Late Archean Mantle Composition and Crustal Growth in the Western Superior Province of Canada: Neodymium and Lead Isotopic Evidence from the Wawa, Quetico, and Wabigoon Subprovinces

Geochimica et Cosmochimica Acta, Vol. 62, No. 1, pp. 143–157, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/98 $19.00 1 .00

Pergamon

PII S0016-7037(97)00324-4

Late Archean mantle composition and crustal growth in the Western Superior Province of Canada: Neodymium and lead isotopic evidence from the Wawa, Quetico, and Wabigoon subprovinces PHILIPPE HENRY,* ROSS K. STEVENSON, and CLE´ MENT GARIE´ PY GEOTOP, Universite´ du Que´bec a` Montre´al, C.P. 8888, Succ. Centre-ville, Montre´al H3C 3P8, Canada (Received September 25, 1996; accepted in revised form September 2, 1997)

Abstract—Neodymium and lead isotopes are presented for late Archean rocks from the Wawa, Quetico, and Wabigoon subprovinces (Sp) in the Western Superior Province (Ontario, Canada). The isotopic compositions were determined on greenstone volcanic sequences, pre-tectonic (2.73-2.69 Ga) trondhjemite-tonalite-granodiorite (TTG) suites, metasedimentary rocks, and post-tectonic (2.69-2.67 Ga) dioritic to granitic plutons in order to characterize the mantle and crustal reservoirs involved in the evolution of the southern part of the Western Superior Province. Although derived from a depleted mantle, almost all the greenstone volcanism and pre-tectonic TTG suites record contamination by crustal material as old as 3.2 Ga. Neodymium and lead isotopic compositions of the pre-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 that the isotopic compositions of these subducted sediments varied as a result of the addition of new ca. 2.7 Ga juvenile material, in agreement with neodymium isotopic differences recorded by sedimentary units of the Quetico 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 an isotopic trend which is distinct from that of the TTG suites and which is interpreted to reflect the importance of crustal assimilation in the evolution of these suites. Furthemore, such plutons found in the Wawa Sp have more juvenile neodymium and lead isotopic compositions and higher Nd contents than those intruded into the Quetico and Wabigoon Sp which have more crustal isotopic signatures and lower Nd concentrations. This geographical difference could be due either to differences in Nd contents of initial Archean sanukitoid magmas, lesser assimilation in the Wawa Sp or, more probably, because the Wawa crust is on average more juvenile than the Quetico and Wabigoon crusts in agreement with the fact that the geographical difference mirrors that recorded by metasedimentary rocks deposited in Quetico and Wawa Sp. Copyright © 1998 Elsevier Science Ltd 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 been divided into a series of elongated, broadly E-W trending subprovinces, based upon similarities in lithologic assemblages, structural traits, and metamorphic grades (e.g., Card and Ciesielski, 1986). The alternating pattern of subprovinces within the Superior Province and their characteristic elongated nature suggest that the craton was formed by accretion of crustal segments in much the same way as Phanerozoic orogens formed (e.g., Langford and Morin, 1975; Corfu et al., 1985; Blackburn et al., 1985; Ludden et al., 1986), i.e., in a convergent plate setting with terrane accretion and concomitant to late-collisional magmatism. Radiogenic isotope studies are responsible for important advances in our understanding of material mass transfer between the different reservoirs of the Earth involved in Phanerozoic convergent plate settings (e.g., Samson et al., 1989; Frost and Coombs, 1989). In many late Archean terranes, such studies are frequently hampered by the lack of a precise geochronological framework and by difficulties in establishing reliable isotopic compositions for the different reservoirs involved, notably because the period of crustal evolution was relatively short, typically less than a few hundred million years. This makes mass transfer modeling between the different reservoirs

1. INTRODUCTION

The Late Archean was a major period of continental crust formation during which very different geological assemblages were tectonically assembled and rapidly stabilized to form large cratonic areas. In the Canadian Shield, this is exemplified by the formation of the Superior and the Slave Provinces which now lie at the core of the North American continent. Such late Archean cratons are generally regarded as alternating tectonostratigraphic assemblages made of granite-greenstone and of tonalite-trondhjemite-granodiorite (TTG) terranes, with intermingled terranes dominated by metasedimentary rock packages. By comparison with the present tectonic regime of the Earth, numerous studies have made links between greenstonedominated terranes and different tectono-magmatic environments active in oceanic plate settings, as well as links between Archean TTG terranes and modern, subduction-related TTG suites (e.g., Barker and Arth, 1976; Maaloe, 1982). Indeed, the overall petrological and geochemical similarity of Archean and Phanerozoic TTG suites suggests that the majority of these rocks were derived from amphibolitic/eclogitic melt precursors

