Late Archean Mantle Composition and Crustal Growth in the Western Superior Province of Canada:...

PII S0016-7037(97)00324-4

Late Archean mantle composition and crustal growth in the Western SuperiorProvince of Canada: Neodymium and lead isotopic evidence from the Wawa, Quetico,

and Wabigoon subprovinces

PHILIPPE HENRY,* ROSS K. STEVENSON, and CLEMENT GARIEPY

GEOTOP, Universite´ du Quebec aMontreal, C.P. 8888, Succ. Centre-ville, Montre´al H3C 3P8, Canada

(Received September25, 1996;accepted in revised form September2, 1997)

Abstract—Neodymium and lead isotopes are presented for late Archean rocks from the Wawa, Quetico, andWabigoon subprovinces (Sp) in the Western Superior Province (Ontario, Canada). The isotopic compositionswere determined on greenstone volcanic sequences, pre-tectonic (2.73-2.69 Ga) trondhjemite-tonalite-grano-diorite (TTG) suites, metasedimentary rocks, and post-tectonic (2.69-2.67 Ga) dioritic to granitic plutons inorder to characterize the mantle and crustal reservoirs involved in the evolution of the southern part of theWestern Superior Province.

Although derived from a depleted mantle, almost all the greenstone volcanism and pre-tectonic TTG suitesrecord contamination by crustal material as old as 3.2 Ga. Neodymium and lead isotopic compositions of thepre-tectonic TTG can be modeled as products of the melting of amphibolitic crust (e.g., subducted basalts)contaminated by 1-10% of subducted sediments. Moreover, lead isotopes of TTG suites provide evidence thatthe isotopic compositions of these subducted sediments varied as a result of the addition of new ca. 2.7 Gajuvenile material, in agreement with neodymium isotopic differences recorded by sedimentary units of theQuetico Sp (50:50 mix of young:old crusts) and Wawa Sp (75:25 young:old).

Neodymium and lead isotopes of the post-tectonic REE-enriched Archean sanukitoid suites define anisotopic trend which is distinct from that of the TTG suites and which is interpreted to reflect the importanceof crustal assimilation in the evolution of these suites. Furthemore, such plutons found in the Wawa Sp havemore juvenile neodymium and lead isotopic compositions and higher Nd contents than those intruded into theQuetico and Wabigoon Sp which have more crustal isotopic signatures and lower Nd concentrations. Thisgeographical difference could be due either to differences in Nd contents of initial Archean sanukitoidmagmas, lesser assimilation in the Wawa Sp or, more probably, because the Wawa crust is on average morejuvenile than the Quetico and Wabigoon crusts in agreement with the fact that the geographical differencemirrors that recorded by metasedimentary rocks deposited in Quetico and Wawa Sp.Copyright © 1998Elsevier Science Ltd

1. INTRODUCTION

The Late Archean was a major period of continental crustformation during which very different geological assemblageswere tectonically assembled and rapidly stabilized to formlarge cratonic areas. In the Canadian Shield, this is exemplifiedby the formation of the Superior and the Slave Provinces whichnow lie at the core of the North American continent. Such lateArchean cratons are generally regarded as alternating tectono-stratigraphic assemblages made of granite-greenstone and oftonalite-trondhjemite-granodiorite (TTG) terranes, with inter-mingled terranes dominated by metasedimentary rock pack-ages. By comparison with the present tectonic regime of theEarth, numerous studies have made links between greenstone-dominated terranes and different tectono-magmatic environ-ments active in oceanic plate settings, as well as links betweenArchean TTG terranes and modern, subduction-related TTGsuites (e.g., Barker and Arth, 1976; Maaloe, 1982). Indeed, theoverall petrological and geochemical similarity of Archean andPhanerozoic TTG suites suggests that the majority of theserocks were derived from amphibolitic/eclogitic melt precursors

in subduction zone settings (Barker and Arth, 1976; Maaloe,1982; Martin, 1987, 1994; Rapp et al., 1991, amongst others).

The Superior Province of the Canadian Shield has beendivided into a series of elongated, broadly E-W trending sub-provinces, based upon similarities in lithologic assemblages,structural traits, and metamorphic grades (e.g., Card andCiesielski, 1986). The alternating pattern of subprovinceswithin the Superior Province and their characteristic elongatednature suggest that the craton was formed by accretion ofcrustal segments in much the same way as Phanerozoic orogensformed (e.g., Langford and Morin, 1975; Corfu et al., 1985;Blackburn et al., 1985; Ludden et al., 1986), i.e., in a conver-gent plate setting with terrane accretion and concomitant tolate-collisional magmatism.

Radiogenic isotope studies are responsible for importantadvances in our understanding of material mass transfer be-tween the different reservoirs of the Earth involved in Phanero-zoic convergent plate settings (e.g., Samson et al., 1989; Frostand Coombs, 1989). In many late Archean terranes, such stud-ies are frequently hampered by the lack of a precise geochro-nological framework and by difficulties in establishing reliableisotopic compositions for the different reservoirs involved,notably because the period of crustal evolution was relativelyshort, typically less than a few hundred million years. Thismakes mass transfer modeling between the different reservoirs

* Present address:Geosciences, Universite´ de Franche Comte´, 16route de Gray, 25030 Besanc¸on cedex, France.

Pergamon

Geochimica et Cosmochimica Acta, Vol. 62, No. 1, pp. 143–157, 1998Copyright © 1998 Elsevier Science LtdPrinted in the USA. All rights reserved

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143

difficult to evaluate because the endmembers are isotopicallyquite similar.

This study reports Sm-Nd and Pb-Pb isotope data for awide variety of rock units within three subprovinces of theWestern Superior craton (Fig. 1). The rock units includeboth mafic and felsic volcanic units, metasedimentary de-posits, pre-tectonic TTG assemblages and post-tectonicgranitoid intrusions. The large data base of precise U-Pb agedeterminations in the Western Superior Province allowsaccurate determination of initial isotopic compositions andmakes it possible to probe, at the scale of a few millions ofyears, the geochemical evolution of the craton, the processesinvolved in its formation and assembly, the tectonic envi-ronment of the different terranes, and the amount of netjuvenile additions to the crust.

2. GEOCHRONOLOGICAL BACKGROUNDAND SAMPLING

To the west of Lake Superior, the Western Superior Provinceconsists, from south to north, of the Wawa, Quetico, andWabigoon subprovinces (Fig. 1a). The Wawa and Wabigoonsubprovinces (Sp) are low metamorphic grade, volcano-plu-tonic assemblages comprising several intervening greenstonebelts and some metasedimentary sequences intruded by mas-sive granitoid bodies. In contrast, the Quetico Sp is dominatedby clastic metasedimentary sequences that were intruded bylate-kinematic granitoids, frequently two-mica bearing.

The oldest rock units in the Western Superior Province wereformed at ca. 3.17 Ga (Corfu, 1988) and are preserved withinthe Winnipeg River Sp, to the northwest of the Wabigoon Sp.

Fig. 1. (a) A map of the southern part of the Western Superior Province (after Stern et al., 1989) with the locations ofsamples from the Wabigoon, Quetico. and eastern Wawa Sp (numbers and symbols are defined in Table 1 and Fig. 2,respectively). (b) inset from (a) showing the Shebandowan greenstone belt (after Corfu and Stott, 1986) with the locationsof the volcanic units, the Timiskaming-type sediments, the Shebandowan Lake Pluton (SLP), and the Burchell Lake Pluton(BLP).

144 P. Henry, R. K. Stevenson, and C. Garie´py

Two younger episodes of crustal growth have also been recog-nized, corresponding to (1) the formation of the Lumby Lakegreenstone belt and the Marmion Lake batholith, in the centralpart of the Wabigoon Sp at 3.0 Ga (Davis and Jackson, 1988)and (2) a period between 2.76 and 2.69 Ga corresponding to themain period of crustal growth with the generation of numerousgreenstone assemblages and associated TTG suites, such asthose characterizing the Wawa and Wabigoon Sp (e.g., Davisand Edwards, 1985; Corfu and Stott, 1986). A period of gra-nitoid intrusion lasting until ca. 2.65 Ga followed the formationof the greenstone assemblages. The two periods of crustalgrowth at ca. 3.0 and 2.7 Ga are also recognized in metasedi-mentary rocks of the Quetico Sp through the presence ofabundant detrital zircons yielding U-Pb ages between 2698 and3009 Ma (Davis et al., 1990). The following section reviews thegeochronological database of the subprovinces from whichsamples were obtained (Table 1 and Fig. 1).

