nor-mative corundum but the trend shows a widescatter.

Oxygen-isotope compositions of the pegmatiticgranites are remarkably homogeneous within,but vary greatly between individual bodies. Mostof the data are correlatable with 8180 valuesfound for enclosing metasedimentary and meta-volcanic rocks. The SttO is low in the GreerLake and Eaglenest Lake intrusions (8.1-8.7and 8.6-9.3, respectively), and seems relatedto the low 8180 of the enclosing metabasalt.

The 8'8O ranges for the Tin Lake and OsisLake bodies (10.3-10.9 and ll.l-12.4, respect-ively) closely correspond to those of their r8O-

rich metasedimentary hosts (Longstafte et al.1 9 8 1 ) .

Mineralogy

The chemical composition and structural char-acteristics of rock-forming and accessory min-erals have been studied, mainly in the potassicpegmatite phase of the intrusions, to facilitatedirect comparison with neighboring pegmatitegroups. However, these data also contribute tothe definition of geochemical characteristics andinternal fractionation of the pegmatitic granitesthemselves.

Bulk compositions of the blocky K-feldsparsare quite uniform in all four intrusions. Theyaverage very close to OrzsAbrzi the An compo-nent is low in the south and higher in the north-ern occurrences (Greer Lake Ano.o:r, EaglenestLake Ano.or, Tin Lbke Ano.oe, Osis Lake Ano.os).Conversely, rare-alkali fractionation of theGreer Lake and Eaglenest Lake bodies is higherthan that of the northern intrusions (Fig. 7).Obliquities of K-feldspars commonly range be-tween 0.90 and 1.00, except for the Osis Lake

\s\*Cs,wLt

Fto. 7. Selected compositional characteristics ofblocky K-feldspar from the potassic pegmatitethe pegmatitic leucogranite phase (b) and platytassic pegmatite phase (cd,e).

o l u ,wt i J

rock-forming minerals:phase (a), garnet frommuscovite from t}te po-

PERALUMINOUS PEGMATITIC GRANITES. MANITOBA

intrusion, in which the values cover a wide span:0.66-1.00. lo o

Plagioclase is albitic (Ans-Ane) and of lowsfructural state. The An content is slightly higherin the Tin Lake and Osis Lake intrusions thanin the southern trodies. No systematic variationwas found among plagioc)ases from, the differentrock types within individual intrusions. 60

Coarse platy muscovite uniformly crystallizedin the zMr polytype. , Li and Rb are highlyenriched in this species in the Greer Lake andEaglenest Lake intrusions, but the ranges, ofCs contents are largely overlapping (Fig. 7). oThe total Fe (expressed as FezOa) and MgO gocontents are low (1.24-3.53 and 0.03-0.48 wt.70, respectively), as are those of F (0.17-0.40wt.'/r), relative to the celadonitic nature of mus-covite in many two-mica granites (Miller et a/. 201981, Anderson & Rowley 1981).

Within a single intrusion, the Mn enrichmentis evident in the garnets from potassic pegmatitelayers, but the compositions of other paragenetictypes of garnet largely overlap. Among dif-ferent intrusions, garnets are Mn-poor in theTin Lake body (Spz-za), intermediate in theGreer Lake and Eaglenest Lake intrusions(Spu-l and Spt"-en, respectively) and mos[Mn-enriched in the Osis Lake body (Spro,so: Cernyet al. l98l), In terms of garnets from the peg-matitic leucogranite phase, however, the com-positions of the Greer Lake and Osis Lake gar-nets overlap (Fig. 7). MgO and CaO are dis-tinctly higher in the garnets from the northernintrusions (Fig. 7), and PrOr is particularlyenriched in the Osis Lake body (average 0.23wt. Vo), whereas the Greer Lake and EaglenestLake garriets are enriched in Y:O, (0.10-0.25wL Vo ) .

Accessory minerals from the potassic pegma-tite layers display relatively primitive composi-tions. Alkali contents of greenish beryl are low(up to 0.45 wt. /6 LizO and 0.20 wt. % CszO),and disordered columbite is poor in Mn and Ta(about Feo.zMno.gTtu.sNbr.rOo). However. frac-tionation reached in the Annie Claim and Silver-leaf pegmatite pods along the southern marginof the Greer Lake intrusion is very advanced(K-feldspar with up to 1.20 wt. Vo Rb,O; 2Mr> lM lithian muscovite with 3.77 wt. % LizO,2.14 wt. % Rbso and 0.44 wt. % CszO; berylwith 1.05 wt. Vo Ligo and 2.30 wt. /o CsuO;and disordered columbite-tantalite reachingcompositions of Feo.aMno.?Nbo.oTa..oOe and Feo.ooMno.ssNbr.oTar.oOu).