* Present address: Ge´osciences, Universite´ de Franche Comte´, 16 route de Gray, 25030 Besanc¸on cedex, France. 143

144

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

Fig. 1. (a) A map of the southern part of the Western Superior Province (after Stern et al., 1989) with the locations of samples 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 locations of the volcanic units, the Timiskaming-type sediments, the Shebandowan Lake Pluton (SLP), and the Burchell Lake Pluton (BLP).

difficult to evaluate because the endmembers are isotopically quite similar. This study reports Sm-Nd and Pb-Pb isotope data for a wide variety of rock units within three subprovinces of the Western Superior craton (Fig. 1). The rock units include both mafic and felsic volcanic units, metasedimentary deposits, pre-tectonic TTG assemblages and post-tectonic granitoid intrusions. The large data base of precise U-Pb age determinations in the Western Superior Province allows accurate determination of initial isotopic compositions and makes it possible to probe, at the scale of a few millions of years, the geochemical evolution of the craton, the processes involved in its formation and assembly, the tectonic environment of the different terranes, and the amount of net juvenile additions to the crust.

2. GEOCHRONOLOGICAL BACKGROUND AND SAMPLING

To the west of Lake Superior, the Western Superior Province consists, from south to north, of the Wawa, Quetico, and Wabigoon subprovinces (Fig. 1a). The Wawa and Wabigoon subprovinces (Sp) are low metamorphic grade, volcano-plutonic assemblages comprising several intervening greenstone belts and some metasedimentary sequences intruded by massive granitoid bodies. In contrast, the Quetico Sp is dominated by clastic metasedimentary sequences that were intruded by late-kinematic granitoids, frequently two-mica bearing. The oldest rock units in the Western Superior Province were formed at ca. 3.17 Ga (Corfu, 1988) and are preserved within the Winnipeg River Sp, to the northwest of the Wabigoon Sp.

Late Archean crustal growth, Ontario, Canada

Two younger episodes of crustal growth have also been recognized, corresponding to (1) the formation of the Lumby Lake greenstone belt and the Marmion Lake batholith, in the central part 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 the main period of crustal growth with the generation of numerous greenstone assemblages and associated TTG suites, such as those characterizing the Wawa and Wabigoon Sp (e.g., Davis and Edwards, 1985; Corfu and Stott, 1986). A period of granitoid intrusion lasting until ca. 2.65 Ga followed the formation of the greenstone assemblages. The two periods of crustal growth at ca. 3.0 and 2.7 Ga are also recognized in metasedimentary rocks of the Quetico Sp through the presence of abundant detrital zircons yielding U-Pb ages between 2698 and 3009 Ma (Davis et al., 1990). The following section reviews the geochronological database of the subprovinces from which samples were obtained (Table 1 and Fig. 1). The geochronological data for the Wawa Sp consist of U-Pb ages from the Shebandowan greenstone belt (Corfu and Stott, 1986). Ages for units sampled outside the Shebandowan greenstone belt towards Thunder Bay (Fig. 1a) were estimated on the basis of field relationships and lithological similarities. The Shebandowan greenstone belt (Fig. 1b) comprises an older calc-alkaline sequence of mainly mafic-ultramafic tholeiites dated at 2733 6 3 Ma (Corfu and Stott, 1986) and locally abundant intermediate to felsic flows (samples Nos. 1-6 and 12-15) for which a slightly younger age of ca. 2710 Ma was assigned because they lie above the older mafic sequence but are cut by a younger intrusive suite. The intrusive suite is deformed and is represented by the trondhjemite-tonalite Shebandowan Lake pluton (No. 7), dated at 2696 6 2 Ma (Corfu and Stott, 1986). These rocks are unconformably overlain by alkaline magmatic units, dated at 268913/22 Ma (Corfu and Stott, 1986), and alluvial-fluvial sediments (Nos. 8-9, 16-17) comparable to the Timiskaming units of the Abitibi Sp (Williams et al., 1991), which represent late extensional phases in the evolution of the Superior Province (Corfu et al., 1991). The above units were affected by two deformational events accompanied by greenschist-facies metamorphism (Corfu and Stott, 1986). The greenstone belt was intruded by the undeformed and unmetamorphosed Burchell Lake pluton (Nos. 10, 11), dated at 2684 Ma16/23 (Corfu and Stott, 1986). This is the youngest intrusion dated in the belt and along with the older Shebandowan Lake pluton is used to assign an age of 2690 Ma to the Timiskaming-type sediments. The Burchell pluton is composed of diorite-tonalite and granodiorite and is considered by Stern et al. (1989) to be a member of the sanukitoid suite. Archean sanukitoids, which were first identified in the Western Superior Province by Shirey and Hanson (1984), are rocks characterized by 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 Lake bodies) thought to have sanukitoid affinities (Stern et al., 1989) were collected near Lake Superior. These stocks were assigned ages of ca. 2.67 Ga based on their undeformed character and their similarity to post-tectonic stocks in the Quetico and Wabigoon Sp (see below). The Quetico Sp is largely composed of wackes interpreted to have been deposited in an accretionary prism on the south

