Nd and Pb isotopes from the Lake of the Woodsgreenstone belt, northwestern Ontario:implications for mantle evolution and theformation of crust in the southern SuperiorProvince

John A. Ayer and Jaroslav Dostal

Abstract: Nd and Pb isotopes from the Lake of the Woods greenstone belt indicate the presence of three distinct reser-voir sources: old enriched crust (>3.0 Ga); pre-2.7 Ga, homogeneous depleted mantle; and post-2.70 Ga heterogeneousmantle. EpsilonNd values of +1.1 to +2.3 for ultramafic to felsic metavolcanic rocks (2.74–2.72 Ga) indicate derivationfrom depleted mantle. TheεNd value of –0.9 for younger turbidite (2.71 Ga), in conjunction with detrital zircon agesranging from 2.72 to 3.0 Ga, indicates detritus from local greenstone belt sources (depleted mantle) mixed with anolder crustal source. Post-2.70 Ga heterogeneity is demonstrated byεNd values ranging from –0.4 to +0.4 inshoshonitic to calc-alkaline metavolcanic rocks and +2.1 in a coeval ultrapotassic pluton. Pb isotopes from the plutonindicate derivation from a depleted mantle reservoir with an initial207Pb/204Pb of 14.52, an initial206Pb/204Pb of 13.29,andµ1 of 7.86. Isotopic comparison with post-2.70 Ga potassic suites from across the Superior Province indicateswidespread mixing between depleted mantle and enriched end members. The enriched end member has isotopic charac-teristics of rocks derived from old crustal terrains, such as the Winnipeg River and Opatica subprovinces. This type ofisotopic heterogeneity could be the result of crustal contamination or derivation from metasomatized mantle. Contami-nation of the mantle wedge by influx of fluids derived from partial melting of isotopically evolved, subducted sedi-ments is favoured for the Superior Province potassic suite, because elevated concentration of Sr, Nd, and Pb inconjunction with primitive Mg#s suggest only limited crustal contamination has occurred.

Résumé: Des isotopes Nd et Pb de la ceinture de roches vertes Lake of the Woods indiquent la présence de troissources réservoir distinctes : une vieille croûte enrichie (>3,0 Ga), un manteau appauvri homogène, pré-2,7 Ga; unmanteau hétérogène post-2,70 Ga. Les valeursεNd de +1,1 à +2,3 pour les roches ultramafiques à métavolcaniques fel-siques (2,74–2,72 Ga) indiquent une provenance d’un manteau appauvri. Une valeurgNd de –0,9 pour des turbiditesplus récentes (2,71 Ga), en conjonction avec des âges sur des zircons détritiques allant de 2,72 à 3,0 Ga, indique desdétritus de sources locales de la ceinture de roches vertes (manteau appauvri) mêlés à une source crustale plus an-cienne. L’hétérogénéité post-2,70 Ga est démontrée par des valeursεNd allant de –0,4 à +0,4 dans des roches métavol-caniques shoshonitiques à calco-alcalines et +2,1 dans un pluton contemporain ultrapotassique. Les isotopes Pb dupluton indiquent une provenance d’un réservoir de manteau appauvri ayant des rapports initiaux de207Pb/204Pb de14,52 et206Pb/204Pb de 13,29 et une valeur µ1 de 7,86. Une comparaison isotopique avec les suites potassiques post-2,70 Ga de partout à travers la province du Supérieur indique un mixage répandu entre le manteau appauvri et les ter-mes extrêmes enrichis. Le terme extrême enrichi possède les caractéristiques de roches dérivées d’anciens terrains crus-taux tels les sous-provinces de Winnipeg River et Opatica. Ce genre d’hétérogénéité isotopique pourrait résulter d’unecontamination crustale ou d’une dérivation d’un manteau métasomatisé. La contamination d’un biseau du manteau parun influx de fluides dérivés d’une fusion partielle de sédiments subductés et isotopiquement évolués est privilégiéepour la suite potassique de la province du Supérieur parce qu’une concentration élevée de Sr, Nd et Pb en conjonctionavec des Mg#s primitifs suggère que la croûte a seulement été partiellement contaminée.

[Traduit par la Rédaction] Ayer and Dostal 1689

Introduction

The isotopic characteristics of Archean reservoirs can beevaluated by using the Sr, Nd, and Pb isotopic compositionsof igneous rocks. This approach is similar to the one that ledto the recognition of heterogeneity in Phanerozoic mantlereservoirs (e.g., Zindler and Hart 1986). Temporal and spa-tial variations in mantle composition permit a better under-standing of crust–mantle interaction and aid in thecharacterization of different geodynamic environments. Ourdata from the Lake of the Woods greenstone belt within the

Can. J. Earth Sci.37: 1677–1689 (2000) © 2000 NRC Canada

1677

Received November 22, 1999. Accepted June 7, 2000.Published on the NRC Research Press Web site onNovember 10, 2000.Paper handled by Associate Editor K. Ansdell.

J.A. Ayer.1 Ontario Geological Survey, 933 Ramsey LakeRoad, Sudbury, ON P3E 6B5, Canada.J. Dostal. Department of Geology, Saint Mary’s University,Halifax, NS, B3H 3C3, Canada.

1Corresponding author (e-mail: john.ayer@ndm.gov.on.ca).

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Wabigoon subprovince indicate isotopic homogeneity in thepre-2.7 Ga mantle and heterogeneity in the post-2.7 Ga man-tle. These signatures are widespread across the southern Su-perior Province. Interpretation of Archean isotopic results ismore complex than in the modern environments, because theArchean rocks are typically more altered and metamor-phosed, and it is difficult to evaluate the extent to which theisotopic systems have remained closed since the time ofcrystallization (Machado et al. 1986). Additionally, the Su-perior Province isotopic reservoirs were shorter lived thanmodern-day mantle reservoirs and because of less time forradiogenic growth, the fractionation trends are less pro-nounced and closer to the margin of error of the measure-ments.

Radiogenic isotopes are particularly useful as petrogenetictracers when the composition of potential end members arewell characterized and when a precise time reference isavailable. The Lake of the Woods greenstone belt has an es-tablished stratigraphy, U–Pb geochronology has provided ahighly precise framework for the timing of geologicalevents, and the geochemical compositions of the igneoussuites have been studied in detail (Ayer and Davis 1997;Ayer 1998, 1999). Thus a Nd and Pb isotopic study of thisbelt provides additional insight into the characteristics of thelate Archean crust and mantle and provide the basis for com-parisons of reservoirs across the southern Superior Province.