187

6000

t o o o

500

200

Frc. 8. Fraclionation trends and discordances inthe fine grained leucogranite phases of the peg"matitic granite intrusions related to the K/Rbratio. Element contents in ppm.

Internal lractionation trends

Some fractionation trends are expressed byregional changes in accessory mineralogy. Sucha trend is shown for the Tin Lake and OsisLake intrusions in Table 2, indicating changesin abundance of B, Li, P, As and Mo. Muchbetter evidence is available from regional varia-tions in whole rock K/Rb, blocky K-feldspar

O L E N L

1 8 8 THE CANADIAN MINERALOGIST

K/Rb and K/Cs, muscovite K/Rb, Mg/Li andBe, and garnet Fe/Mn ratios. Fractionation in-creases from central parts of the Greer Lakeand Eaglenest Lake intrusions to their westernand eastern extremities. and from the easterntermination of the Osis Lake body to the westand northwest. No vectorial trend can be de-tected in the Tin Lake pegmatitic granite; how-ever, the part of this body south of the shearzone (Fig. 1) tends to be slightly more fraction-

eted than the northern segment (Goad 1981,Cen! et al. l98l).

M u ual f rac tionatiott relationships

The four intrusions of pegmatitic granites areobviously closely related in general terms ofemplacement, internal diversity and composi-tional characteristics. However, they do not con-stitute a simple fractionation series, as is readilyevident from the irregular distribution of acces-sory minerals (Table 2). Figure 8 illustratesdiverse geochemical indicators plotted againstaverages of the K/Rb ratio. It is evident fromthis graph that the Osis Lake intrusion is themost anomalous, deviating from trends that aregenerally (but not ideally) followed by the threeother bodies. This behavior matches some of thepreviously established anomalies that also con-tradict simple igneous fractionation, notably theenrichment in Mn, P and B, the extreme pera-luminous compositions and the slight deviationin the pattern of REE abundances.

Pscrueilrr Gnoups

Each of the pegmatitic granites is flanked orpartly surrounded by a swarm of pegmatitedykes (Fig. 1). Besides this slose spatial rela-tionship, several other features of the individualpegmatitic granites and their respective pegma-tite aureoles imply a common genetic link: (1)Accessory mineral assemblages (Table 4). Forexample, the Greer Lake and Eaglenest Lakepegmatitic granites are virtually free of B-bear-ing minerals and are very low in P-bearing min-erals, as are their pegmatite aureoles. In con-trast, these two elements are enriched in the TinLake and particularly Osis Lake intrusions, andthis is also reflected in the abundance of tour-maline and phosphates in the adjacent pegmatitegroups. The Greer Lake intrusion and the peg-matite group adjacent to it are the only onesin the district that carry pseudomorphs aftercordierite that are studded with garnet. (2)Texture and zoning: Bands and schlieren of amineralized potassic pegmatite phase that occur

within the pegmatitic granite display the samezonal sequence and textural relationships as doindividual bodies in adjacent pegmatite swarrn$.This is particularly true of the Greer Lake'Eaglenest Lake and Tin Lake intrusions. Thelast of these is transitional into the Birse Lakepegmatite group in such a gradual manner thatonly an arbitrary boundary can be drawn be-tween the fingering-out pegmatitic granite andpegmatites proper. Even the most complexlyzoned pegmatite types of the Rush Lake andBernic Lake groups have their petrological andparagenetic analogues inside the pegmatitic gran-ites, in the Li-, Rb-, Cs-, Be-, Nb-Ta- and F-richSilverleaf and Annie Claim pegmatitic nodulesthat represent local, gradually evolving faciesin the Greer Lake pegmatitic granite. (3) Geo-chemical relations: The rock-forming mineralsfrom the individual major intrusions and fromtheir respective pegmatite aureoles indicatecommon fractionation relationships. This isillustrated in Figure 9, where K/Rb and Csof blocky K-feldspars from the potassic peg-matite phase of three pegmatitic granites andfrom their pegmatite aureoles exhibit continu-ous fractionation schemes. Similar sequentialtrends are shown by other fractionation-sensi-tive elements in K-feldspars and muscovites.Rough parallelism in averaged geochemicalcharacteristics of the pegmatitic granites andtheir pegmatite aureoles is also shown in Table