145

facing arc of the Wabigoon Sp. The prism was subsequently buried and metamorphosed during its collision with the Wawa Sp, resulting in a symmetrical metamorphic zonation grading from greenschist-facies near the margins to high grade facies with 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 taken from the low grade margins adjacent to the Wawa (Nos. 25-29) and the Wabigoon Subprovinces (Nos. 30-32). Two main types of syn- to post-tectonic plutons are found in the Quetico Sp. This includes S-type granitoids formed by partial melting of Quetico sediments (Nos. 36-40) which have been dated between 2687 and 2671 Ma (Percival and Sullivan, 1988). The second type includes plutons having dioritic to granodioritic compositions of sanukitoid affinity (Stern et al., 1989) such as the undeformed and unmetamorphosed Blalock pluton (Nos. 33-35) dated at 268864 Ma (Davis et al., 1990). Samples from the Wabigoon Sp were derived from the Western and Central portions of this subprovince. The Western Wabigoon is dominated by 2.76-2.73 Ga old greenstone belts, such as the Kakagi-Rowan Lakes and the Eagle-WabigoonManitou Lakes greenstone belts (Davis et al., 1982). These belts also contain TTG suites such as the 2732 6 3 Ma old Atikwa tonalite (Nos. 41, 42) which were intruded closely in time with host greenstone supracrustals (Davis et al., 1982; Blackburn et al., 1991). In contrast, the Central Wabigoon is dominated by deformed ovoid intrusive complexes (e.g., Revell, Indian Lake, and White Otter Lake batholiths, Nos. 43-48) ranging in composition from tonalite and trondhjemite to granodiorite, quartz monzonite and granite (Blackburn et al., 1991). These plutons were assigned an age of ca. 2730 Ma on the basis on their correlation with the 2732 Ma Atikwa batholith. Deformation of the pre-tectonic TTG suites resulted in foliation at their margins, but left the cores largely massive with no evidence of recrystallization. Undeformed, post-tectonic, diorite-monzodiorite-granodiorite plutons (i.e., the EyeDashwa and Smirch Lake plutons, Nos. 49-54) were assigned an age of ca. 2670 Ma based on the work of Zartman and Kwak (1990). These post-tectonic plutons are LILE- and REE-enriched intrusions which likely belong to the Archean sanukitoid-type suites (Stern et al., 1989). Older terranes have been recognized in the Central Wabigoon such as the Marmion Lake batholith (Nos. 55-57) which consists of metaplutonic gneisses dated at 3003 6 5 Ma (Davis and Jackson, 1988). A felsic volcanic unit in the Lumby Lake greenstone belt was dated at 2999 6 1 Ma (Davis and Jackson, 1988) and was sampled for this study (No. 58). These units constitute the oldest crust exposed in the southern Western Superior Province and provide a means to test the recycling of older materials into the younger plutonic and greenstone supracrustals. 3. ANALYTICAL METHODS Samples for Sm-Nd analysis were crushed using a hydraulic press and a representative fraction was powdered in an agate-lined shatterbox. About 0.1-0.2 g of the powders were weighed out in a teflon pressure-vessel to which a 149Sm-150Nd mixed tracer solution and HF-HNO3 acids were added. The mixture was reacted under pressure in an oven at 150°C for 1 week. The resulting fluoride salts were converted to chlorides by redissolving and drying the samples in 6 N HCl. Chemical separation of Sm-Nd was done following the procedure

146

P. Henry, R. K. Stevenson, and C. Garie´py Table 1. Sample descriptions and whole rock Sm-Nd data from Wawa, Quetico and Wabigoon Subprovinces. 147

Age Ma

a

[ref] b

Nd c

Sm Nd c

144

Sm c

143

Nd Nd

144

(2s)

eNdt d

WAWA SUBPROVINCE Shebandowan Greenstone Belt 1 WS-21b 2 (Wawa-g) WS-21a 3 WS-19 4 WS-17b 5 WS-17c 6 (Wawa-a) WS-20 7 (SLP) C83-38 8 WS-15 9 WS-16 10 (BLP) WS-22c 11 (BLP) WS-22d

V. mafic Granite (sill in 21b) Basalt V. intermediate V. felsic Andesitic tuff Tonalite (She. L. P.) Timiskaming-sedt Timiskaming-sedt Grd (Burchell L. P.) Ton (Burchell L. P.)