Within the Superior Province, evolved Sr, Nd, and Pb iso-topes generally occur in the middle Archean granitic ter-rains, such as the Winnipeg River and Opatica subprovinces,and in late Archean metasedimentary terrains, such as theQuetico and Pontiac subprovinces (Gower et al. 1983; Corfu1988; Beakhouse and McNutt 1991; Carignan et al. 1993;Henry et al. 1998). The late Archean mafic–ultramafic rocksand volcanic associated sulphide minerals from the Abitibiand Wabigoon subprovinces have shown the existence of arelatively homogeneous pre-2.7 Ga (i.e., 2.70 to 2.75 Ga)depleted mantle across the southern Superior Province(Tilton 1983; Morrison et al. 1985; Shirey and Hanson 1986;Machado et al. 1986; Dupré and Arndt 1990; Vervoort et al.1994; Carignan et al. 1995). However, a different mantlesource is evident from the degree of isotopic heterogeneityin the post-2.7 Ga mantle-derived potassic suites (Shirey andHanson 1986; Tilton and Kwon 1990; Stern et al. 1989;Stern and Hanson 1991; Tilton and Bell 1994).

An interesting problem addressed by this paper is: whatgeodymanic process(es) were responsible for developmentof the heterogeneity in the post 2.7 Ga potassic suite? Somehave suggested that it is the result of contamination of isoto-pically depleted magma by evolved isotopic sources withinthe crust (Henry et al. 1998; Stevenson et al. 1999). How-ever, it is significant that similar types of isotopic heteroge-neity also occur in modern subduction-related potassicsuites, where the two main schools of thought about thesources for contamination are: (1) an upper crustal source or(2) a mantle wedge source (see e.g., Conticelli andPeccerillo 1992; Foley and Peccerillo 1992; Nelson 1992;Müller and Groves 1997). While the evidence is as yetequivocal, it is herein suggested that a metasomatized man-tle wedge source is a viable hypothesis for the isotopic char-acteristics of Superior Province potassic suites.

Regional setting

The Lake of the Woods greenstone belt is situated alongthe northwestern margin of the Wabigoon subprovince, anortheast-trending metavolcanic–plutonic belt about 150 kmwide exposed over a length of about 900 km in the SuperiorProvince (Fig. 1). The subprovince is bounded to the north bythe Winnipeg River and English River subprovinces and tothe south by the Quetico subprovince (Blackburn et al. 1991).

Extensive detailed and synoptic geological mapping pro-jects, as well as geochemical and geochronological studieshave resulted in subdivision of the Lake of the Woodsgreenstone belt into three supracrustal assemblages (Fig. 2)(Ayer and Davis 1997): (1) the Lower Keewatin assemblage,(2) the Upper Keewatin assemblage, and (3) the Electrumassemblage. Metamorphism is within greenschist-facies con-ditions for most supracrustal rocks, but has achieved loweramphibolite-facies conditions in proximity to the largeplutons. For the sake of brevity the prefix meta will be omit-ted.

The Lower Keewatin assemblage consists of five geo-graphically isolated mafic volcanic groups correlated on thebasis of stratigraphy, lithological and geochemical similari-ties, and geochronology. Its maximum preserved strati-graphic thickness is about 10 km. Geochemical analyses ofleast altered tholeiitic basalt from this assemblage indicateflat to slightly enriched light rare earth elements (LREE) andflat heavy rare earth elements (HREE) at about 10 timeschondritic values, and undepleted high field strength ele-ments (HFSE) on extended trace-element diagrams (Ayerand Davis 1997). Zircon grains from the upper part of theassemblage yield an upper age limit of 2737.9 ± 2.0 Ma(Ayer and Davis 1997).

The Upper Keewatin assemblage is the most areally ex-tensive assemblage within the Lake of the Woods greenstonebelt. It comprises a diversity of stratigraphically and struc-turally interdigitated rock units having an estimated strati-graphic thickness of about 10 km. Three distinctive rockassociations compose the assemblage: (1) mafic to felsicvolcanic rocks of calc-alkaline affinity, which have enrichedLREE, depleted HREE and moderately depleted HFSE onchondrite-normalized REE plots and extended trace-elementdiagrams (Ayer and Davis 1997); (2) tholeiitic mafic volca-nic rocks and komatiite occur in sequences up to 2 km thick(intercalated with the calc-alkaline sequences), which havegenerally higher Mg#, flat to slightly depleted LREE andflat HREE at about 10 times chondrite, and slightly more en-riched HFSE than the Lower Keewatin assemblage maficvolcanic rocks; and (3) turbiditic sedimentary sequences.U Pb− zircon ages from felsic volcanic rocks in the lowerstratigraphy range from 2723 to 2719 Ma, whereas zircongrains from felsic pyroclastic rocks intercalated with theturbidite in the upper part of the assemblage have a U–Pbage of 2712 ± 2 Ma (Ayer and Davis1997). Earlysynvolcanic intrusions consist of tonalite and granodiorite instocks and batholiths with zircon ages ranging from 2717 to2724 Ma, coeval with early volcanism in the UpperKeewatin assemblage. Syn- to late-tectonic granodiorite in-trusions postdate Upper Keewatin volcanism, ranging in agefrom 2709 to 2711 Ma (Blackburn et al. 1991).

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1678 Can. J. Earth Sci. Vol. 37, 2000

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The Electrum assemblage consists of the Crowduck andthe White Partridge groups which are restricted to the north-ern part of the Lake of the Woods greenstone belt (Fig. 2).The assemblage unconformably overlies the Lower and Up-per Keewatin assemblages and consists of clastic sedimen-tary rocks and subordinate shoshonite to calc-alkalinevolcanic units with highly enriched LREE, depleted HREE,and depleted HFSE patterns; detrital zircon grains indicatedeposition after 2699 ± 2 Ma (Ayer and Davis1997). A latemagmatic event represented by late-tectonic to posttectonicpotassic to granitic plutons is coeval with Electrum assem-blage volcanism. The Falcon Island ultrapotassic pluton is amember of this group and has an age of 2695 ± 3 Ma (Ayerand Davis 1997). The units of the pluton have highly en-riched REE and depleted HFSE patterns similar to theElectrum assemblage shoshonitic volcanic rocks (Ayer andDavis 1997).