The most fractionated and complex group ofpegmatites in the district, the Bernic Lakegroup, which includes the Tanco deposit(Crouse et al. 1979), app€ars to have no par-ent body exposed at the present erosional level.The distance from the westernmost Parts of theOsis Lake pegmatitic granite is excessive (Fig.1 ), and the geochemistry of feldspars and micasin the Rush Lake and Bernic Lake groups sug-gests a parallel but not sequential fractionation ofthese pegmatite groups (Fig. 9). A source hid-den beneath the central parts of the area pop-ulated by the Bernic Lake pegmatites is sug-gested by the directions of lateral fractionptionirends within individual pegmatite dykes (eernfer al. l98l). Such a hypothetical source wouldfit the extreme fractionation and enrichment inrare elements attained by this group: in an ideal-ized 3-dimensional pegmatite aureole flankingand topping a parental intrusion, extreme en-richment in volatile comPonents and associatedrare elements should be expected in tlre topmostpart. A pegmatitic granite of the Greer Laketype may be envisaged as a parental body. TheSilverleaf and Annie Claim pods in the Greer

PERALUMINOUS PEGMATITIC GRANITES, MANITOBA

TAB|..E 5. CORRELATION OF ACCESSORY MINEMI. CONTENT A|{O SOI,IE GEOOIS4ICALCHARACTERISIICS OF PARENT TNTRUSIONS AND IHEIR PEd'IATITE AUREOLES

Parent lntroslon Assoclated pe$ntites

lE9

Typlcal acessonfnlnerals

zlrcon, all i l ' l te,nonazlte, ((gamet))

gnmet ((touroalim))

Localnlnera l I zatlon

LAC-I t -BoNNET IEU@GRNITE(Ld8)

IIN LA(E PEGI'IATI.TIC cRAl,lIIE(n)

Avg. whole rcckK/Rb,Vcs

t49lLlTyplBl accessory

nlnemls

gadollnite, casslterlte

touml lne, b€ryl

garnet, beryl

Rare-elmnt6socl atlon

Avg. K/Rb,Cs(ppn) ln btocky

K-fel dspars

bery l , gamet , topaz , a l lan i te , JBe,Nb-Ta,Sn,nonazite, zircon, thorlte, uran- |REE,Y,U,Ih,ln l t€ ,co lmb l te - tan ta ' l l t€ , lZ r -Hfeuxenlte, yttrctanta'l l te, I

SHATFORD LA|G GROUP(SHL)

BIR5E LAKE GROI'P(BIS)Be( (Ll , i lb-Ta) )

66 ;

GREER LAKE PEGI4ATITIC CMflITE(Gt) GnEER tA$ GR0UP(6I)

EAGLENESI LAKE CROI'P(ENL)

Be

RUSH LArc GROUP(RT)

BERIIIC LA(E GROUP(BL)

ganet , @rd ie r l t€ , (gahn l te , l8e ,Nb-Ta(L i ,Rb,beryl) ((colunblte-tantalite, I Cs,Zn)monazlte)) |

87 i 5 ,1 104 . 7

42, 250

50; 265

9.5. '1262

EACLENEST'LArc PECI4ATITIC 6RANITE(ENI)

OSIS LAKE PEGI'IATITIC GMNITE(Ot)garnet , touml ine apat i te , l (L l ,P ,As)(trlphylite,areenopyrite) I

(unexposed; presmd pegnatltlc granite)

( ) subordlnaG; (( )) rare

Lake intrusion are not as fractionated as theBernic Lake pegmatite group, but they exhibitthe same low-P assemblage (petalite as theprimary Li-aluminosilicate) and similar para-genesis.

DrscussroN

Although the genetic relationship between thepegmatitic granites and their pegmatite aureolesappears to be established, the internal evolutionof the pegmatitic granites and their origin aremuch less evident, and they should be examinedin the light of the data presented earlier.