2733 2733 2710 2710 2710 2710 2696 2690 2690 2684 2684

* [1] * * * * [1] [1] [1] [1] [1]

8.21 2.03 2.89 2.14 3.55 2.11 2.88 3.78 4.33 4.77 7.16

41.91 10.42 15.81 6.83 20.78 11.26 17.79 21.14 24.80 32.70 47.63

0.1184 0.1176 0.1104 0.1897 0.1032 0.1130 0.0978 0.1081 0.1056 0.0882 0.0903

0.511375 0.511357 0.511211 0.512640 0.511052 0.511256 0.510994 0.511158 0.511101 0.510797 0.510805

(7) (24) (20) (9) (9) (11) (10) (12) (51) (26) (7)

2.9 2.9 2.3 2.5 1.7 2.3 2.3 1.8 1.6 1.6 1.0

Eastern Wawa 12 13 14 15 16 17 18 (P­) 19 (P­) 20 21 22 23 24 (B­)

95 TB 1 95 TB 2 WS-11 WS-12 WS-13b WS-13a WS-1a WS-1c WS-1b WS-5b WS-6 WS-7 WS-10

Gabbro Gabbro V. felsic Andesite Timiskaming-sedt Timiskaming-sedt Diorite (Penassen ­ Diorite (Penassen ­) Granite (sill in 1a) Granodiorite Granodiorite Diorite Grd (Barnum ­)

2730 2730 2710 2710 2690 2690 2670 2670 2665 2670 2670 2670 2670

* * * * * * * * * * * * *

3.21 2.02 3.74 6.52 16.76 3.79 16.85 15.13 5.05 15.26 11.97 12.37 8.19

10.33 6.72 20.12 22.31 61.92 15.46 82.82 76.79 33.79 105.84 79.40 82.87 54.06

0.1880 0.1819 0.1122 0.1765 0.1636 0.1482 0.1230 0.1191 0.0903 0.0872 0.0911 0.0902 0.0915

0.512587 0.512499 0.511208 0.512402 0.512203 0.511827 0.511393 0.511346 0.510838 0.510788 0.510876 0.510919 0.510864

(8) (10) (11) (6) (9) (15) (10) (12) (11) (12) (9) (9) (10)

2.1 2.5 1.6 2.5 3.0 1.0 1.0 1.5 1.4 1.6 1.9 3.1 1.6

WS-4 WS-5a WS-8a WS-8c WS-14c WS-23a WS-23b WS-23c WS-26a WS-26c WS-26b WS-2 WS-24 WS-25 WS-14a WS-14b

Quetico-sediment Quetico-sediment Quetico-sediment Quetico-sediment Quetico-sediment Quetico-sediment Quetico-sediment Quetico-sediment Tonalite (Blalock) Granodiorite (Blalock) Grd dyke (in 26c) two micas Granite bio-Leucogranite ms-Leucogranite Aplitic sill (in 14c) Granitic sill (in 14c)

24.24 26.32 23.78 28.14 22.75 24.22 22.60 18.24 44.32 54.38 3.88 66.88 42.05 38.48 2.39 2.90

0.1055 0.1038 0.1063 0.1032 0.1054 0.1056 0.1047 0.1169 0.0840 0.0849 0.0878 0.0876 0.0834 0.1111 0.1053 0.0978

0.511022 0.510998 0.511052 0.511018 0.511060 0.511055 0.511058 0.511246 0.510689 0.510708 0.510821 0.510724 0.510728 0.511152 0.511135 0.510934

(15) (10) (14) (9) (11) (6) (12) (9) (10) (8) (7) (14) (8) (12) (21) (10)

0.1 20.1 0.4 0.8 0.8 0.7 1.1 0.5 1.0 1.0 2.2 0.4 1.7 0.4 2.3 1.0

WS-32 WS-33 WS-31 WS-34 WS-35 WS-36 WS-37 WS-29 WS-27a WS-27b WS-28a WS-28b WS-30a WS-30b

WABIGOON SUBPROVINCE Ton (Atikwa) 2732 [4] 2.79 Grd (Atikwa) 2732 [4] 1.55 Grd (Revell) 2730 * 3.02 Ton (Revell) 2730 * 1.89 Ton (Indian Lake) 2730 * 2.84 Grd (Indian Lake) 2730 * 3.04 Grd (Indian Lake) 2730 * 3.98 Grd (White Otter Lake) 2730 * 1.85 Grd (Eye-Dashwa) 2665 [5] 5.75 Dio (Eye-Dashwa) 2665 [5] 5.00 Grd (Eye-Dashwa) 2665 [5] 6.98 Grd (Eye-Dashwa) 2665 [5] 7.36 Diorite (Smirch Lake) 2670 * 6.03 Pegmatite (in 30a) 2670 * 2.43