The geochronological data for the Wawa Sp consist of U-Pbages from the Shebandowan greenstone belt (Corfu and Stott,1986). Ages for units sampled outside the Shebandowan green-stone belt towards Thunder Bay (Fig. 1a) were estimated on thebasis of field relationships and lithological similarities. TheShebandowan greenstone belt (Fig. 1b) comprises an oldercalc-alkaline sequence of mainly mafic-ultramafic tholeiitesdated at 27336 3 Ma (Corfu and Stott, 1986) and locallyabundant intermediate to felsic flows (samples Nos. 1-6 and12-15) for which a slightly younger age of ca. 2710 Ma wasassigned because they lie above the older mafic sequence butare cut by a younger intrusive suite. The intrusive suite isdeformed and is represented by the trondhjemite-tonalite She-bandowan Lake pluton (No. 7), dated at 26966 2 Ma (Corfuand Stott, 1986). These rocks are unconformably overlain byalkaline magmatic units, dated at 268913/22 Ma (Corfu andStott, 1986), and alluvial-fluvial sediments (Nos. 8-9, 16-17)comparable to the Timiskaming units of the Abitibi Sp (Wil-liams et al., 1991), which represent late extensional phases inthe evolution of the Superior Province (Corfu et al., 1991). Theabove units were affected by two deformational events accom-panied by greenschist-facies metamorphism (Corfu and Stott,1986).

The greenstone belt was intruded by the undeformed andunmetamorphosed Burchell Lake pluton (Nos. 10, 11), dated at2684 Ma16/23 (Corfu and Stott, 1986). This is the youngestintrusion dated in the belt and along with the older Sheban-dowan Lake pluton is used to assign an age of 2690 Ma to theTimiskaming-type sediments. The Burchell pluton is composedof diorite-tonalite and granodiorite and is considered by Sternet al. (1989) to be a member of the sanukitoid suite. Archeansanukitoids, which were first identified in the Western SuperiorProvince by Shirey and Hanson (1984), are rocks characterizedby LILE- and REE-enrichments along with high Mg/(Mg1Fe)ratios. Other small diorite-monzodiorite-granodiorite stocks(Nos. 18-24 including the Penassen Lake and Barnum Lakebodies) thought to have sanukitoid affinities (Stern et al., 1989)were collected near Lake Superior. These stocks were assignedages of ca. 2.67 Ga based on their undeformed character andtheir similarity to post-tectonic stocks in the Quetico and Wabi-goon Sp (see below).

The Quetico Sp is largely composed of wackes interpreted tohave been deposited in an accretionary prism on the south

facing arc of the Wabigoon Sp. The prism was subsequentlyburied and metamorphosed during its collision with the WawaSp, resulting in a symmetrical metamorphic zonation gradingfrom greenschist-facies near the margins to high grade facieswith migmatites and granites in the axis of the sedimentary belt(Percival and Sullivan, 1988; Percival and Williams, 1989).Samples of the Quetico metasedimentary rocks were takenfrom the low grade margins adjacent to the Wawa (Nos. 25-29)and the Wabigoon Subprovinces (Nos. 30-32). Two main typesof syn- to post-tectonic plutons are found in the Quetico Sp.This includes S-type granitoids formed by partial melting ofQuetico sediments (Nos. 36-40) which have been dated be-tween 2687 and 2671 Ma (Percival and Sullivan, 1988). Thesecond type includes plutons having dioritic to granodioriticcompositions of sanukitoid affinity (Stern et al., 1989) such asthe undeformed and unmetamorphosed Blalock pluton (Nos.33-35) dated at 268864 Ma (Davis et al., 1990).

Samples from the Wabigoon Sp were derived from theWestern and Central portions of this subprovince. The WesternWabigoon is dominated by 2.76-2.73 Ga old greenstone belts,such as the Kakagi-Rowan Lakes and the Eagle-Wabigoon-Manitou Lakes greenstone belts (Davis et al., 1982). Thesebelts also contain TTG suites such as the 27326 3 Ma oldAtikwa tonalite (Nos. 41, 42) which were intruded closely intime with host greenstone supracrustals (Davis et al., 1982;Blackburn et al., 1991). In contrast, the Central Wabigoon isdominated by deformed ovoid intrusive complexes (e.g., Rev-ell, Indian Lake, and White Otter Lake batholiths, Nos. 43-48)ranging in composition from tonalite and trondhjemite togranodiorite, quartz monzonite and granite (Blackburn et al.,1991). These plutons were assigned an age of ca. 2730 Ma onthe basis on their correlation with the 2732 Ma Atikwa batho-lith. Deformation of the pre-tectonic TTG suites resulted infoliation at their margins, but left the cores largely massive withno evidence of recrystallization. Undeformed, post-tectonic,diorite-monzodiorite-granodiorite plutons (i.e., the Eye-Dashwa and Smirch Lake plutons, Nos. 49-54) were assignedan age of ca. 2670 Ma based on the work of Zartman and Kwak(1990). These post-tectonic plutons are LILE- and REE-en-riched intrusions which likely belong to the Archean sanuki-toid-type suites (Stern et al., 1989).

Older terranes have been recognized in the Central Wabi-goon such as the Marmion Lake batholith (Nos. 55-57) whichconsists of metaplutonic gneisses dated at 30036 5 Ma (Davisand Jackson, 1988). A felsic volcanic unit in the Lumby Lakegreenstone belt was dated at 29996 1 Ma (Davis and Jackson,1988) and was sampled for this study (No. 58). These unitsconstitute the oldest crust exposed in the southern WesternSuperior Province and provide a means to test the recycling ofolder materials into the younger plutonic and greenstone su-pracrustals.

3. ANALYTICAL METHODS

Samples for Sm-Nd analysis were crushed using a hydraulic pressand a representative fraction was powdered in an agate-lined shatter-box. About 0.1-0.2 g of the powders were weighed out in a teflonpressure-vessel to which a149Sm-150Nd mixed tracer solution andHF-HNO3 acids were added. The mixture was reacted under pressure inan oven at 150°C for 1 week. The resulting fluoride salts were con-verted to chlorides by redissolving and drying the samples in 6 N HCl.Chemical separation of Sm-Nd was done following the procedure

145Late Archean crustal growth, Ontario, Canada

Table 1. Sample descriptions and whole rock Sm-Nd data from Wawa, Quetico and Wabigoon Subprovinces.

aAgeMa

[ref]b Sm c Nd c

147Sm144Nd

c

143Nd144Nd (2s) eNd

t d

WAWA SUBPROVINCEShebandowan Greenstone Belt

1 WS-21b V. mafic 2733 * 8.21 41.91 0.1184 0.511375 (7) 2.92 (Wawa-g) WS-21a Granite (sill in 21b) 2733 [1] 2.03 10.42 0.1176 0.511357 (24) 2.93 WS-19 Basalt 2710 * 2.89 15.81 0.1104 0.511211 (20) 2.34 WS-17b V. intermediate 2710 * 2.14 6.83 0.1897 0.512640 (9) 2.55 WS-17c V. felsic 2710 * 3.55 20.78 0.1032 0.511052 (9) 1.76 (Wawa-a) WS-20 Andesitic tuff 2710 * 2.11 11.26 0.1130 0.511256 (11) 2.37 (SLP) C83-38 Tonalite (She. L. P.) 2696 [1] 2.88 17.79 0.0978 0.510994 (10) 2.38 WS-15 Timiskaming-sedt 2690 [1] 3.78 21.14 0.1081 0.511158 (12) 1.89 WS-16 Timiskaming-sedt 2690 [1] 4.33 24.80 0.1056 0.511101 (51) 1.6

10 (BLP) WS-22c Grd (Burchell L. P.) 2684 [1] 4.77 32.70 0.0882 0.510797 (26) 1.611 (BLP) WS-22d Ton (Burchell L. P.) 2684 [1] 7.16 47.63 0.0903 0.510805 (7) 1.0

Eastern Wawa12 95 TB 1 Gabbro 2730 * 3.21 10.33 0.1880 0.512587 (8) 2.113 95 TB 2 Gabbro 2730 * 2.02 6.72 0.1819 0.512499 (10) 2.514 WS-11 V. felsic 2710 * 3.74 20.12 0.1122 0.511208 (11) 1.615 WS-12 Andesite 2710 * 6.52 22.31 0.1765 0.512402 (6) 2.516 WS-13b Timiskaming-sedt 2690 * 16.76 61.92 0.1636 0.512203 (9) 3.017 WS-13a Timiskaming-sedt 2690 * 3.79 15.46 0.1482 0.511827 (15) 1.018 (P) WS-1a Diorite (Penassen 2670 * 16.85 82.82 0.1230 0.511393 (10) 1.019 (P) WS-1c Diorite (Penassen) 2670 * 15.13 76.79 0.1191 0.511346 (12) 1.520 WS-1b Granite (sill in 1a) 2665 * 5.05 33.79 0.0903 0.510838 (11) 1.421 WS-5b Granodiorite 2670 * 15.26 105.84 0.0872 0.510788 (12) 1.622 WS-6 Granodiorite 2670 * 11.97 79.40 0.0911 0.510876 (9) 1.923 WS-7 Diorite 2670 * 12.37 82.87 0.0902 0.510919 (9) 3.124 (B) WS-10 Grd (Barnum) 2670 * 8.19 54.06 0.0915 0.510864 (10) 1.6