The Winnipeg River subprovince lies immediately to thenorth of the Lake of the Woods greenstone belt (Figs. 1, 2).It is a predominantly granitic terrane, which was tectonicallyjuxtaposed with the Lake of the Woods greenstone by

2709 ± 4 Ma, as indicated by the age of a granodiorite sillintruded along the contact between the two exotic terrains(Corfu 1988). It consists of gneissic and plutonic rocks span-ning a large range of ages, subdivided into two groups: anolder (2830–3170 Ma) gneissic sodic group intruded byposttectonic potassic plutons (2660–2709 Ma) (Beakhouse1991). It is likely that old detrital zircon ages and evolvedisotopic signatures found in some of the Lake of the Woodsgreenstone belt units were derived from the Winnipeg Riversubprovince.

Analytical techniques

Nd isotopes (Table 1) were determined for the Lake of theWoods greenstone belt samples at the Geoservices Labora-tory of the Ontario Ministry of Northern Development andMines, Sudbury and for the Falcon Island pluton samples atCarleton University, Ottawa. Whole-rock powder was dis-solved and analyzed utilizing techniques described inCousens (1996).

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Ayer and Dostal 1679

Fig. 1. Geological map of the Superior Province in Ontario showing distribution of the subprovinces and location of the map in Fig. 2(Ayer and Davis 1997).

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The total procedural blanks for Nd are <50 pg at theGeoservices Laboratory. Internal 2σ uncertainties of themeans of 143Nd/ 144Nd isotopic analyses were typically0.002%, and external reproducibilities based on replicate

runs were 0.004% based on reproducibility of United StatesGeological Survey rock standard BCR-1 (n = 7), whichyields Sm = 6.61 ± 0.46 ppm, Nd = 28.74 ± 1.65 ppm, and143Nd/ 144Nd = 0.512643 ± 15, and an internal rock standard

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1680 Can. J. Earth Sci. Vol. 37, 2000

Fig. 2. General geology of the Lake of the Woods greenstone belt with isotope sample locations (Ayer and Davis 1997). Sample num-bers, rock names, and stratigraphic unit or intrusion names are indicated in Table 1. Details on the mineralogy and chemical composi-tion of the samples are available in Ayer and Davis (1997) and Ayer (1998, 1999).

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(MRB-29). Concentrations are precise to ±1%, but147Sm/144Nd ratios are reproducible to 0.5%. Seven runs of the LaJolla Nd standard average143Nd/ 144Nd = 0.511839 ± 19 arewithin error of the literature value of 0.511860 ± 10 (Shireyand Hanson 1986), so no correction was applied.

The total procedural blanks for Nd are <75 pg at CarletonUniversity. The internal 2σ uncertainties of the means of Ndanalyses were typically 0.002% and external reproducibili-ties based on replicate runs were 0.004%. Isotope ratios arenormalized to 146Nd/ 144Nd = 0.72190. Analyses of theUnited States Geological Survey standard BCR-1 yield Nd =29.02 ppm, Sm = 6.68 ppm, and143Nd/ 144Nd = 0.512668 ±20 (n = 4). Thirty four runs of the La Jolla standard average143Nd/ 144Nd = 0.511875 ± 18 (September 1992 – July1996).

Lead isotope ratios were determined at Carleton Univer-sity. Alkali feldspar grains from four samples of the FalconIsland pluton were hand picked by binocular microscope af-ter crushing and separation using a Frantz magnetic separa-tor. Following the proceedure of Gariépy and Allègre(1985), the selected grains were acid washed to remove theouter layer of the feldspar and surface contaminants and topreferentially remove as much labile radiogenic Pb in thefeldspar structure as possible. The fractions were washed in6N HCl for one hour at 120°C, then partially dissolved in amix of HBr and HF for one hour. The remaining feldspargrains were then processed to extract Pb following the tech-niques of Cousens (1996).

Total procedural blanks for Pb are <150 pg. Analysis ofUnited States Geological Survey Standard BCR-1 yields206Pb/ 204Pb = 18.818,207Pb/ 204Pb = 15.633, and208Pb/

204Pb = 38.633 (average of two runs, see Tatsumoto et al.1972). All mass spectrometer runs are corrected for fraction-ation using National Institute of Standards and Technology(NIST) SRM981. The average ratios measured for SRM981are 206Pb/ 204Pb = 16.890 ± 0.012,207Pb/ 204Pb = 15.429 ±0.014, and208Pb/ 204Pb = 36.502 ± 0.048 (2σ), based on 38runs between September 1992 and February 1996. The frac-tionation correction based on the values of Todt et al. (1984)is +0.13%/amu.

Nd isotope results

Samples for Sm–Nd isotope analysis were selected to rep-resent the range of ages and compositions of the supracrustalrocks in the Lake of the Woods greenstone belt. The FalconIsland pluton is a mantle-derived alkalic complex that is co-eval with volcanic rocks from the Electrum assemblage. Itwas sampled to determine the isotopic characteristics of thepost-2.7 Ga mantle and for comparison with the other mag-matic suites of the belt. The sample locations are shown onFig. 2, and results are presented in Table 1. Linear regres-sions were calculated using “ISOPLOT,” a program of Lud-wig (1992) utilizing the least-squares regression techniquesof York (1968).