13.20 8.96 17.91 13.44 18.92 18.45 26.17 18.17 39.85 34.74 47.55 49.45 35.52 9.72

0.1275 0.1044 0.1020 0.0849 0.0906 0.0996 0.0920 0.0614 0.0872 0.0870 0.0888 0.0900 0.1025 0.1389

0.511500 0.511062 0.511039 0.510644 0.510831 0.510953 0.510656 0.510386 0.510766 0.510745 0.510769 0.510801 0.511012 0.511733

(14) (14) (7) (28) (8) (7) (9) (11) (11) (10) (9) (7) (9) (11)

2.1 1.7 2.1 0.4 2.1 1.3 21.9 3.6 1.0 0.7 0.6 0.8 1.0 2.4

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

41 42 43 44 45 46 47 48 49 50 51 52 53 54

(Bl) (Bl) (Q-g) (Q-g)

(A) (A) (R) (R) (IL) (IL) (IL) (WO) (E-D) (E-D) (E-D) (E-D) (SL)

QUETICO SUBPROVINCE 2690 * 4.23 2690 * 4.52 2690 * 4.18 2690 * 4.81 2690 * 3.97 2693 [2] 4.23 2693 [2] 3.92 2693 [2] 3.53 2688 [2] 6.16 2688 [2] 7.64 2685 * 0.564 2686 [3] 9.70 2671 [3] 5.81 2671 [3] 7.07 2687 * 0.417 2687 [3] 0.470

Late Archean crustal growth, Ontario, Canada

147

Table 1. (Continued) Sample descriptions and whole rock Sm-Nd data from Wawa, Quetico and Wabigoon Subprovinces. 147

a Marmion Lake Batholith and Lumby Lake greenstone belt 55 (M) 95 TB 4 Gneiss 56 (M) 95 TB 5A Orthogneiss 57 (M) 95 TB 6A Gneiss 58 (LL) 95 TB 8 Felsic tuff

Age Ma

[ref] b

Sm c

Nd c

Sm Nd c

3003 3003 3003 2999

[6] [6] [6] [6]

1.39 3.06 3.21 3.22

8.51 13.46 14.53 18.48

0.0989 0.1373 0.1335 0.1051

144

143

Nd Nd

144

(2s)

eNdt d

0.510829 0.511584 0.511506 0.510985

(11) (14) (15) (14)

2.5 2.4 2.4 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 et al. 1982, [5] Zartman and Kwak 1990, [6] Davis and Jackson 1988, *: age based on field relationships (see text). c- Sm and Nd concentrations and Sm/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 depending to each Sm/Nd ratio, and with 143Nd/144Nd and 147Sm/144Nd ratios of 0.512638 and 0.1967, respectively, for today CHUR.

described by Richard et al. (1976). Samarium and neodymium were loaded on a double Re-Ta filament and analyzed in static and dynamic multi-collector mode, respectively, on a VG Sector 54 mass spectrometer. During the course of this study, the La Jolla Nd standard yielded a 143Nd/144Nd ratio of 0.511849 6 12 (2s mean on twenty-one analyses) and total blanks for Nd or Sm were less than 50 pg and negligible. The lead isotopic compositions were measured on K-feldspar separates from plutonic rock samples. K-feldspar contains little U and its lead isotopic composition generally closely approximates the initial composition. Only suites which showed no evidence of K-feldspar recrystallization 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 were washed overnight in aqua regia and rinsed in distilled water. The residual powder was leached with a mixture of a dilute HF-HBr for 30 min.; the supernatant and the residue were recovered, dissolved in HF and processed separately through anion-exchange separation (Manhe`s et al., 1980). The leaching treatments are carried out to enhance, inasmuch as possible, removal of labile radiogenic Pb held in crystalline defects, cleavage surfaces, or nonsilicate impurities. Lead was analyzed in static multicollection mode on a VG Sector 54 mass spectrometer. The raw data were corrected for instrumental mass fractionation using a factor of 0.09% amu21. Total blanks were smaller than 30 pg and negligible. Replicate analyses of the NBS SRM-981 standard yielded a reproducibility of 60.05% amu21 (1s) for filament loads ranging from 10 to 100 ng. 4. RESULTS