QUETICO SUBPROVINCE25 WS-4 Quetico-sediment 2690 * 4.23 24.24 0.1055 0.511022 (15) 0.126 WS-5a Quetico-sediment 2690 * 4.52 26.32 0.1038 0.510998 (10)20.127 WS-8a Quetico-sediment 2690 * 4.18 23.78 0.1063 0.511052 (14) 0.428 WS-8c Quetico-sediment 2690 * 4.81 28.14 0.1032 0.511018 (9) 0.829 WS-14c Quetico-sediment 2690 * 3.97 22.75 0.1054 0.511060 (11) 0.830 WS-23a Quetico-sediment 2693 [2] 4.23 24.22 0.1056 0.511055 (6) 0.731 WS-23b Quetico-sediment 2693 [2] 3.92 22.60 0.1047 0.511058 (12) 1.132 WS-23c Quetico-sediment 2693 [2] 3.53 18.24 0.1169 0.511246 (9) 0.533 (Bl) WS-26a Tonalite (Blalock) 2688 [2] 6.16 44.32 0.0840 0.510689 (10) 1.034 (Bl) WS-26c Granodiorite (Blalock) 2688 [2] 7.64 54.38 0.0849 0.510708 (8) 1.035 WS-26b Grd dyke (in 26c) 2685 * 0.564 3.88 0.0878 0.510821 (7) 2.236 (Q-g) WS-2 two micas Granite 2686 [3] 9.70 66.88 0.0876 0.510724 (14) 0.437 WS-24 bio-Leucogranite 2671 [3] 5.81 42.05 0.0834 0.510728 (8) 1.738 (Q-g) WS-25 ms-Leucogranite 2671 [3] 7.07 38.48 0.1111 0.511152 (12) 0.439 WS-14a Aplitic sill (in 14c) 2687 * 0.417 2.39 0.1053 0.511135 (21) 2.340 WS-14b Granitic sill (in 14c) 2687 [3] 0.470 2.90 0.0978 0.510934 (10) 1.0

WABIGOON SUBPROVINCE41 (A) WS-32 Ton (Atikwa) 2732 [4] 2.79 13.20 0.1275 0.511500 (14) 2.142 (A) WS-33 Grd (Atikwa) 2732 [4] 1.55 8.96 0.1044 0.511062 (14) 1.743 (R) WS-31 Grd (Revell) 2730 * 3.02 17.91 0.1020 0.511039 (7) 2.144 (R) WS-34 Ton (Revell) 2730 * 1.89 13.44 0.0849 0.510644 (28) 0.445 (IL) WS-35 Ton (Indian Lake) 2730 * 2.84 18.92 0.0906 0.510831 (8) 2.146 (IL) WS-36 Grd (Indian Lake) 2730 * 3.04 18.45 0.0996 0.510953 (7) 1.347 (IL) WS-37 Grd (Indian Lake) 2730 * 3.98 26.17 0.0920 0.510656 (9) 21.948 (WO) WS-29 Grd (White Otter Lake) 2730 * 1.85 18.17 0.0614 0.510386 (11) 3.649 (E-D) WS-27a Grd (Eye-Dashwa) 2665 [5] 5.75 39.85 0.0872 0.510766 (11) 1.050 (E-D) WS-27b Dio (Eye-Dashwa) 2665 [5] 5.00 34.74 0.0870 0.510745 (10) 0.751 (E-D) WS-28a Grd (Eye-Dashwa) 2665 [5] 6.98 47.55 0.0888 0.510769 (9) 0.652 (E-D) WS-28b Grd (Eye-Dashwa) 2665 [5] 7.36 49.45 0.0900 0.510801 (7) 0.853 (SL) WS-30a Diorite (Smirch Lake) 2670 * 6.03 35.52 0.1025 0.511012 (9) 1.054 WS-30b Pegmatite (in 30a) 2670 * 2.43 9.72 0.1389 0.511733 (11) 2.4


described by Richard et al. (1976). Samarium and neodymium wereloaded on a double Re-Ta filament and analyzed in static and dynamicmulti-collector mode, respectively, on a VG Sector 54 mass spectrom-eter. During the course of this study, the La Jolla Nd standard yieldeda 143Nd/144Nd ratio of 0.5118496 12 (2s mean on twenty-one anal-yses) and total blanks for Nd or Sm were less than 50 pg and negligible.

The lead isotopic compositions were measured on K-feldspar sepa-rates from plutonic rock samples. K-feldspar contains little U and itslead isotopic composition generally closely approximates the initialcomposition. Only suites which showed no evidence of K-feldsparrecrystallization were studied to avoid problems of Pb remobilization.Sample treatment followed the procedure outlined by Carignan et al.(1993) with the following modifications. Powdered K-feldspars werewashed overnight in aqua regia and rinsed in distilled water. Theresidual powder was leached with a mixture of a dilute HF-HBr for 30min.; the supernatant and the residue were recovered, dissolved in HFand processed separately through anion-exchange separation (Manhe`set al., 1980). The leaching treatments are carried out to enhance,inasmuch as possible, removal of labile radiogenic Pb held in crystal-line defects, cleavage surfaces, or nonsilicate impurities. Lead wasanalyzed in static multicollection mode on a VG Sector 54 massspectrometer. The raw data were corrected for instrumental mass frac-tionation using a factor of 0.09% amu21. Total blanks were smallerthan 30 pg and negligible. Replicate analyses of the NBS SRM-981standard yielded a reproducibility of60.05% amu21 (1s) for filamentloads ranging from 10 to 100 ng.

4. RESULTS

The whole rock Sm-Nd compositions (Table 1) show signif-icant geochemical differences between the older (2.733-2.695Ga) and younger (2.690-2.665 Ga) plutonic suites. These arebest illustrated when the147Sm/144Nd ratios and the Nd con-centrations of the rocks are examined as a function of the agesof the units (Fig. 2). The pre-tectonic tonalite plutons havemean147Sm/144Nd ratios and Nd concentrations of;0.10 and;15 ppm, respectively. These values are in good agreementwith those reported in other studies of Archean TTG suites(e.g., Martin, 1987), where the TTG geochemical characteris-tics were consistent with;20% partial melting of an amphi-bolite source with a garnet-rich residue. The post-tectonic plu-tons also yielded low147Sm/144Nd ratios, but their Ndconcentrations are systematically higher (30-110 ppm). Stern etal. (1989) suggested that the REE- and LILE-enriched nature ofArchean sanukitoid suites, such as the Burchell Lake, Blalock,Barnum, Penassen, Eye-Dashwa, and Smirch Lake plutons,were derived from partial melting of a metasomatized and traceelement-enriched mantle.

In Fig. 3a, theeNdt values are plotted against the ages of

sample crystallization/sedimentation. Also shown, for compar-

ison, are reference lines depicting the evolution, between 2.74and 2.66 Ga, of a depleted mantle and of continental crustsformed at 3.0 and 3.2 Ga. The model mantle compositions arederived from a present-day depleted mantle having143Nd/144Nd and147Sm/144Nd ratios of 0.51310 and 0.2136, respec-tively (e.g., Jacobsen, 1988). This corresponds to aneNd

2.7Ga

value of13 which is in good agreement with several publishedestimates for the neodymium isotopic composition of the lateArchean depleted mantle in the Superior Province and else-where (e.g., Machado et al., 1986; Tilton and Kwon, 1990;Stevenson, 1995; Vervoort et al., 1996 and global compilationin Shirey and Hanson, 1986). The Nd evolution lines forancient continental crusts are calculated for materials extractedfrom this same depleted mantle, at 3.2 and 3.0 Ga, with anaverage147Sm/144Nd ratio of 0.115.