Internal evolution

In terms of internal composition of eachphase, the four rock types distinguished in allfour intrusions show the following relationships:( I ) The fine grained and pegmatitic leucogran-ite phases have not been found in mutual con-tact, and the difference between them was prob-ably established below the final level ofemplacement, prior to solidification. The differ-ences in their texture, bulk chemistry and traceelement contents suggests that the fine grained

toumal lne , b€ry I , spodwene, l t i ,Rb, (cs ) ,8e ,petalite, amblygon'lte, triphy- lSn,Nb-Ta,F,B,Pllte, (Nb,Ta-oxlde minerals Icass i te r i te , su l f ides) |( (po luc l te ) ) |

touml lne , bery l , pe ta l l te l l i ,Rb,Cs,Be(spod@ne, eucryp t l t€ ) ' lep ido- lSn 'Nb-Ta 'F 'B ,Pl i te , po l luc l te , ess i te r i te , I

minerals, anblygonlt€,

phase crystallized from a relatively dry melt,whereas the pegmatitic phase originated from amore fractionated and volatile-oversaturatedmelt, which did not reach a stage of large-scalespatial separation of supercritical fluids fromresidual magma. According to Jahns & Burn-ham ( 1969), graphic K-feldspar + quartz mega-crysts may have grown from local concentra-tions of supercritical fluids that did not coalesce.(2) Potassic pegmatite bands develop locally bygradual or abrupt change in mineral assemblageand texture of pegmatitic leucogranite. Thiscorresponds to incipient segregation of super-critical fluids into larger pods, preferentially ex-tracting K, Si and some trace elements. (3)Fine grained leucogranites (and, rarely, pegma-titic leucogranites) grade into layered assem-blages of garnetiferous sodic aplite alternatingwith potassic pegmatite. Two interpretations maybe applied to this association. One is that pro-posed by Jahns & Burnham (1969), who con-sider the sodic aplite to have crystallized fromresidual melt impoverished in silica, potassiumand most rare elements owing to extraction intoa separated supercritical fluid that gave rise tothe pegmatitic bands. The other possibility is ametasomatic origin of the banded garnetiferous

190 THE CANADIAN

C ! , i l I

FIc. 9. K/Rb versas Cs in blocky K-feldspars of' the potassic pegmatite phase of the pegmatiticgranites, and of the pegmatite groups adjacentto their individual intrusions (c1., Fig. 1).

aplite assemblage, analogous to ,saccharoidal al-bitization fronts in complex pegmatites (e.g.,Beus 1961).

Textural and mineral-compositional character-istics of the banded aplite-pegmatite associa-tion favor the first mechanism: (l) the sur-face of aplitic bands 'commonly serves as asubstrate for crystallization of pegmatitic as-semblages; (2) pegmatitic assemblages cross-cutand cement broken bands of sodic aplite, butreverse c,lses ale extremely rare; (3) the Ancontent of albite in the sodic aplite is about thesame as in the other units composing pegmatiticgranite intrusions, whereas metasomatic albitecould be expected to be extremely An-poor; (4)

the Mn content of garnets is also about the sameas, if not lower than, tlat of garnets in finegrained and pegmatitic leucogranites; enrich-ment in Mn could be expected in garnet asso-ciated with albitization because of continuedFe-Mn fractionation during late metasomatism.

The only feature of the sodic aplites pointingto an analogy with albitization is the assemblageof accessory minerals, which is . close to thatof many typical albitized units'in pegmatites.However, if the layered aplitic assemblages weregenerated through sodic metasomatism, thentheir textural relationships with the pegmatiticassemblages noted above would indicate totalrecrystallization of original rock type(s). Thecompositional uniformity o"f albite, garnet andmuscovite (Goad 1981 , Ceny et al. 1981)would also suggest a thorough metasomatic re-working of precursor lithologies whose onlyremnants might be represented by the finegrained leucogranites. Such a profound meta-somatism and recrystallization would suggest ananalogy with apogranites of Soviet authors, whichare, however, decidedly sodic in bulk composi-tions, extremely enriched in Li, Rb, Cs, Be andF and not associated with mineralized pegmatiteswarms (Beus et al. 1968).

Thus the concept of igneous and supercriticalcrystallization from melts and fluids, triggeredby a pressure quench in dilation zones, is pre-ferred for all four phases of the pegmatiticgranite intrusions, as modeled by Jahns & Burn-ham ( 1969).