The whole rock Sm-Nd compositions (Table 1) show significant geochemical differences between the older (2.733-2.695 Ga) and younger (2.690-2.665 Ga) plutonic suites. These are best illustrated when the 147Sm/144Nd ratios and the Nd concentrations of the rocks are examined as a function of the ages of the units (Fig. 2). The pre-tectonic tonalite plutons have mean 147Sm/144Nd ratios and Nd concentrations of ;0.10 and ;15 ppm, respectively. These values are in good agreement with those reported in other studies of Archean TTG suites (e.g., Martin, 1987), where the TTG geochemical characteristics were consistent with ;20% partial melting of an amphibolite source with a garnet-rich residue. The post-tectonic plutons also yielded low 147Sm/144Nd ratios, but their Nd concentrations are systematically higher (30-110 ppm). Stern et al. (1989) suggested that the REE- and LILE-enriched nature of Archean 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 trace element-enriched mantle. In Fig. 3a, the eNdt values are plotted against the ages of sample crystallization/sedimentation. Also shown, for compar-

ison, are reference lines depicting the evolution, between 2.74 and 2.66 Ga, of a depleted mantle and of continental crusts formed at 3.0 and 3.2 Ga. The model mantle compositions are derived from a present-day depleted mantle having 143Nd/ 144 Nd and 147Sm/144Nd ratios of 0.51310 and 0.2136, respectively (e.g., Jacobsen, 1988). This corresponds to an eNd2.7Ga value of 13 which is in good agreement with several published estimates for the neodymium isotopic composition of the late Archean depleted mantle in the Superior Province and elsewhere (e.g., Machado et al., 1986; Tilton and Kwon, 1990; Stevenson, 1995; Vervoort et al., 1996 and global compilation in Shirey and Hanson, 1986). The Nd evolution lines for ancient continental crusts are calculated for materials extracted from this same depleted mantle, at 3.2 and 3.0 Ga, with an average 147Sm/144Nd ratio of 0.115. The lead isotopic compositions are given in Table 2. For each sample, the Pb/Pb ratio obtained for the leachate analysis is greater than or equal, within error, to that obtained for the residue analysis. The residue analysis are consistent with calculated Archean lead isotopic compositions (see Fig. 5 and discussion below) and, thus represent excellent estimates for the initial lead isotopic compositions of the plutons. Figure 3b shows the 207Pb/204Pb results obtained on the K-feldspar residues plotted as a function of pluton emplacement age. Evolution curves depicting the lead isotopic evolution of the mantle and crustal reservoirs are difficult to model because they strongly depend on the final lead isotope composition of the Bulk Silicate Earth after accretion and core formation (e.g., Alle`gre et al., 1996; Galer and Goldstein, 1996) and on the 238 U/204Pb (m) of the mantle and crustal reservoirs. The data are compared to a single-stage Pb evolution model assuming a Pb-Pb age of 4.48 Ga for the Earth. This age represents the time at which the Bulk Silicate Earth acquired its U/Pb ratio after core formation (Alle`gre et al., 1996; Galer and Goldstein, 1996). The depleted mantle curve is then calculated using initial compositions of the Canyon Diablo meteorite (Tatsumoto et al., 1973) and a m value of 8.5. This model yields 207 Pb/204Pb ratios that are comparable to other lead isotopic compositions reported for the late Archean mantle (Tilton and Kwon, 1990; Carignan et al., 1995). The Pb evolution curves of 3.2 and 3.0 Ga old crustal reservoirs are shown for arbitrarily chosen m values of 12. Both the neodymium and lead isotope data show significant deviation from the model depleted mantle evolution which suggests the involvement of a reservoir char-

148

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

Fig. 2. Sm-Nd evolution with time. (a) 147Sm/144Nd vs. ages. There appears to be a slight decrease in the 147Sm/144Nd ratios from pre- to post-tectonic plutons with the latter having lower and more homogeneous 147Sm/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- and post-tectonic eras are separated in time by a short period (less than 10 Ma) corresponding to the deposition of 2.69 Ga Quetico and Timiskaming-type sediments in Quetico and Wawa Sp, respectively.