The lead isotopic compositions are given in Table 2. Foreach sample, the Pb/Pb ratio obtained for the leachate analysisis greater than or equal, within error, to that obtained for theresidue analysis. The residue analysis are consistent with cal-culated Archean lead isotopic compositions (see Fig. 5 anddiscussion below) and, thus represent excellent estimates forthe initial lead isotopic compositions of the plutons. Figure 3bshows the207Pb/204Pb results obtained on the K-feldspar resi-dues plotted as a function of pluton emplacement age. Evolu-tion curves depicting the lead isotopic evolution of the mantleand crustal reservoirs are difficult to model because theystrongly depend on the final lead isotope composition of theBulk Silicate Earth after accretion and core formation (e.g.,Allegre et al., 1996; Galer and Goldstein, 1996) and on the238U/204Pb (m) of the mantle and crustal reservoirs. The dataare compared to a single-stage Pb evolution model assuming aPb-Pb age of 4.48 Ga for the Earth. This age represents the timeat which the Bulk Silicate Earth acquired its U/Pb ratio aftercore formation (Allegre et al., 1996; Galer and Goldstein,1996). The depleted mantle curve is then calculated usinginitial compositions of the Canyon Diablo meteorite (Tat-sumoto et al., 1973) and am value of 8.5. This model yields207Pb/204Pb ratios that are comparable to other lead isotopiccompositions reported for the late Archean mantle (Tilton andKwon, 1990; Carignan et al., 1995). The Pb evolution curves of3.2 and 3.0 Ga old crustal reservoirs are shown for arbitrarilychosenm values of 12. Both the neodymium and lead isotopedata show significant deviation from the model depleted mantleevolution which suggests the involvement of a reservoir char-

Table 1. (Continued) Sample descriptions and whole rock Sm-Nd data from Wawa, Quetico and Wabigoon Subprovinces.

aAgeMa

[ref]b Sm c Nd c

147Sm144Nd

c

143Nd144Nd (2s) eNd

t d

Marmion Lake Batholith and Lumby Lake greenstone belt55 (M) 95 TB 4 Gneiss 3003 [6] 1.39 8.51 0.0989 0.510829 (11) 2.556 (M) 95 TB 5A Orthogneiss 3003 [6] 3.06 13.46 0.1373 0.511584 (14) 2.457 (M) 95 TB 6A Gneiss 3003 [6] 3.21 14.53 0.1335 0.511506 (15) 2.458 (LL) 95 TB 8 Felsic tuff 2999 [6] 3.22 18.48 0.1051 0.510985 (14) 3.2

a- Number in first column refer to Fig. 1 and text. b- [1] Corfu and Stott 1986, [2] Davis et al. 1990, [3] Percival and Sullivan 1988, [4] Davis etal. 1982, [5] Zartman and Kwak 1990, [6] Davis and Jackson 1988, *: age based on field relationships (see text). c- Sm and Nd concentrations andSm/Nd ratios with an accuracy of 0.5%. d- calculated at age of crystallisation or sedimentation with an accuracy of 0.4 to 0.8 epsilon units dependingto each Sm/Nd ratio, and with143Nd/144Nd and147Sm/144Nd ratios of 0.512638 and 0.1967, respectively, for today CHUR.


acterized by radiogenic Pb and loweNdt values such as conti-

nental crusts.The Sm-Nd data for pre-2.7 Ga volcanic rocks sampled in

Wawa Sp (Nos. 1, 3, 12, 13) yieldedeNdt values between12.9

and12.1 and a granite sill (No. 2), dated at 2733 Ma (Corfuand Stott, 1986) has aneNd

t value of12.96 0.5, similar to thatof the host mafic volcanics. K-feldspar from this granite yieldeda 207Pb /204Pb ratio of 14.58. These values are consistent witha magmatic derivation from the depleted mantle reservoir. Theca. 2710 Ma calc-alkaline, intermediate to felsic volcanics(Nos. 4-6, 14-15) and the syn-tectonic Shebandowan Lakepluton (No. 7), both from Wawa Sp, haveeNd

t values from12.5 to 11.6. The K-feldspar separated from a calc-alka-line tuff and from the Shebandowan Lake pluton yielded207Pb/204Pb ratios of 14.65 and 14.52, respectively. Some ofthese Nd and Pb values for the pre-tectonic TTG suites are

significantly different from those estimated for the depletedmantle reservoir (Fig. 3a,b). In the Wabigoon Sp, the 2732 Maold Atikwa batholith (Nos. 41, 42), which is isolated from otherlarge intrusive complexes by the Kakagi-Rowan Lakes and theEagle-Wabigoon-Manitou Lakes greenstone belts, has isotopiccompositions closer to the mantle endmember witheNd

t 11.96 0.5 and207Pb/204Pb 5 14.53 6 0.03. Conversely, the ca.2.73 Ga Indian Lake batholith (Nos. 45-47), which intrudes agranitoid-gneiss terrane, has more crustal-like isotopic compo-sitions witheNd

t between12 and22, and207Pb/204Pb5 14.766 0.03. Therefore, the isotopic compositions of the pre-tectonictonalites appear to reflect their particular crustal environment.The question of just how the crustal signatures were incorpo-rated in the TTG suites is addressed in section 5.

The Timiskaming-type sediments, deposited in Wawa Sp,haveeNd

t values in the range of13 to 11. The two samples

Fig. 2. Sm-Nd evolution with time. (a)147Sm/144Nd vs. ages. There appears to be a slight decrease in the147Sm/144Ndratios from pre- to post-tectonic plutons with the latter having lower and more homogeneous147Sm/144Nd ratios. (b)Neodymium concentrations vs. ages. The high Nd concentrations of the post-tectonic dioritic to granodioritic plutons,compared to the lower Nd contents in the pre-tectonic TTG suites, suggest a major change in magmatism. The pre- andpost-tectonic eras are separated in time by a short period (less than 10 Ma) corresponding to the deposition of 2.69 GaQuetico and Timiskaming-type sediments in Quetico and Wawa Sp, respectively.


from the Shebandowan greenstone belt (Nos. 8 and 9, Table 1)yielded similar eNd

t values (11.8 and 11.6) and have Ndcontents (21.1 and 24.8 ppm) and147Sm/144Nd ratios (0.108and 0.105) consistent with well-mixed sediments (e.g., Alle`greand Rousseau, 1984; Taylor and McLennan, 1985; McLennanand Hemming, 1992). Conversely, sediments with higher andlower values were sampled in the eastern Wawa Sp (Nos. 16and 17, Table 1) and have different Nd contents (61.9 and 15.5ppm) and147Sm/144Nd ratios (0.164 and 0.148). The variationbetween the samples may reflect the greater influence of localsources in the eastern Wawa Sp leading to more variableisotopic compositions. The coeval Quetico sediments (Nos.25-32, Table 1) yield a narrow range ofeNd

t values from11.1to 20.1. These homogeneous values are associated with narrowranges in both the Nd concentrations (18.2-28.1 ppm) and the147Sm/144Nd ratios (0.103-0.117) which imply well-mixedsources. Moreover, the narrow range of initial neodymiumisotopic compositions indicates that the greenschist-faciesmetamorphism did not significantly affect the rare earth ele-ments in these sediments because such alteration would havemodified the parent/daughter (Sm/Nd) ratios, producing a dis-persion in calculated initialeNd values. Overall, the data fromboth the Timiskaming-type Wawa and Quetico sediments dem-onstrate that, at the time of their deposition, the eroded crusts

were composed of mixtures of ca. 2.7 Ga juvenile terranes andolder crusts, in agreement with the U-Pb ages of detrital zirconsfrom Quetico sediments (Davis et al., 1990).

The Quetico granitic rocks (Nos. 36-40) yield a range ofeNdt

values (12.3 to 10.4) larger than that obtained from theQuetico sediments. The two S-type granites (Nos. 36, 38) haveeNd

t values similar to those of the sediments and yield207Pb/204Pb ratios of 14.64 and 14.67 which could represent anestimate of the average207Pb/204Pb ratio of the crustal materialeroded at 2.69 Ga. The younger intrusive rocks of the WawaSp, including the post-tectonic Burchell Lake Pluton (Nos. 10,11) and post-tectonic diorites and granodiorites (Nos. 18-24)yieldedeNd

t values from13.1 to11.0 and207Pb/204Pb ratiosof 14.52-14.67. Two samples of tonalite-granodiorite from the2688 Ma Blalock Pluton (Nos. 33, 34) emplaced on the bound-ary between Quetico and Wabigoon Sp, haveeNd

t values of11.0 and207Pb/204Pb ratios of 14.63. One dyke cutting theBlalock tonalite (No. 35) has aneNd

t value of12.2. HighereNdt

values found for the dykes and sills may be derived by partialmelting of material more juvenile than the enclosing rocks(possibly a mafic precursor). The post-tectonic and REE-richgranodioritic to dioritic rocks from the Eye-Dashwa (Nos.49-52) and Smirch Lake (No. 53) plutons yielded a restrictedrange ofeNd

t (10.6 to 11.0) and207Pb/204Pb (14.63-14.68)

Fig. 3. Neodymium and lead isotopic compositions vs. time. (a)eNdt vs. ages. TheeNd

t values are compared with thoseof the depleted mantle, 3.0 Ga Marmion Lake gneisses and a felsic tuff from the 3.0 Ga Lumby Lake greenstone belt (crosssymbols). The evolution curves represent felsic crusts created at 3.0 and 3.2 Ga with147Sm/144Nd 5 0.115 and illustratethat older crustal components are recorded by the Wawa, Quetico, and Wabigoon rocks. (b)207Pb/204Pb vs. ages.207Pb/204Pb ratios are compared with depleted mantle havingm1 (238U /204Pb ) value of 8.5 and 3.0 to 3.2 Ga felsic crustsevolving with am2 value of 12. In agreement with neodymium isotopes, lead isotopic compositions of K-feldspars suggestthe presence of an older crustal component. Symbols are as Fig. 2.