Figure 3 displays the secular variation of the Nd isotopevalues of individual samples. Tholeiitic basalt samples fromthe Lower Keewatin assemblage (2.74 Ga) yieldεNd valuesof +1.7 and +2.3. The Upper Keewatin assemblage volcanicsamples (2.72 Ga) yieldεNd values of +1.1 to +2.1 and arewithin error (±0.5 epilson unit) of the Lower Keewatin as-semblage tholeiite samples (Table 1). This relatively narrow

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Ayer and Dostal 1681

LocationNo.(on Fig. 2)

SampleNo. Rock type Strata

Sm(ppm)±1%

Nd(ppm)±1%

147

144

SmNd

±0.5%

143

144

NdNd

±0.00002εNd(T)±0.5

Ta

(Ma)

Supracrustal Rocks1 87-839 Tholeiitic basalt LKA 1.93 5.59 0.209 0.51294 1.7 27382 93-23 Tholeiitic basalt LKA 1.57 4.61 0.206 0.51292 2.3 27383 93-36 Calc-alkalic andesite UKA 3.17 15.10 0.127 0.51146 1.3 27204 93-37 Tholeiitic basalt UKA 1.50 4.40 0.206 0.51288 1.7 27205 93-18 High silica rhyolite UKA 6.87 43.8 0.095 0.51086 1.1 27196 93-29 Calc-alkalic rhyolite UKA 1.99 12.0 0.101 0.51102 2.1 27197 93-30 Komatiite UKA 0.72 1.90 0.229 0.51328 1.3 27208 93-26 Komatiitic basalt UKA 1.39 4.35 0.193 0.51263 1.2 27209 93-27 Evolved basalt UKA 3.55 18.8 0.114 0.51124 1.6 2720

10 89-1116 Wacke UKA 4.60 26.3 0.106 0.51096 –0.9 271211 93-17 Shoshonitic andesite EA 5.44 28.0 0.117 0.51125 0.4 269912 93-35 Calc-alkalic rhyolite EA 1.88 13.8 0.082 0.51058 –0.4 2699

Falcon Island Pluton13 88-182 Clinopyroxenite 11.8 62.8 0.113 0.51126 2.1 269514 88-190 Peraluminous syenite 9.76 58.4 0.101 0.51104 2.1 "15 88-195 Monzodiorite 22.8 124.2 0.111 0.51120 1.8 "16 88-261 Peraluminous syenite 7.36 55.1 0.081 0.51073 1.9 "17 93-31 Syenite 8.53 50.4 0.102 0.51107 2.3 "18 93-32 Melasyenite 20.8 121.3 0.104 0.51109 2.0 "19 93-33 Melasyenite 12.3 74.2 0.100 0.51103 2.2 "

Note: LKA, Lower Keewatin assemblage; UKA, Upper Keewatin assemblage; EA, Electrum assemblage.aEstimated age based on correlation with U–Pb geochronology (Ayer and Davis 1997).

Table 1. Whole-rock Sm–Nd data from Lake of the Woods greenstone belt and the Falcon Island pluton.

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range ofεNd values is interesting considering the wide rangeof compositions from ultramafic to felsic and their differingtrace-element patterns (Ayer and Davis 1997).

Wacke from the Upper Keewatin assemblage depositedaround 2.71 Ga yields anεNd value of –0.86, which is con-siderably lower than that of the pre-2.7 Ga Lake of theWoods volcanic samples. Relatively enriched Nd isotopicsources are also evident in shoshonitic andesite (εNd = +0.4)and calc-alkaline rhyolite (εNd = –0.4) of the post-2.7 GaElectrum assemblage (Table 1). On the other hand, the Fal-con Island ultrapotassic pluton emplaced around 2695 Mayields elevatedεNd values ranging from +1.8 to +2.3 (avg. =+2.1).

Pb isotope results of the Falcon Islandpluton

Alkali feldspar has low-U concentrations and conse-quently very low abundances of radiogenic Pb. Thus, analy-sis of the Pb isotopes of alkali feldspar provide a useful toolto estimate the initial Pb isotopic ratios of magma sources(Gariépy and Allègre 1985). Acid leaching removes surfacecontaminants and loosely bound Pb (presumably radiogenicin origin). Any remaining sources of radiogenic Pb will mostlikely be from fine-grained mineral or fluid inclusions thatcontain U or Th as well as Pb.

Lead isotope analyses of leached alkali feldspar separatesfrom four different locations within the Falcon Island plutonare tabulated in Table 2. Four different splits of melasyenitesample 93-33 (Table 2) were run to check the analytical un-certainty and the efficiency of acid leaching in removal of

radiogenic Pb. Two leached splits, free of any visible con-tamination, yielded results that are slightly above the quoted2σ error, which could be the result of analytical error or aheterogeneous distribution of small inclusions. Thus an esti-mate of the uncertainty is ± 0.02 for207Pb/ 204Pb and ± 0.1for both 206Pb/ 204Pb and208Pb/ 204Pb. A third leached splitof sample 93-33 contained small visible biotite inclusions. Ityielded more radiogenic Pb isotopes than the visually cleansplits, but it is still slightly less radiogenic than theunleached split (Table 2).

The results from the four different alkali feldspar splits ofsample 93-33 show that there is a radiogenic Pb componentin the feldspars, thus the initial Pb isotopic composition cannot be directly determined by leaching alone. That the fourdifferent splits of sample 93-33 and the leached alkali feld-spars from three other samples all fall along an isochronwith a slope yielding an age of 2700 ± 100 Ma (Fig. 4), sim-ilar to the U–Pb crystallization age of the pluton (2695 ±3 Ma), implies that the radiogenic Pb component in the feld-spar minerals was produced by in situ decay of U and Thand that the Pb isotopic system has remained closed sincecrystallization.

Leached alkali feldspar from syenite sample 88-190 yieldsthe least radiogenic Pb values with a single stage207Pb/204Pb and206Pb/204Pb model age of 2618 Ma, closest to thecrystallization age of the pluton (Table 2). The leached feld-spar from melasyenite samples 93-32 and 93-33 yieldslightly more radiogenic Pb isotopes, while the leached feld-spar from syenite sample 93-31 is significantly more radio-genic than all of the other samples, including the biotite-bearing and unleached fractions. It is displaced a consider-

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Fig. 3. An εNd vs. time diagram for samples from the Lake of the Woods greenstone belt, Falcon Island pluton (this study), and theAulneau and Lount Lake batholiths (Beakhouse and McNutt 1991). Depleted mantle evolution curves from DePaolo (1987) and 3.2 Gacontinental crust evolution curves for the southwestern Superior Province from Henry et al. (1998). EA, Electrum assemblage; LKA,Lower Keewatin assemblage; UKA, Upper Keewatin assemblage.

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able distance from the 4.55–2.70 Ga geochron (Fig. 4) andhas a model age of 981 Ma, which is considerably youngerthan the crystallization age of the pluton. Its elevated208Pb/204Pb and206Pb/204Pb values suggests that this sample

may contain significant Th as well as U. This sample is fromthe margin of the pluton and unlike the massive internalunits, it is foliated and contains epidote and andraditeovergrowths indicative of late-stage alteration (Ayer 1998).