acterized by radiogenic Pb and low eNdt values such as continental crusts. The Sm-Nd data for pre-2.7 Ga volcanic rocks sampled in Wawa Sp (Nos. 1, 3, 12, 13) yielded eNdt values between 12.9 and 12.1 and a granite sill (No. 2), dated at 2733 Ma (Corfu and Stott, 1986) has an eNdt value of 12.9 6 0.5, similar to that of the host mafic volcanics. K-feldspar from this granite yielded a 207Pb /204Pb ratio of 14.58. These values are consistent with a magmatic derivation from the depleted mantle reservoir. The ca. 2710 Ma calc-alkaline, intermediate to felsic volcanics (Nos. 4-6, 14-15) and the syn-tectonic Shebandowan Lake pluton (No. 7), both from Wawa Sp, have eNdt values from 12.5 to 11.6. The K-feldspar separated from a calc-alkaline tuff and from the Shebandowan Lake pluton yielded 207 Pb/204Pb ratios of 14.65 and 14.52, respectively. Some of these Nd and Pb values for the pre-tectonic TTG suites are

significantly different from those estimated for the depleted mantle reservoir (Fig. 3a,b). In the Wabigoon Sp, the 2732 Ma old Atikwa batholith (Nos. 41, 42), which is isolated from other large intrusive complexes by the Kakagi-Rowan Lakes and the Eagle-Wabigoon-Manitou Lakes greenstone belts, has isotopic compositions closer to the mantle endmember with eNdt 11.9 6 0.5 and 207Pb/204Pb 5 14.53 6 0.03. Conversely, the ca. 2.73 Ga Indian Lake batholith (Nos. 45-47), which intrudes a granitoid-gneiss terrane, has more crustal-like isotopic compositions with eNdt between 12 and 22, and 207Pb/204Pb 5 14.76 6 0.03. Therefore, the isotopic compositions of the pre-tectonic tonalites appear to reflect their particular crustal environment. The question of just how the crustal signatures were incorporated in the TTG suites is addressed in section 5. The Timiskaming-type sediments, deposited in Wawa Sp, have eNdt values in the range of 13 to 11. The two samples

Late Archean crustal growth, Ontario, Canada

149

Fig. 3. Neodymium and lead isotopic compositions vs. time. (a) eNdt vs. ages. The eNdt values are compared with those of the depleted mantle, 3.0 Ga Marmion Lake gneisses and a felsic tuff from the 3.0 Ga Lumby Lake greenstone belt (cross symbols). The evolution curves represent felsic crusts created at 3.0 and 3.2 Ga with 147Sm/144Nd 5 0.115 and illustrate that older crustal components are recorded by the Wawa, Quetico, and Wabigoon rocks. (b) 207Pb/204Pb vs. ages. 207 Pb/204Pb ratios are compared with depleted mantle having m1 (238U /204Pb ) value of 8.5 and 3.0 to 3.2 Ga felsic crusts evolving with a m2 value of 12. In agreement with neodymium isotopes, lead isotopic compositions of K-feldspars suggest the presence of an older crustal component. Symbols are as Fig. 2.

from the Shebandowan greenstone belt (Nos. 8 and 9, Table 1) yielded similar eNdt values (11.8 and 11.6) and have Nd contents (21.1 and 24.8 ppm) and 147Sm/144Nd ratios (0.108 and 0.105) consistent with well-mixed sediments (e.g., Alle`gre and Rousseau, 1984; Taylor and McLennan, 1985; McLennan and Hemming, 1992). Conversely, sediments with higher and lower values were sampled in the eastern Wawa Sp (Nos. 16 and 17, Table 1) and have different Nd contents (61.9 and 15.5 ppm) and 147Sm/144Nd ratios (0.164 and 0.148). The variation between the samples may reflect the greater influence of local sources in the eastern Wawa Sp leading to more variable isotopic compositions. The coeval Quetico sediments (Nos. 25-32, Table 1) yield a narrow range of eNdt values from 11.1 to 20.1. These homogeneous values are associated with narrow ranges in both the Nd concentrations (18.2-28.1 ppm) and the 147 Sm/144Nd ratios (0.103-0.117) which imply well-mixed sources. Moreover, the narrow range of initial neodymium isotopic compositions indicates that the greenschist-facies metamorphism did not significantly affect the rare earth elements in these sediments because such alteration would have modified the parent/daughter (Sm/Nd) ratios, producing a dispersion in calculated initial eNd values. Overall, the data from both the Timiskaming-type Wawa and Quetico sediments demonstrate that, at the time of their deposition, the eroded crusts