Table 2. Pb isotopes of K-feldpars from plutonic rocks from Wawa, Quetico and Wabigoon Subprovinces.

a b

206Pb/204Pbc

207Pb/204Pbc

208Pb/204Pbc

WAWA SUBPROVINCEShebandowan greenstone belt

2 Wawag (WS-21a) R 13.49 14.58 33.26l 13.56 14.61 33.35

6 Wawaa (WS-20) R 13.97 14.65 33.60l 14.31 14.63 33.64

7 SLP (C 83-38) R 13.71 14.52 33.48l 14.17 14.63 33.90

10 BLP (WS-22c) R 13.51 14.60 33.29l 13.51 14.62 33.36

11 BLP (WS-22d) R 13.48 14.59 33.72l 13.54 14.66 33.45

Eastern Wawa18 Pd (WS-1a) R 13.61 14.63 33.38

l 13.62 14.66 33.4819 Pd (WS-1c) R 13.59 14.66 33.38

l 13.74 14.76 33.4921 (WS-5b) R 13.56 14.63 33.40

l 13.65 14.66 33.4522 (WS-6) R 13.43 14.54 33.28

l 13.46 14.55 33.3023 (WS-7) R 13.46 14.53 33.25

l 13.49 14.55 33.3024 Bd (WS-10) R 13.54 14.62 33.33

l 13.79 14.69 33.55

QUETICO SUBPROVINCE33 Bl (WS-26a) R 13.59 14.64 33.34

l 13.84 14.73 33.5834 Bl (WS-26c) R 13.53 14.63 33.32

l 13.55 14.64 33.4236 Q-g (WS-2) R 13.59 14.64 33.37

l 14.04 14.77 33.7338 Q-g (WS-25) R 13.62 14.67 33.33

l 13.72 14.69 33.4440 (WS-14b) R 13.56 14.57 33.33

l 16.02 15.11 33.75

WABIGOON SUBPROVINCE41 A (WS-32) R 13.44 14.51 33.25

l — — —42 A (WS-33) R 13.49 14.55 33.24

l 13.52 14.54 33.2243 R (WS-31) R 13.49 14.59 33.31

l 13.56 14.60 33.3844 R (WS-34) R 13.56 14.65 33.48

l 13.59 14.63 33.4445 IL (WS-35) R 13.77 14.76 33.54

l 13.82 14.75 33.5246 IL (WS-36) R 13.75 14.77 33.48

l 13.82 14.77 33.5047 IL (WS-37) R 13.76 14.74 33.56

l 13.84 14.73 33.5548 WO (WS-29) R 13.55 14.58 33.31

l 13.60 14.61 33.4349 E-D (WS-27a) R 13.63 14.68 33.40

l 13.60 14.65 33.3850 E-D (WS-27b) R 13.58 14.65 33.36

l 13.63 14.67 33.4351 E-D (WS-28a) R 13.61 14.67 33.41

l 13.62 14.66 33.3952 E-D (WS-28b) R 13.59 14.66 33.36

l 13.62 14.67 33.3953 SL (WS-30a) R 13.54 14.64 33.31

l 13.57 14.68 33.4554 (WS-30b) R 13.59 14.58 33.27

l 13.69 14.61 33.38

a: numbers refer to Table 1, Fig. 1 and text. b: R5 residues, l5 leachates. c: 1 sigma for residue analyses is 0.015, 0.020 and 0.060 for206Pb/204Pb,207Pb/204Pb and208Pb/204Pb, respectively, and must be doubled in the case of leachate analyses.


values. Finally, and as was the case for Quetico dykes, apegmatite (No. 54) intruded in the Smirch Lake diorite has amore radiogenic isotopic composition than its host lithology,with an eNd

t value of 12.4 and a207Pb/204Pb ratio of 14.58.Overall, with the exception of one dioritic body near ThunderBay and some minor dykes and sills, the post-tectonic rocks ofsanukitoid affinity yielded homogeneous neodymium and leadisotopic compositions of abouteNd

t 11 and 207Pb/204Pb 514.65, respectively (Fig. 3a,b). These values could correspondto mixtures between depleted and enriched mantle reservoirsproduced in a subarc mantle through metasomatism of themantle wedge (Shirey and Hanson, 1984; Stern and Hanson,1991). Alternatively, the values could be produced by theintrusion of a depleted mantle-derived magma into an enrichedcrustal reservoir and subsequent assimilation of this crust.

Finally, the 3.0 Ga old gneisses from the Marmion Lakebatholith (Nos. 55-57) and a felsic tuff from the Lumby Lakegreenstone belt (No. 58) yieldedeNd

3Ga values between13.2and 12.4, which are consistent with these rocks being directmantle derivatives at their time of formation. However, at thetime of the main crustal growth episode in the Wabigoon Sp,these rocks would have hadeNd

2.7Ga values in the range of21.3 to 10.1. These values will be used as a first-orderapproximation for an old crustal reservoir potentially presentover the whole terrane.

In comparison with the evolution curves of both depleted andenriched crustal reservoirs (Fig. 3), the data from this studyshow that there is no unique isotopic evolution trend for the2.74-2.66 Ga old terranes in the different subprovinces. Thedispersion of the data could be due to mixtures of differentreservoirs such as depleted mantle, enriched mantle, and con-tinental crust. Differences in Nd concentrations and the Nd-Pbisotopic compositions between the pre-tectonic and post-tec-

tonic suites could also reflect progressive thickening of thecrust through volcanic and tectonic processes. For example,pre-tectonic tonalites intruded into a thin crust would not formlarge magma chambers, producing only limited amounts ofhighly fractionated derivatives. In contrast, post-tectonic dior-itic melts intruded into a thickened crust may develop largermagma chambers allowing for greater fractionation and moreabundant REE-rich granitoids. In addition, assimilation and/oranatexis of thickened crustal segments would also result in agreater petrological diversity of granitoid suites.

5. DISCUSSION

5.1. Mixing Relationships

The neodymium and lead isotope data strongly implicate theinteraction between depleted and enriched reservoirs in theformation of the late Archean Western Superior Province. Theidentity of these reservoirs is further investigated with the aidof the Sm-Nd and Pb-Pb isochron diagrams in Figs. 4 and 5,respectively.

In Fig. 4, the Nd data are plotted in aeNdt vs. 147Sm/144Nd

diagram to clarify the mixing relationships between ca. 2.7 Gajuvenile component(s) derived from a depleted mantle andolder crustal endmember(s). The juvenile endmember encom-passes the depleted mantle as well as any recent crustal addi-tions derived from it. Thus it comprises rock units havingeNd

t

values .12.5, but highly variable147Sm/144Nd ratios: forexample,;0.19 for basalts and mafic rocks extracted directlyfrom the depleted mantle and between 0.13-0.11 for graniticdykes and intermediate to felsic volcanics formed either bypartial melting of a basaltic precursor, by fractional crystalli-zation of mantle-derived magma, or both. The147Sm/144Ndratio of this juvenile component may even be as low as 0.09, asexemplified by a ca. 2.67 Ga diorite from the eastern Wawa Sp

Fig. 4. eNdt vs.147Sm/144Nd. The distribution of the data may reflectmixing between depleted and enriched reservoirs as illustrated by thelines drawn on the diagram. The depleted mantle endmember (DM)corresponds to materials ultimately derived from a depleted mantlewith eNd

t 5 13 and variable147Sm/144Nd ratios defined by basalts (b),intermediate to felsic rocks (g) or diorites to granodiorites (d). Theintersection of the mixing lines is consistent with a single crustalendmember (CM) havingeNd

t ; 22 and147Sm/144Nd ; 0.08. Such alow Sm/Nd ratio may result from partial melting of the CM endmemberprior to mixing with mantle-derived magmas. Note that, at 2.7 Ga theCM endmember has aeNd value slightly lower than that calculated fromthe 3.0 Ga terranes (samples from Central Wabigoon region). Thiscould indicate recycling of crustal material slightly older than 3.0 Ga.Symbols as Fig. 2.