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Sample No. Rock type

206

204

PbPb

207

204

PbPb

208

204

PbPb

206/207Model Age(Ma)

88-190 (leached) Syenite 13.533 14.576 33.368 261893-31 (leached) Syenite 15.897 15.004 36.150 98193-32 (leached) Melasyenite 13.687 14.590 33.565 245293-33 (leached) Melasyenite 13.748 14.603 33.417 240393-33 (leached duplicate) Melasyenite 13.856 14.618 33.528 230293-33 (leached fraction w.

biotite)Melasyenite 14.229 14.689 33.913 2031

93-33 (unleached fraction) Melasyenite 14.235 14.708 34.122 2044

Table 2. Pb isotopes of alkali feldspar separates, Falcon Island pluton.

Fig. 4. A 207Pb/204Pb vs.206Pb/204Pb diagram showing the secondary isochron for feldspar separates from the Falcon Island pluton.Leached feldspars data from various samples as indicated in Table 2. A single stage isochron for evolution from 4.55 to 2.70 Ga withCanyon Diablo meteorite compositions (Tatsumoto et al. 1973) withµ1 values is also shown, as is the Stacey and Kramer (1975)crustal evolution curve (S–K). Fields of initial207Pb/204Pb vs.206Pb/204Pb for various suites from the southern Superior Province afterCarignan et al. (1995) are (1) depleted mantle as represented by mafic to ultramafic flows and intrusions; (2) calc-alkalinetrondhjemite–tonalite–granodiorite suites from the Abitibi subprovince; (3) alkaline and LREE-enriched plutons in the Abitibi,Wabigoon, and Wawa subprovinces; (4) gneisses and granitic plutons from the Pontiac, Abitibi, and Opatica subprovinces; (5) gneissand granitic plutons from the Winnipeg River subprovince. The star represents the initial Pb composition of the Falcon Island pluton.

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This suggests excess radiogenic Pb may have been acquiredby contamination or during a multistage history (see Faure1986). It is likely that contamination occurred around thetime of emplacement of the pluton, as the207Pb/ 204Pb and206Pb/ 204Pb ratios of this feldspar sample lie on the same2.7 Ga isochron as the less radiogenic feldspars.

The 207Pb/ 204Pb and206Pb/ 204Pb values of the feldsparsamples all plot well below the S–K crustal evolution curve(Fig. 4), indicating derivation from a large ion lithophile ele-ment depleted mantle source. Regressing the Falcon Islandsecondary isochron to intersection with a single-stagegrowth curve geochron from 4.55 to 2.70 Ga with CanyonDiablo troilite Pb isotopic composition (Tatsumoto et al.1973) yields initial207Pb/ 204Pb = 14.52 and206Pb/ 204Pb =13.29, corresponding to a reservoir with aµ1 = 7.86.

Discussion

Timing of isotopic heterogeneityThe εNd values of pre-2.7 Ga Lake of the Woods volcanic

sequences are similar to those from rocks of similar agefrom other parts of the Western Wabigoon subprovince, in-cluding tholeiite, intermediate and felsic volcanic rocks, andanorthosite of the Rainy Lake greenstone belt and theMulchay mafic to ultramafic intrusion (Ashwal et al. 1983;Morrison et al. 1985; Shirey and Hanson 1986). They arealso similar to depleted mantle estimates derived from clino-pyroxene and plagioclase mineral separates from volcanicrocks in the Abitibi greenstone belt (i.e.,εNd = +2.5 ± 0.3,Machado et al. 1986). The lack of any systematic change inεNd with increasing SiO2 (or variable degrees of REE frac-tionation) for the Lower and Upper Keewatin volcanic sam-ples is consistent with derivation from relativelyhomogeneous Nd isotopic reservoir(s).

Komatiite from the Lake of the Woods differ from thoseof the southern Abitibi greenstone belt in that they are volu-metrically less abundant and compositionally less primitive(i.e., <23% MgO) (Ayer and Davis 1997). Abitibi komatiiterocks also have typically higherεNd values (i.e., >+3) andhave been attributed to deep mantle plume sources (Cattellet al. 1984; Xie et al. 1993; Lehaye et al. 1995; Dostal andMueller 1997). Ayer and Davis (1997) indicated that theLake of the Woods komatiite rocks have similar REE values,LREE depletion, and chondritic Al2O3/TiO2 ratios (i.e.,about 20) to typical alumina-undepleted Archean komatiiteof the Abitibi subprovince and suggested that they were de-rived by higher degrees of partial melting at relatively shal-low depths of a mantle source that may have also been thesource for the associated basalts. The similar isotopic char-acteristics of the Lake of the Woods komatiite and basalt isconsistent with this hypothesis.

The Aulneau batholith is a large plutonic complex that in-trudes the Lower Keewatin assemblage in the southern partof the Lake of the Woods greenstone belt (Fig. 2). Thebatholith consists of early tonalite dated at2717 3

5−+ Ma and

late granodiorite with an age of2710 24

−+ Ma (Davis and Ed-

wards 1986). The tonalites have highly fractionated REE anddepleted HREE typical of trondhjemite–tonalite–grano-diorite (TTG) suites (Beakhouse and McNutt 1991). Isotopicanalysis of the Aulneau tonalite yields mantle-like initial87Sr/86Sr values (0.7008) andεNd values (+1.4 and +1.5)

(Beakhouse and McNutt 1991). Thus the tonalite is similarin both age and Nd isotopic characteristics to the UpperKeewatin assemblage volcanic samples (Fig. 3). The uniquetrace-element characteristic of TTG suites have been attrib-uted to melting of hydrated tholeiitic basalt under conditionsin which both garnet and amphibole are retained in the resid-uum (Martin 1987; Drummond and Defant 1990). Martin(1993) hypothesized that because subducted oceanic crust inthe Archean was younger and hotter than in the present-daysubduction zones, conditions favourable for the genesis ofTTG-type magma were probably more common in the past.Thus the age, isotopic, and geochemical evidence all suggestthe Aulneau TTG suite may have been derived by melting ofsubducted basaltic crust with isotopic characteristics similarto the Lower and Upper Keewatin assemblage volcanic se-quences.