Conditions ol crystallization

An estimate of the P{ regime under whichthe pegmatitic granite intrusions crystallized ishampered by several compositional character-istics of the different rock types. The proceduresapplied by Castle & Theodore (1972) and Harris(1974) to similar rocks cannot be used rigor-ously in our case because of compositional com-plexities introduced by the presence of Li, Band F. Wyllie & Tuttle (1964) proved the flux-ing role of Li, and Chorlton & Martin (1978)established the same for B. The profound in-fluence of F on the granitic solidus, depressingit partly by dissolving more HsO, was discussedrecently by Bailey (1977) and Wyllie (1979).Thus any estimates based on simple experimentalsystems lacking Li, B and F would yield exces-sively high P-T values, partly because of anunrealistic granite solidus and partly also be-cause of Mg-, Fe- and F-induced shifts in thestability field of (OH)-muscovite (Miller er a/.t981, Anderson & RowleY 1981).

I'ERALUMINOUS PEGMATITIC GRANITES. MANITOBA 191

Of the four phases constituting the pegmatiticgranite intrusions, the fine grained leucograniteis the most simple and "primitive" rock type,least affected by supercritical leaching and otherprocesses that could have disturbed its originaligneous characteristics. In the granite-crystal-lization grid of Anderson & Cullers (1978), alarge part of the fine grained leucogranite com-positions clusters near the 0.5-l kbar minima inthe granite system. Considering the lregionametamorphic grade, this low pressure is prob-ably misleading as to the depth of consolidation(24 km), but it may be indicative of the con-ditions prevalent in large-scale dilation zonesinto which the pegmatitic granites were em-placed. Regarding the temperature of crystalliza-tion, the available experimental grids are evenmore difficult to use. as indicated earlier. How-ever, the two-feldspar geothermometer of Whit-ney & Stormer (1977) suggests a reasonabletemperature range between 64'0 and 600oC forthe microcline-plagioclase solvus [for I to 2kbar P(HsO)1, and even lower if the K-feld-spar equilibrated in a partly disordered stbte.

Petrogenesis

Derivation of the melts that gave rise to the

pegmatitic granites is still controversial. Thedata base and some details of the psesent stateof argument given in Goad (1981), Cern! er al.(1981) and Longstaffe et al. (1981) may com-plement.the brief review presented here.

Fractionation of batholithic tonalites and bio-tite granites of the district and single stage par-tial melting from a variety of possible sourcesare highly improbable processes because of theincompatible fraction4lion . trends and REEabr-rndinces (Fie. l0; ("rn{

"t a/. 1981). Also,

in a scheme of direct partial anatexis, the ex-treme fractionation shown by whole-rock andmineral compositions of the pegmatitic graniteswould require negligible percentages of melt,with consequent problems in restite-melt separa-tion. However, the data collected so far suggestthat the examined pegmatitic granites couldhave formed by either of the following two-stage processes: (l ) igneous differentiationfrom a juvenile source modified by interactionwith the greenstone-belt rocks (contaminatedI-type magma) or (2) partial melting of thegreenstone metasediments followed by igneousand fluid fractionation (differentiated S-typemagma). Both processes appear to be compatiblewith the rather constant bulk composition, er-ratic accessory mineralogy and trace-element

lrlFE.oz.or l Oo

lrjJo-=(t

LoCe Nd SmEu Dy Yb Lu Lo Cs Nd Sm Eu Dy Yb Lu

Ftc. 10. Comparison of REE abundances demonstrating genetic independence [A] between the early leuco-granite and late biotite granite of the I-ac-du-Bonnet batholith, and [Bl between the metaturbidites of theBooster Lake Formation (BRGB) as a possible source for direct partial melting of the three relativelyuncontaminated pegmatitic granite intrusions.

G L , E N L , T L

r92 THE CANADIAN MINERALOGIST

content, variations in normative corundum. R.EEabundances and oxygen-isotope ratios. However,testing these two processes by quantitative trace-element modeling is not feasible since experi-mental data on granitic systems oversaturatedwith volatile components and on partitioning ofREE and other trace elements between melt andan evolving supercritical fluid are not available.