Fig. 5. 207Pb/204Pb vs. 206Pb/204Pb. The distribution of the leadisotope ratios suggests at least two endmembers corresponding todepleted (DM) and enriched (CM) reservoirs withm values lower andhigher than that the Bulk Silicate Earth (BSE,m ; 9), respectively.Therefore, the mixing array (gray array on the figure) corresponds tothe equation BSE5 DM 1 CM where the depleted component (DM)is consistent with the isotopic composition of the Mulcahy intrusion(Carignan et al., 1995). The enriched component (CM) could be ex-plained by an older crust isolated from the depleted mantle at ca. 3.2 Gaand evolving with an averagem value of about 12. Only, two rocksfrom Shebandowan greenstone belt have high206Pb/204Pb ratios ex-plained by in situ decay of U, in agreement with a 2.7 Ga isochron. ThePb-Pb evolution lines are defined in text. Symbols as Fig.2.


(No. 23, Table 1) which yielded aneNdt value of 13.1 sug-

gesting a depleted mantle source. A granodiorite from theWhite Otter Lake batholith (No. 48) with a higheNd

t value of13.6 calculated from an even lower measured147Sm/144Ndratio of 0.0614 may be similarly derived through fractionationfrom a mafic precursor.

The data from the plutonic samples, irrespective of theiractual location in the Western Superior Province, fan towards acommon endmember (Fig. 4) having aneNd

2.7Gavalue of; 22and a147Sm/144Nd of ; 0.08. These values were arbitrarilychosen to represent the crustal endmenber. At 2.7 Ga, theisotopic composition of that endmember would be slightlylower than that of the 3.0 Ga Marmion Lake gneisses and theLumby Lake felsic tuff, albeit its Sm/Nd ratio is clearly lower.The lower Sm/Nd ratio of the crustal component could resultfrom the partial melting of rocks similar in composition to theMarmion and Lumby Lakes samples, producing magmas withsteeper LREE-enrichment profiles. This does not mean thatMarmion Lake materials were incorporated into the Wawa Sp,but it implies the presence of materials with similar age andcomposition. Older crustal contributions to the Wawa Sp mayactually originate farther east, in the Michipicoten region,where 2.9 Ga old rocks have been described (Turek et al.,1992). Nevertheless, Fig. 4 illustrates that both the pre- andpost-tectonic samples have interacted with older crustal mate-rials including ancient terranes isolated from the depleted man-tle at 3.0-3.2 Ga.

The lead isotopic compositions of K-feldspar residues fromthe plutonic samples are shown in a conventional207Pb/204Pbvs. 206Pb/204Pb diagram (Fig. 5). Only an andesitic tuff (No. 6,Table 2) and the Shebandowan Lake pluton (No. 7, Table 2),both from the Shebandowan greenstone belt, show clear evi-dence of in situ growth of radiogenic Pb from U decay or topost-crystallization reequilibration. For the other analyses, al-though in situ growth of radiogenic Pb may be responsiblesome very slight scatter in the data, the preponderant trenddefined by the majority of the data points clearly has a steeperslope than a 2.7 Ga isochron (Fig. 5). The steep array likelyrepresents, as was the case for the neodymium isotopic results,mixtures between depleted and enriched reservoirs.

The least radiogenic samples, which plot to the left of the 2.7Ga geochron can only reflect the participation of a depletedendmember (DM) which evolved with a time-integrated238U/204Pb (m) ratio inferior to that of the Bulk Silicate Earth.Indeed, the axis of the trend of the K-feldspars (shaded area,Fig. 5) intersects the initial lead isotopic composition definedfor the Mulcahy mafic-ultramafic intrusion in the Wabigoon Sp(Carignan et al., 1995) as well as those determined for the lateArchean depleted mantle underneath the Superior Province(Dupreet al., 1984; Tilton and Kwon, 1990). The most radio-genic samples can only reflect the involvement of crustal end-members (CM) with time-integratedm values superior to thatof the Bulk Silicate Earth. Most of the samples lie at interme-diate positions between DM and CM (Fig. 5) suggesting, aswas the case for the neodymium isotopic results, that mostsamples are mixtures of both depleted and enriched reservoirs.Within the single-stage model illustrated on Fig. 5, these sam-ples lie close to a reference growth curve with am 5 9, a valuethat is close to that estimated for the Bulk Silicate Earth (e.g.,Galer and Goldstein, 1996). Assuming that the age of the Bulk

Silicate Earth is 4.48 Ga and that the endmember labeled CMon Fig. 5 represents the minimum lead isotopic composition ofan ancient crustal reservoir derived in a simple two-stageprocess from a depleted mantle with U-Pb isotopic character-istics comparable to that of the DM endmember (m 5 8.5), onecan estimate from the207Pb/204Pb vs.206Pb/204Pb systematicsa minimum extraction age of ca. 3.2 Ga for that crustal end-member. This agrees with the estimate determined above fromthe Sm-Nd isotopes.

The mixing relationships are further investigated in terms ofthe pre- and post-tectonic plutonic series with the aid of Fig. 6which plots the neodymium and lead isotopic compositions ofthe suites as a function of their Nd concentrations. Two trendsare apparent: (1) the neodymium and lead isotopic composi-tions of the pre-tectonic rocks with less than 30 ppm of Nd areconsistent with simple, binary mixing (trend 1 on Fig. 6a,b)between a juvenile endmember, for example an oceanic crustderived from a depleted mantle, and older crustal segmentsformed at 3.2 Ga; (2) in contrast, the post-tectonic plutons are

Fig. 6. Neodymium and lead isotopes vs. Nd concentrations. Thesemixing diagrams demonstrate that for pre-tectonic TTG suites having[Nd] , 30 ppm, mixing between the depleted mantle (DM) and a 3.2Ga old felsic crust (CM) adequately models our data. This mixing isconsistent with a concentration ratio of r5 10 (see text) as shown bytrend (1) in b. In the case of the Nd-rich post-tectonic plutons, there arelarge variations in Nd concentrations for rocks having similar neody-mium or lead isotopic compositions (trends 2). This can be due to amixing with a Nd-rich endmember (r; 1) or to Nd enrichmentresulting from processes such as partial melting, fractional crystalliza-tion, or metasomatism. But, regardless of the real process, three distinctisotopic compositions can be identified. One, labeled C1 having (eNd

t,207Pb/204Pb ) values of (13, 14.53) in agreement with depleted mantleat 2.67 Ga. The two other crustal endmembers, labeled C2 (11.8,14.60) and C3 (10.6, 14.68) have neodymium isotopic compositionsconsistent with those of metasedimentary rocks deposited in Wawa andQuetico Sp, respectively.


characterized by increasing Nd contents with relatively cons-tant neodymium and lead isotopic compositions. These trendsbranch off from the main data array (trend 2 on Figs. 6a,b) toform three parallel trends labeled C1, C2, and C3 which haveeNd

t: 207Pb/204Pb isotopic compositions of13.0: 14.53,11.8:14.60, and10.6: 14.68, respectively. These trends suggest theexistence of parental magmas with variable initial isotopiccompositions that were affected by fractionation processes,thus explaining the large ranges of Nd concentrations (30-110ppm, Fig. 6). Conversely, the C1 to C3 compositions may alsobe interpreted as reflecting mixtures of juvenile componentsderived from a depleted mantle-like with several Nd-rich end-members. These Nd-rich endmembers could correspond to thepost-tectonic Archean sanukitoid suites whose REE-rich char-acter is thought to be derived from an enriched or metasoma-tized mantle (Shirey and Hanson, 1984; Stern et al., 1989; Sternand Hanson, 1991).

The 207Pb/204Pb vs. eNdt diagrams of Fig. 7 discriminate

between the different possibilities outlined above. Figure 7ashows the isotopic results obtained on all pre-tectonic intru-sives with superimposed mixing curves between ca. 2.73 Gajuvenile materials derived from a depleted mantle (DM) and a3.2 Ga old crustal endmember (CM). The Nd and Pb data of thepre-tectonic TTG suites do not lie along a linear trend betweenthe crustal and mantle reservoirs. The curvature of the mixingcurves between CM and DM is controlled by the ratio r5CDM/CCM (e.g., Langmuir et al., 1978), where C is the [Nd]/[Pb] concentration ratio of the DM and CM endmembers,respectively. For example, a sedimentary mixing line with r51(Fig. 7a) can be drawn to represent mixtures of crusts orsediments of juvenile and ancient origin, which have similar Ndand Pb concentrations. However, when 1-10% of a crustalcomponent such as the Quetico sediments (CCM ; [20]/[10] 52; Zindler and Hart, 1986; Fralick and Purdon, 1995; this study)is directly mixed with either a depleted mantle (CDM ;[1]/[0.01] 5 100; r5 50; Zindler and Hart, 1986) or a basalticcomponent (CDM ;[10]/[0.5] 5 20; r 5 10), the result is acurve where the207Pb/204Pb ratio of the mixture rapidly ap-proaches that of the crustal contaminant, whereas theeNd

t valueis lowered by only a few epsilon units. This is particularly wellillustrated in Fig. 7a by the three analyses from the Indian Lakebatholith (IL on Fig. 7a) which have a207Pb/206Pb ratio ofabout 14.75 but theeNd

t values range from12.1 to21.9. Thisis because lead isotopes are much more sensitive to crustalcontamination compared to neodymium isotopes, due to thehigh concentration of Pb in the crust. However, the range oflead and neodymium isotopic compositions for all the TTGsuites cannot be represented by a single mixing curve betweenthe DM and CM endmembers. The distribution of the datarequire at least three crustal endmembers (CM, Q, and T on Fig.7a) which produce different curves when mixed with depletedmantle-like material.