The depleted mantle evolution curve of DePaolo (1981)corresponds well with the maximum observed depleted man-tle conditions underlying the Lake of the Woods greenstonebelt between 2.69 and 2.74 Ga (Fig. 3). The curve corre-sponds to anεNd

2.7 Ga value of +2.2, which is similar to thatof the Falcon Island pluton samples, and is in general agree-ment with Archean sources in the Superior Province andelsewhere (Shirey and Hanson 1986; Stevenson 1995;Vervoort et al. 1996). This curve is utilized to derive the en-riched continental crust evolution curve shown on Fig. 3, byusing its εNd

3.2 Ga value and a147Sm/144Nd ratio of 0.115(Henry et al. 1998). A starting age of 3.2 Ga is chosen as itrepresents the oldest age found to date within the WinnipegRiver subprovince (Corfu 1988). This enriched crustal evolu-tion curve has anεNd

2.7 Ga of –3.5, which is close to the mostenriched sample from the Lount Lake batholith. The LountLake batholith occurs in the central part of the WinnipegRiver subprovince and its enriched isotopic values are attrib-uted to partial melting of a crustal source that is 200–400 Ma older than the U–Pb zircon age of the batholith(2702 3

4−+ Ma) (Beakhouse and McNutt 1991).

In general, the Sm–Nd isotopic signatures of fine clasticsediments record the overall mean characteristics of theircrustal sources (Taylor and McLennan 1985). Wacke fromthe Upper Keewatin assemblage (sample 89-1116) has Ndconcentrations (26 ppm) and147Sm/144Nd ratios (0.1057)(Table 1) similar to worldwide averages for mature sedi-ments (i.e., Nd = 20 ppm and147Sm/144Nd = 0.118; Jahn andCondie 1995). Davis (1996) has determined that the UpperKeewatin assemblage turbidite contains 3.0 Ga detrital zir-cons, as well as detrital zircons similar in age to the UpperKeewatin assemblage volcanics, and is consistent with juxta-position of the Wabigoon and Winnipeg River subprovincesby the time of Upper Keewatin assemblage sedimentation(i.e., 2.71 Ga). AnεNd value of –0.8 for the Upper Keewatinassemblage wacke is considerably lower than that of the pre-2.7 Ga volcanic rocks, but is within the range of the LountLake batholith samplesεNd values (i.e., –0.4 to –2.6)(Fig. 3). Using a two-component mixing model (Pushkar etal. 1972; Patchett and Bridgewater 1984), theεNd value ofthis wacke implies mixing of about 35% detritus from an oldcontinental crust source withεNd of –3.5 (the value of the3.2 Ga continental evolution curve at 2.71 Ga, Fig. 3) and atypical upper continental crust Nd concentration of 26 ppm(Taylor and McLennan 1985), with 65% locally derived de-

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tritus from Lake of the Woods supracrustal rocks (andsynvolcanic intrusions) predating the wacke (i.e., avg.εNd =+1.6 and avg. Nd = 12 ppm, Table 1).

Post-2.7 Ga potassic suites in the Lake of the Woodsgreenstone belt have variable isotopic signatures. TheElectrum assemblage shoshonite rocks are LREE enrichedwith elevated Mg# (Ayer and Davis 1997; Ayer 1998) andhave affinity with the Superior Province sanukitoid suitesthat have been attributed to metasomatized mantle sources(Stern et al. 1989; Stern and Hanson 1991). Electrum assem-blage shoshonite and calc-alkaline rhyolite yieldεNd valuesof +0.4 and –0.4, respectively. These values are significantlylower than the pre-2.7 Ga Lake of the Woods volcanic andthe Aulneau batholith TTG samples and are close to theεNdvalue of the Upper Keewatin assemblage wacke discussedabove. However,εNd values that average +2.1 for the FalconIsland pluton samples also provide evidence for the exis-tence of a post-2.7 Ga depleted mantle component in theLake of the Woods greenstone belt.

The εNd values of the pre-2.7 Ga Lake of the Woodsgreenstone belt volcanic and plutonic samples indicate thathomogenous depleted sources existed until around 2.7 Ga.Some of the pre-2.7 Ga samples are slightly less depletedthan the depleted mantle evolution curve (Fig. 3) and couldbe the result of minor early contamination of the mantle res-ervoir by ancient crust (Carignan et al. 1993, 1995; Vervoortet al. 1994; Henry et al. 1998). However, the post-2.7 Ga ig-neous samples with lowεNd values are unequivocally fromisotopically evolved sources.

Detecting crustal contamination using Pb isotopesstrongly depends on the U/Pb ratios of the sources and thecrustal residence times. Significant variations in the produc-tion of 207Pb compared to206Pb occurs in Archean reservoirshaving different U/Pb ratios as a result of the shorter halflife of 235U relative to 238U. Thus melts derived from juve-nile mantle, mantle contaminated by older crust, or the oldercrust itself, should have progressively higher207Pb/204Pb val-ues (Gariépy and Allègre 1985; Carignan et al. 1993, 1995).The Falcon Island pluton has a similar206Pb/204Pb ratio tothat of Wabigoon and Abitibi depleted mantle, but is slightlyhigher in its 207Pb/204Pb ratio. In this latter ratio, it is mostsimilar to the TTG plutons and associated calc-alkaline vol-canic suites of the southern Abitibi subprovince (Fig. 4),with slightly elevated ratios attributed to minor contamina-tion of the mantle sources (Carignan et al. 1993, 1995).

Source of heterogeneity for post-2.7 Ga potassic suites:crustal vs. mantle contamination

The elevated LREE, depleted HREE and HFSE, and vari-able isotopic signatures of the post-2.7 Ga igneous suitehave two possible sources: (1) assimilation of enriched ma-terial in crustal-level magma chambers, or (2) direct or indi-rect derivation from metasomatized mantle sources. Ingeneral, potassic suites are considered to be very good indi-cators of the isotopic composition of their sources becausetheir Sr, Nd, and Pb concentrations are at least an order ofmagnitude greater than that of surrounding crustal rocks,making them relatively insensitive to contamination of theseisotopes (Nelson 1992; Conticelli and Peccerillo 1992).Based on simple mass balance calculations, the elevated Mg,Ni, and Cr, and low SiO2 abundances of all but the most

evolved units preclude significant assimilation of felsiccrustal material (Conticelli and Peccerillo 1992; Foley andPeccerillo 1992; Hattori et al. 1996; Müller and Groves1997). Thus, geochemical and isotopic signatures indicatingcrustal contamination are evident only in the most evolvedparts of alkalic suites (Fowler 1988; Laflèche et al. 1991;Chen et al. 1994; Landoll and Foland 1996).