In the first model, variable contamination ofa fractionated I-type melt by different supra-crustal lithologies and their low-temperaturepartial melts explains the regional distributionof accessory minerals and the devialions of sub-ordinate and trace elements from ideal fractiona-tion trends. Equilibration of juvenile melts withdifferent host rocks explains the variations inoxygen-isotope ratios among the four intrusionsThe highest degree of hybridization displayed bythe Osis Lake body is in accord with its re-action with the metaturbidite host. which isobserved even at the emplacement level andconceivably more extensive at depth. The com-positional data suggest a genetic link of the peg-matitic granites with a metarhyolite formationin the greenstone belt and with the early leuco-granite of the Lac-du-Bonnet batholith (Cerny1980a, b). These rocks appear to constitute afractionation series, very close to the compo-sitions of anorogenic, potassic and extremelyfractionated rhyolites from eastern Australia(Ewart et al. 1976, 1977), which are inter-preted as products of tholeiitic magma differ-entiation that were modified by sialic contami-nation. The compositions are analogous also tothe anorogenic coarse and fine Belongia gran-ites of Wisconsin, interpreted as derivatives byigneous fractionation of granitic magmas formedby partial melting of deep tonalitic crust (An-derson & Cullers 1978), Juvenile origin of com-positionaly similar leucogranites is also claimedby Kolbe & Taylor (1966), Proctor & El-Etr(1968), Taylor et al. (1968) and McKenzie &Clarke {1975).

The second model, fractionation of S-typemelts mobilized from the greenstone-belt meta-sediments, is analogous to that proposed byBreaks et al. (1978) and Drury (1979) forsimilar occurrences in northwestern Ontario andin the Yellowknife area: it has been tested inthe Winnipeg River district by (ernf et at.(1978). Partial melting of metasedimentarymaterial is also favored for compositionally sim-ilar leucogranites and pegmatitic granites byDidier & Lameyre (1969), Dickson (1974),Harris (1974), Dostal (1975) and is experi-mentally supported by Winkler et al. (1975).This model offers a simple explanation for the

relationship between individual pegmatitic gran-ites and their host rocks. The northern intrusionscould have been generated at depth from sub-vertical extensions of their volcaniclastic andmetaturbidite hosts and adjacent units, and thesouthern ones from similar sources and meta-rhyolitic lithologies; however, these were some-what modified by reaction with metabasalt hosts.The model is in better agreement with restrictedmobility of wet granitic magmas than the firstinterpretation. The apparent rhyolite-leucogran-ite-pegmatitic granite suite quoted above wouldhave to be dismissed as an incidental conver-gence.

In either model, the peraluminous nature ofthe pegmatitic granites seems to be enhanced byloss of alkalis through a vapor phase to the hostrocks. over and above the direct effect of meta-sedimentary hybridization or mobilization. Thisis supported by the mildly peraluminous toslightly diopside-normative composition of thelargely nonpegmatitic Lac-du-Bonnet leucogran-ite, whose consanguinity with the pegmatiticgranites is postulated by both models (Table 1.Fig. 3). Significantly, in this respect, the coarsegrained phases of the pegmatitic granites(i.e., the pegmatitic leucogranites and potassicpegmatites) are the most peraluminous, and thatthe Osis Lake intrusion. which shows the mostconspicuous exocontact effects, is the most pera-luminous as a whole.

The predominantly pegmatitic character ofthe intrusions implies large volumes of volatiles,specifically H:O, involved during and after theiremplacement and crystallization. In view of theclose relationship between the pegmatitic gran-ites and major fault-systems and their protractedepisodic activity, the melting mechanism pro-posed by Strong & Hanmer ( 1981 ) for the leuco-granites in Brittany may also have operated inthe Winnipeg River district.

Crystal-melt fractionation alone can hardlybe responsible for the high enrichment in Li,Rb,CsoBe,Ga and Sn and depletion in Ba and Sr,as opposed to the relative uniformity of bulk-rock compositions. Liquid-state differentia-tion could have been a factor; depolymer-ization would be very effective in volatile-saturated melts. as well as enrichment ofapical parts of individual intrusions in rare ele-ments and HREE transported in complex anions(Mahood 1979, Crecraft et al. 1979, Miller &Mitrlefehldt 1979. Muecke & clarke l98l).

The final deciphering of the process thatgenerated the Winnipeg River pegmatitic gran-ites would benefit from Sr and Pb isotopestudies, which were beyond the scope of the

PERALUMINOUS PECMATITIC CRANITES, MANITOBA 193

pre$ent research. Detailed examination of otherlocalities of pegmatitic granites, in differentgeological settings and igneous assemblages, isrequired to clarify and generalize their genesis,particularly in Archean and early Proterozoicterrains.

Acruowlt'ocEMENTS

This study is based on the data collected underthe Canada-Manitoba Subsidiary Agreement onMineral Exploration and Development in Mani-toba. The research was also funded byN.S.E.R.C. operating grants and D.E.M.R. re-search agreements to the second author.

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Received May 1980, revised manuscript acceptedDecember 1980.