The fact that the TTG suites lie along curves constructedfrom such mixtures could reflect the contamination of theirsource areas. TTG suites are considered to form in an arcenvironment by the subduction and melting of oceanic crust(Martin, 1987). The contamination of these suites couldresult from the subduction of sediments along with theoceanic crust and mixing during melting. However, theisotopic compositions of these sediments (Pb in particular)

may change from one area to another, producing a family ofmixing curves as a result of greater contributions fromjuvenile orogens for example. Thus, Fig. 7a shows threemixing curves between amphibolite-like material with de-pleted mantle compositions (207Pb/204Pb 5 14.45; eNd

t 513) and different crustal endmembers (207Pb/204Pb5 14.80,14.65, and 14.55 andeNd

t 5 22.0, 10.6, and11.8, respec-tively; r 5 10) similar to those estimated for a ca. 3.2 Ga old

Fig. 7. 207Pb/204Pb vs.eNdt. (a) Nd-Pb mixing model for pre-tectonic

TTG suites. Because lead isotopes are very sensitive to crustal con-tamination, the data dispersion found in pre-tectonic plutons is believedto represent mixtures between depleted (DM) and different enrichedreservoirs (CM, Q, and T) having distinct207Pb/204Pb ratios repre-sented by the mixing curves calculated with r5 10 (see text). Theendmembers Q and T represent Quetico and Timiskaming-type sedi-ments having measured neodymium and estimated lead isotopic com-positions on a sedimentary mixing line (r5 1, see text) between CMand DM. The estimated isotopic composition of the Quetico sedimentsis in agreement with theeNd

t and207Pb/204Pb values of S-type granitesfrom the Quetico Sp (Q-g). In this model Q and T represent mixturesof 50:50 and 75:25, respectively, between DM and CM. Thus, theneodymium and lead isotopic compositions of the TTG suites can beexplained by the partial melting of an amphibolite with some subductedsediments (1-10%) whose the isotopic composition varies as a functionof the proportion of neighbouring young:old eroded terranes. (notationas in Fig. 2). (b) Nd-Pb mixing model for the post-tectonic plutons. At2.67 Ga, the neodymium and lead isotopes of post-tectonic plutonssuggest mixing between normal depleted mantle (DM5 C1) andcrustal compositions (CM, C2, or C3) where r; 1 (see text). The Wawapost-tectonic intrusions have, on average, more juvenile signatures thanthose from the Quetico and Wabigoon Sp. A mixing array where r51, and the fact that C2 and C3 compositions are similar to the compo-sitions of Timiskaming-type (T) and Quetico (Q) sediments, respec-tively, strongly imply that the neodymium and lead isotopic composi-tions of the post-tectonic suites reflect crustal contamination of thedioritic magmas during intrusion into the Wawa, Quetico or Wabigooncrusts (see discussion in text).


felsic crust (CM), Quetico sediments (Q), and Timiskaming-type sediments from the Shebandowan greenstone belt (T).

Conversely, the range of neodymium and lead isotope com-positions of the Nd-rich (.30 ppm), post-tectonic plutonicrocks is much more limited than that of the pre-tectonic TTG.The data do not conform to any of the mixing trends defined bythe TTG but rather define a linear trend (i.e., r;1) betweenDM and CM (Fig. 7b) which overlaps with the neodymium andlead isotopic compositions labeled C1, C2, and C3 in Fig. 6.

The eNd2.67Ga; 207Pb/204Pb compositions of C1 (13; ;

14.53), C2 (11.8; ; 14.60), and C3 (10.6; ; 14.68) rangefrom compositions similar to depleted mantle (DM) at 2.67 Ga,to values estimated for the Timiskaming-type sediments fromthe Wawa Sp (T) and for sediments and two-mica granites fromthe Quetico Sp (Q), respectively. In addition, there is a cleargeographical distribution, with the Archean sanukitoid rocksfrom the Wawa Sp yieldingeNd

2.67Gavalues, on average, moreradiogenic than those from the Quetico and the Wabigoon Sp(Fig. 7b). This geographical distribution mirrors that observedfor the Timiskaming-type sediments in the Wawa Sp whichhave neodymium isotopic compositions more radiogenic thanthose of the Quetico sediments. This suggests that the neody-mium isotopic compositions and the estimated lead isotopiccompositions of the sediments provide reasonable estimates ofthe isotopic compositions of the different crustal endmembersfor the mixing trends in Figs. 6 and 7 for both pre- andpost-tectonic plutons.

5.2. Evolution of the Western Superior Crust andSedimentary Basins

The 207Pb/204Pb signatures of late Archean sediments usedabove can only be indirectly estimated from the isotopic com-positions of S-type Quetico granites produced by the partialmelting of the sediments. Two samples of late-tectonic, two-mica granites (Nos. 36 and 38) from large intrusive bodieswithin the Quetico sediments, haveeNd

t isotopic compositions(10.4) almost identical to that of the sediments (10.6). As-suming that these granites were entirely derived from meltingof intracrustal materials, one could postulate their lead isotopiccomposition (207Pb/204Pb ratio of;14.65) reflects that of thecrust/sediments from which they were derived. On Fig. 7a, suchan endmember plots halfway between the CM and DM andwould correspond to a mixture with an r value of;1. This rvalue is comparable to values determined for modern marinesediments (e.g., Abouchami and Goldstein, 1995).

In principle, the Sm-Nd isotopic signatures of fine clasticsediments record, overall, the mean age of their crustal sourceareas (e.g., Alle`gre and Rousseau, 1984; Taylor and McLennan,1985). However, during intensive episodes of crustal growthsuch as that which occurred during the 2.76-2.70 Ga accretionof the Superior Province, the composition of the crust may besignificantly modified and its mean age considerably reduced.This phenomenon is likely recorded in the neodymium isotopicsignatures of the Quetico and the Timiskaming-type sediments,which have different meaneNd

t values (10.6 and11.8, respec-tively) intermediate between that of the Marmion Lake gneisses(21) and the depleted mantle (13), but similar Nd concentra-tions (;20 ppm) and147Sm/144Nd ratios (;0.11) that aretypical of relatively mature sediments (e.g., Jahn and Condie

1995). If this model is correct, depending on their geographicallocations and on their neighbouring crustal segments, sedimen-tary basins would have received in various proportions detritusfrom pre-existing crustal segments which may be as old as 3.2Ga and detritus from young volcanic arcs and TTG magmatismwhere younger juvenile terranes were initiated. In this scenario,the Quetico sediments record a contribution of;50% fromnew crust and the Timiskaming-type sediments;75% (Fig.7a).

In addition, sedimentary deposits with contrasting isotopiccompositions provide suitable endmembers to explain the ap-parent scattered isotopic compositions of the TTG suites. Thisstrongly suggests that the parental magmas of all pre-tectonictonalites contained, in some proportion, sediments that wererecycled back into the mantle. Thus, the Indian Lake batholithwould record the largest contribution from older crustal mate-rials whereas the Revell pluton would record higher propor-tions of detritus derived from juvenile crust. The ShebandowanLake and Atikwa batholiths would essentially record the par-ticipation of juvenile, ca. 2.7 Ga old terranes, an interpretationwhich is in agreement with those of Edwards and Davis (1984)and Davis and Edwards (1985).

The model invoking sediment subduction to explain theNd-Pb isotopic systematics is preferred to alternative explana-tions calling upon intracrustal melting and/or crustal contami-nation of the parental TTG magmas. This is because in the caseof the latter, the [Nd]/[Pb] ratios of the parental TTG magmasare expected to be quite close to those characterizing the crustalendmember which should produce mixing trends around amean r value of 1. However, two samples, one from the Revelland one from the Indian Lake batholiths (Nos. 44 and 47), have207Pb/204Pb ratios comparable to other samples from the sameplutons, but significantly lowereNd

t values, and plot close amixing line with r 5 1 (Fig. 7a). In this situation, crustalcontamination/intracrustal melting may be responsible for theirneodymium isotopic signatures.