Combined initial Nd, Pb, and Sr isotopic data for a num-ber of well-documented, mantle-derived, post-2.7 Gapotassic and ultrapotassic suites from the southern SuperiorProvince are shown on Fig. 5. The field of pre-2.7 Ga juve-nile depleted mantle (DM) is derived from Wabigoon andAbitibi subprovinces sources, including the Mulcahy maficto ultramafic intrusion (Carignan et al. 1995; Morrison et al.1985); U-poor sulphides in komatiite flows (Brèvart et al.1986), galena, and whole-rock analyses associated withvolcanogenic massive sulphides (Deloule et al. 1989;Vervoort et al. 1994); and mineral separates and whole-rockanalyses from komatiite and basalt (Machado et al. 1986;Lehaye and Arndt 1996). The field of old continental crust(CC) is derived from plutonic suites from the WinnipegRiver, Quetico, Pontiac, and Opatica subprovinces (Gower1983; Corfu 1988; Beakhouse and McNutt 1991; Carignanet al. 1993; Bédard and Ludden 1997; Henry et al. 1998).

Of the post-2.7 Ga potassic suites, the Falcon Islandpluton, Otto syenite pluton, Lac Shortt, and Dolodaucarbonatite dikes (Tilton and Bell 1994) all haveεNd and ini-tial 207Pb/204Pb,206Pb/204Pb, and87Sr/86Sr ratios that are onlyslightly more evolved than the pre-2.7 Ga depleted mantlefield (Fig. 5). Other post-2.7 Ga complexes, such as the Stur-geon Lake syenite pluton (Tilton and Bell 1994), the PoobahLake syenite pluton (Tilton and Bell 1994), the RoaringRiver monzodiorite to granodiorite pluton (Stern and Hanson1991), and Eye-Dashwa diorite to granodiorite pluton(Henry et al. 1998), have more evolved Sr, Nd, and Pb isoto-pic values and lie in a diffuse linear array between the fieldsof depleted mantle and old crust. Isotopic comparisons ofthis nature should be viewed with a degree of caution, how-ever, because different geological environments were sam-pled; the post-2.7 Ga intrusions may vary in age by up to20 Ma; the isotopic variation may be relatively close to itserror; and different techniques have been used for determin-ing the initial Pb isotopes, such as regression of a secondaryisochron to the 2.7 Ga geochron for the Falcon Islandpluton; Pb ratios normalized to a common age of 2.7 Ga as-suming a source reservoir having aµ of 8 (Vervoort et al.1994); and Pb ratios, which are the measured ratios cor-rected for in situ decay of radiogenic Pb estimated from theU/Pb ratio and the age of the material analysed (Tilton andBell 1994; Carignan et al. 1995).

Using Pb–Sr–Nd isotopic studies of modern oceanic bas-alts, Zindler and Hart (1986) interpreted isotopic correlationas mixing curves between different isotopic components inthe mantle. They identified the following four end-membercomponents: (1) depleted mid oceanic ridge basalt mantle;(2) high U/Pb with elevated U/Pb ratios and no parallel in-crease in Rb/Sr and Sm/Nd ratios; (3) enriched mantle I pos-sessing low U/Pb coupled with a low Sm/Nd and moderateRb/Sr ratios; and (4) enriched mantle II marked by highRb/Sr, low Sm/Nd, and moderate to high U/Pb ratios similarto upper crustal isotopic compositions. The isotopic ratios of

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modern potassic suites show a wide range of Sr, Nd, and Pbisotopic ratios that have been characterized into two distinc-tive trends: (1) values ranging from near bulk Earth toenriched mantle II for subduction-related suites, lamphyro-phyre, kimberlite, and lamproite, and (2) values rangingfrom enriched mantle I to high U/Pb for continental alkalicsuites (see e.g., Conticelli and Peccerillo 1992; Nelson 1992;Simonetti and Bell 1995).

Mixing between the relatively short-lived reservoirs of thesouthern Superior Province (ca. 300 Ma) yielded consider-

ably less evolved trends than in the long-lived reservoirs ofthe modern mantle. In addition, the degree of isotopic evolu-tion varied for the different Archean isotopic systems. Forexample, depleted mantle became fractionated from CHUR(the chondritic uniform reservior) for initial143Nd/ 144Nd ra-tios very early in Earth’s history (Chase and Patchett 1988;Collerson et al. 1991; Armstrong 1991; Vervoort et al.1996). There is a moderate degree of radiogenic growth evi-dent for Archean initial207Pb/204Pb ratios because of a shorthalf life, but very little for 206Pb/204Pb, with its much longer

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Fig. 5. Comparison of initial Nd, Pb, and Sr isotopic values from a variety of post-2.7 Ga potassic and ultrapotassic igneous suitesacross the southern Superior Province. Data for the Falcon Island pluton (FP); Roaring River pluton (RP) (Stern and Hanson 1991);Eye-Dashwa pluton (EP) (Henry et al. 1998); Sturgeon Lake pluton (SP), Poohbah Lake pluton (PP), Otto pluton (OP) (Ben Othman etal. 1990; Tilton and Bell 1994), Lac Shortt (LC) and Dolodau (DC) carbonatite dykes (Tilton and Bell 1994). The pre-2.7 Ga depletedmantle field (DM) derived from data on the Mulcahy intrusion (Morrison et al. 1985; Carignan et al. 1995); Abitibi subprovince min-eral separates and whole-rock analyses (Machado et al. 1986; Lehaye and Arndt 1995); and Noranda and Matagami whole-rock volca-nic and galena analyses (Vervoort et al. 1994). Old crustal field (CC) derived from plutonic suites from the Winnipeg Riversubprovince (Gower et al. 1983; Corfu 1988, Beakhouse and McNutt 1991); the Opatica and Pontiac subprovinces (Carignan et al.1993; Bédard and Ludden 1997); and the Quetico subprovince (Henry et al. 1998).

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half life. In addition, only minor differentiation is evident inthe Superior Province mantle initial87Sr/86Sr ratios, as thevalues tend to cluster closely around estimates for bulkEarth (Kwon et al. 1989; Tilton and Kwon 1990; Tilton andBell 1994).

Notwithstanding the smaller and variable degrees of dif-ferentiation of the different isotopic systems in the SuperiorProvince, the pre-2.7 Ga southern Superior Provincekomatiite, tholeiite, calc-alkaline, and TTG suites all appearto have been derived from relatively homogeneous depletedmantle reservoirs (i.e., low Pb and Sr and elevated Nd iso-topes), which may be somewhat analogous to present-daydepleted mantle.

The post-2.7 Ga suites range from low Pb and Sr ratiosand elevatedεNd values to higher Pb, Sr ratios and lowerεNdvalues (Fig. 5). This type of variation is indicative of mixingand is similar in trend, but considerably less differentiatedthan that of modern subduction-related potassic suites. Thepost-2.7 Ga suite isotopic variation is, thus, suggestive ofmixing between an Archean depleted mantle-like componentand the Archean equivalent to enriched mantle II. The mix-ing could have occurred as contamination in the crust for themost fractionated members of some of the suites (see e.g.,Landoll and Foland 1996, Stevenson et al. 1999), but wasprobably also the result of derivation from a mantle sourcewith variable isotopic signatures. The authors favour a man-tle source for some of the evolved signatures, because it hasbeen determined that the suite was derived frommetasomatized mantle (Shirey and Hanson 1986; Stern et al.1989; Stern and Hanson 1991), and because some of thesamples with evolved signatures have elevated Mg#s, Nd,Pb, and Sr concentrations making it unlikely that they haveexperienced much crustal contamination (Conticelli andPeccerillo 1992; Foley and Peccerillo 1992; Hattori et al.1996). Isotopic results from Falcon Island demonstrate littleevidence for crustal assimilation during fractionation despitebeing intruded into turbidites (Ayer 1998) with evolved iso-topic signatures. This is indicated byεNd values that remainconstant at around +2 (Table 1) over extreme degrees offractionation from clinopyroxenite cumulates (Mg# = 88) toperaluminous syenite (Mg# = 45) (Ayer 1998, 1999).

Conclusions

Nd isotopes of the pre-2.7 Ga igneous rocks of the Lakeof the Woods greenstone belt yield similarly positiveεNdvalues (+1.1 to +2.1) for basalts of the Lower Keewatin as-semblage (2.74 Ga) and the compositionally diverse volcanicrocks of the Upper Keewatin assemblage (2.72 Ga). The Ndisotopic compositions, thus, suggest that they were derivedfrom isotopically homogeneous mantle with Sm–Nd deple-tion relative to CHUR. TheεNd values in tonalites (+1.4 and+1.5) from the Aulneau Batholith (2.72 Ga) (Beakhouse andMcNutt 1991) suggest they may represent subducted slabmelts of hydrated basalt with isotopic characteristic similarto the pre-2.7 Ga volcanic suites, similar to what has beenhypothesized by Martin (1993) for the origin of otherArchean TTG suites.

A more evolved source is evident in theεNd value of –0.9for wacke from the Upper Keewatin assemblage (2.71 Ga)and theεNd values (avg. –1.9) in the Lount Lake batholith

(2.70 Ga) within the interior of the Winnipeg Riversubprovince to the north (Beakhouse and McNutt 1991). ThenegativeεNd value in the wacke, in conjunction with detritalzircon grains ranging in ages from 2.72 to 3.0 Ga (Davis1996), suggests a mixture of detritus from an old crustalprovenance (35%), such as the Winnipeg River subprovince,and a juvenile depleted mantle source (65%), similar to thatof the pre-2.7 Ga Lake of the Woods volcanic and TTGsuites.

Heterogeneity in the post-2.7 Ga Lake of the Woodsgreenstone belt is indicated by lowεNd values (–0.4 and+0.4) in shoshonitic andesite and calc-alkaline rhyolite ofthe Electrum assemblage (<2.70 Ga) and elevated values(+1.8 to +2.3) in the Falcon Island pluton. The Pb isotopesof leached alkali feldspars from the Falcon Island plutonyield a secondary Pb–Pb isochron with an age of 2700 ±100 Ma which is similar to its U–Pb titanite age (2695 ±3 Ma). The isochron plots below the Stacey and Kramers(1975) crustal evolution curve and has an initial207Pb/204Pbof 14.52 and an initial206Pb/204Pb of 13.29, corresponding toderivation from 2.70 Ga single stage geochron reservoir withµ1 = 7.86. This indicates a mantle Pb source slightly moreenriched in207Pb/204Pb, but similarly depleted in206Pb/204Pbto that of the source of mafic and ultramafic magmas in theWabigoon and Abitibi subprovinces.

Comparison of the Nd, Pb, and Sr initial isotope data froma number of post-2.7 Ga potassic and ultrapotassic unitsacross the Superior Province suggests mixing between a de-pleted end member similar to the values of pre-2.7 Ga mag-mas with an evolved end member approaching the isotopiccharacteristics of middle Archean crustal terrains such as theWinnipeg River and Opatica subprovinces. This could be theresult of potassic magmas being derived from a isotopicallyheterogeneous mantle source and (or) contamination of de-pleted magma in the crust. The heterogenous mantle modelsuggests metasomatism by influx of fluids derived by partialmelting of subducted sediments from evolved isotopicsources, such as the Winnipeg River subprovince.

Acknowledgments

Brian Cousens and Steve Prevec performed the isotopeanalyses and provided helpful advice on interpretation of thedata. We thank Gary Beakhouse, Tony Fowler, BrendanMurphy, Phil Thurston, Jean Carignan, Ross Stevenson, andReg Theriault for their reviews of the manuscript. The ana-lytical costs were supported by Natural Sciences and Engi-neering Research Council of Canada grants to J. Dostal andA. Fowler. The paper is published with the permission of theSenior Manager of the Precambrian Geoscience Section, On-tario Geological Survey, Ministry of Northern Developmentand Mines.

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