All the post-tectonic plutons exhibit isotopic compositionslying along a trend with r;1 (Fig. 7b). This suggests thatcrustal contamination could be important in rocks having sanu-kitoid affinities. In order to reconcile their high Mg# and theirLILE-enriched nature, Stern et al. (1989) suggested that Ar-chean sanukitoid rocks were derived from the melting of ametasomatized mantle wedge above a subduction zone. In sucha model, the geographical distribution observed in Fig. 7bcould mean that the metasomatized mantle wedge beneath theWawa Sp was more radiogenic than that of the Quetico andWabigoon Sp. However in both cases, this requires that themetasomatic fluids and/or the metasomatized endmember werecharacterized by [Nd]/[Pb] ratios comparable to those of nor-mal depleted mantle in order to account for a mixing trend withr values close to unity. Thus, it is unlikely that the isotopicdifferences between the post-tectonic intrusions reflect differ-ences in the mantle isotopic compositions.

Alternatively, the geographical differences in the Nd and Pbcompositions of the post-tectonic plutons intruded in theWawa, Quetico, and Wabigoon crusts, may be imparted to themagmas by interaction with crustal materials. These variationsare clearly less marked than in the pre-tectonic TTG intrusives,because at the time of Archean sanukitoid rock formation, thedifferent crustal segments were dominated by the presence of


juvenile components. The more juvenile isotopic compositionsof Archean sanukitoid suites in the Wawa Sp may result fromdifferences in the type of crust assimilated or differences in theinitial magmas concentrations. Because dioritic to granodioriticplutons found in the Wawa Sp have Nd contents higher thanthose found in Quetico and Wabigoon Sp, their neodymiumisotopic compositions would be less modified by crustal con-tamination. Another assumption could be that the plutons areless contaminated in the Wawa Sp because the younger Wawacrust might be thinner than that of the Wabigoon Sp, and thuslead to lesser assimilation. Finally, and more likely, because theisotopic compositions of the plutons mirror the differencesrecorded by neodymium isotopes of metasedimentary rocksfrom Wawa and Quetico Sp (T and Q on Fig. 7b, respectively),the differences between the Archean sanukitoid plutons fromWawa, Quetico, and Wabigoon Sp reflect the differences be-tween the average isotopic compositions of each crust; theWawa crust being, on average, more juvenile than those ofQuetico and Wabigoon Sp.

6. CONCLUSIONS AND IMPLICATIONS FOR LATEARCHEAN CRUSTAL GROWTH

This neodymium and lead isotopic study of crustal growth inWestern Superior Craton reveals that:

(1) Mafic sequences in 2.73-2.70 Ga greenstone belts of thesouthern portion of the Western Superior Province have isoto-pic compositions consistent with their formation from a de-pleted mantle for which the neodymium isotopic compositionscorrespond closely to those in the depleted mantle models ofDePaolo (1980) and Jacobsen (1988). At 2.7 Ga, this depletedmantle had aneNd value of about13 and lead isotope ratios(207Pb/204Pb of 14.3-14.5) in good agreement with previousestimates for the Superior Province and other Archean cratons(Dupre et al., 1984; Tilton and Kwon 1990; Machado et al.1986; Shirey and Hanson 1986; Carignan et al., 1995; Steven-son, 1995).

(2) All pre- to post-tectonic sequences record the participa-tion of enriched reservoirs mixed with depleted mantle-likematerials. The types of enriched reservoirs which have inter-acted to form Archean crust is often controversial, ranging fromcrustal to enriched mantle reservoirs. For example, a negativecorrelation betweeneNd

t and Pb-Pb ratios (Fig. 7) in Archeanunits has also been remarked upon by Vervoort et al. (1994)who, although they did not discard crustal contamination as acause, suggested the role of an enriched source formed by anancient differentiation event in the mantle. The mixing rela-tionships detailed above demonstrate that the neodymium andlead isotopic compositions of such enriched reservoirs end-members mirror those measured (Nd) and estimated (Pb) formetasedimentary rocks deposited in Quetico or in Wawa Sp.Therefore, crustal contamination processes are thought to ex-plain better the enriched signatures of late Archean rocks.Neodymium and lead model ages also indicate that the recycledcrustal materials include an older component that was isolatedfrom the depleted mantle at ca. 3.2 Ga.

(3) Lead and neodymium mixing models indicate that pre-tectonic tonalites (Nd, 30 ppm, 147Sm/144Nd ;0.10) withages between 2.74 to 2.69 Ga were likely generated by partialmelting of a garnet-bearing amphibolite source, as suggested by

Martin (1987), which was contaminated by the subduction andincorporation of sediments (;1-10%). This contamination var-ied from place to place because the isotopic compositions of thesedimentary basins varied as a result of differences in theisotopic compositions of the eroded sources due to the erosionof 2.73 to 2.69 Ga juvenile terranes.

(4) Post-tectonic magmatism, dominated by Nd(LREE)-en-riched plutonic rocks with sanukitoid affinities, yielded neody-mium and lead isotope data which overlap with those of theQuetico and Timiskaming-type sedimentary units. Further-more, a geographical isotopic distribution is observed with themore mantle-like compositions found in the Wawa Sp and themore crustal-like compositions found in the Wabigoon Sp.Although these magmas are interpreted to be derived from adepleted mantle that was metasomatized before melting (Sternet al., 1989), these observations are thought to reflect theimportance of crustal assimilation with concurrent fractionationin the evolution of the diorite-monzodiorite-granodiorite-gran-ite series of the Archean sanukitoid suite. The Wawa Archeansanukitoid suites reflect the involvement of younger crust, lesscrust, or the same amount of crust if the initial magmas weremore enriched in Nd and Pb and thus less susceptible tocontamination.

Geochemical and volcanological work on present-day oce-anic plateaus have led to the suggestion that mantle plumesmay be important sources of heat for the production and re-working of Archean crust (e.g., Storey et al., 1991; Vervoort etal., 1993; Abbott, 1996; Patchett, 1996). Mantle plumes arecommonly associated with an enriched mantle reservoir and, inthe Archean, komatiites are generally regarded as being ofplume origin (e.g., Campbell et al., 1989). However, almost alllate Archean komatiites have neodymium and lead isotopicsignatures consistent with an origin from a depleted mantle(e.g., Campbell et al., 1989; Stevenson, 1995; Lahaye andArndt, 1996). If there were enriched reservoirs associated withplume generated komatiites, they were not old enough to havegenerated isotopic signatures distinct from those of the depletedmantle; i.e., the enriched reservoirs were more like those hy-pothesized for the formation of Archean sanukitoid suites(Stern and Hanson, 1991).

While plume-tectonics may have been an important catalystin initiating intra-oceanic subduction (Boher et al., 1992;Abouchami et al., 1990), oceanic plateau-like terranes are volu-metrically inferior to the volcanic arc-related suites whichdominate the Western Superior Province (e.g., Thurston andChivers, 1990; Desrochers et al., 1993). The role of plume-plateau volcanism in the creation of felsic crusts in the Archeanalso remains controversial in view of the strong relationshipbetween TTG genesis and volcanic arc environments (Martin,1987, 1994). In this respect, the isotopic data presented here areconsistent with crust formation in arc environments whereextensive interaction could occur with crust from preceedingvolcanic cycles and/or with subducted sediments (Fig. 7).These data also indicate crustal recycling was an importantprocess in the overall construction of the Western SuperiorProvince, but both zircon ages and isotopic Nd and Pb tracerstudies indicate that most of the crust that was recycled was notmuch older than 3.2 Ga; i.e., not excessively older than thecrust presently exposed in the southern Western Superior Prov-ince and in agreement with the oldest 3.17 Ga U-Pb zircon ages


found in Winnipeg-River Sp (Corfu, 1988). Stevenson andPatchett (1990) also noted that there is seldom isotopic evi-dence for the recycling of material much older than that whichis already exposed in the Superior Craton as a whole, as well asin the North Atlantic and Kaapvaal Cratons (see also Jahn andCondie, 1995). However, this contrasts with late Proterozoicand Phanerozoic orogens where there is often evidence forrecycling of much older, isotopically evolved crust which un-derlines the difficulty in identifying crustal recycling of rela-tively less evolved crust in the Archean without a detailedgeochronology database.

Acknowledgments—The authors gratefully thank Fernando Corfu andDon Davis of the Royal Ontario Museum for stimulating discussions,generous help in obtaining samples and for their landmark contribu-tions to the geochronology of the Western Superior Province withoutwhich this study would not have been possible. Jean Carignan isthanked for thought-provoking discussions and help in the Pb labora-tory. L. Donahue, A. Hamel, E. Malka, R. Lapointe, Y. Larbi and F.Robert were of great help in sample preparation and mass spectrometermaintenance. The final manuscript benefited from encouraging reviewsby P.J. Patchett and J. Kramers. This work was funded by LITHO-PROBE, NSERC and FCAR grants to Garie´py and Stevenson. LITHO-PROBE contribution No. 884.

Editorial handling: K. Mezger

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