were composed of mixtures of ca. 2.7 Ga juvenile terranes and older crusts, in agreement with the U-Pb ages of detrital zircons from Quetico sediments (Davis et al., 1990). The Quetico granitic rocks (Nos. 36-40) yield a range of eNdt values (12.3 to 10.4) larger than that obtained from the Quetico sediments. The two S-type granites (Nos. 36, 38) have eNdt values similar to those of the sediments and yield 207 Pb/204Pb ratios of 14.64 and 14.67 which could represent an estimate of the average 207Pb/204Pb ratio of the crustal material eroded at 2.69 Ga. The younger intrusive rocks of the Wawa Sp, including the post-tectonic Burchell Lake Pluton (Nos. 10, 11) and post-tectonic diorites and granodiorites (Nos. 18-24) yielded eNdt values from 13.1 to 11.0 and 207Pb/204Pb ratios of 14.52-14.67. Two samples of tonalite-granodiorite from the 2688 Ma Blalock Pluton (Nos. 33, 34) emplaced on the boundary between Quetico and Wabigoon Sp, have eNdt values of 11.0 and 207Pb/204Pb ratios of 14.63. One dyke cutting the Blalock tonalite (No. 35) has an eNdt value of 12.2. Higher eNdt values found for the dykes and sills may be derived by partial melting of material more juvenile than the enclosing rocks (possibly a mafic precursor). The post-tectonic and REE-rich granodioritic to dioritic rocks from the Eye-Dashwa (Nos. 49-52) and Smirch Lake (No. 53) plutons yielded a restricted range of eNdt (10.6 to 11.0) and 207Pb/204Pb (14.63-14.68)

150

P. Henry, R. K. Stevenson, and C. Garie´py Table 2. Pb isotopes of K-feldpars from plutonic rocks from Wawa, Quetico and Wabigoon Subprovinces. 206

a

b

Pb/204Pb c

207

Pb/204Pb c

208

Pb/204Pb c

WAWA SUBPROVINCE Shebandowan greenstone belt 2 Wawa g

(WS-21a)

6 Wawa a

(WS-20)

7 SLP

(C 83-38)

10 BLP

(WS-22c)

11 BLP

(WS-22d)

Eastern Wawa 18 Pd

(WS-1a)

19 Pd

(WS-1c)

21

(WS-5b)

22

(WS-6)

23

(WS-7)

24 Bd

(WS-10)

33 Bl

(WS-26a)

34 Bl

(WS-26c)

36 Q-g

(WS-2)

38 Q-g

(WS-25)

40

(WS-14b)

41 A

(WS-32)

42 A

(WS-33)

43 R

(WS-31)

44 R

(WS-34)

45 IL

(WS-35)

46 IL

(WS-36)

47 IL

(WS-37)

48 WO

(WS-29)

49 E-D

(WS-27a)

50 E-D

(WS-27b)

51 E-D

(WS-28a)

52 E-D

(WS-28b)

53 SL

(WS-30a)

54

(WS-30b)

R l R l R l R l R l

13.49 13.56 13.97 14.31 13.71 14.17 13.51 13.51 13.48 13.54

14.58 14.61 14.65 14.63 14.52 14.63 14.60 14.62 14.59 14.66

33.26 33.35 33.60 33.64 33.48 33.90 33.29 33.36 33.72 33.45

R l R l R l R l R l R l

13.61 13.62 13.59 13.74 13.56 13.65 13.43 13.46 13.46 13.49 13.54 13.79

14.63 14.66 14.66 14.76 14.63 14.66 14.54 14.55 14.53 14.55 14.62 14.69

33.38 33.48 33.38 33.49 33.40 33.45 33.28 33.30 33.25 33.30 33.33 33.55

R l R l R l R l R l

QUETICO SUBPROVINCE 13.59 13.84 13.53 13.55 13.59 14.04 13.62 13.72 13.56 16.02

14.64 14.73 14.63 14.64 14.64 14.77 14.67 14.69 14.57 15.11

33.34 33.58 33.32 33.42 33.37 33.73 33.33 33.44 33.33 33.75

14.51 — 14.55 14.54 14.59 14.60 14.65 14.63 14.76 14.75 14.77 14.77 14.74 14.73 14.58 14.61 14.68 14.65 14.65 14.67 14.67 14.66 14.66 14.67 14.64 14.68 14.58 14.61

33.25 — 33.24 33.22 33.31 33.38 33.48 33.44 33.54 33.52 33.48 33.50 33.56 33.55 33.31 33.43 33.40 33.38 33.36 33.43 33.41 33.39 33.36 33.39 33.31 33.45 33.27 33.38

WABIGOON SUBPROVINCE R 13.44 l — R 13.49 l 13.52 R 13.49 l 13.56 R 13.56 l 13.59 R 13.77 l 13.82 R 13.75 l 13.82 R 13.76 l 13.84 R 13.55 l 13.60 R 13.63 l 13.60 R 13.58 l 13.63 R 13.61 l 13.62 R 13.59 l 13.62 R 13.54 l 13.57 R 13.59 l 13.69

a: numbers refer to Table 1, Fig. 1 and text. b: R 5 residues, l 5 leachates. c: 1 sigma for residue analyses is 0.015, 0.020 and 0.060 for 206Pb/204Pb, Pb/204Pb and 208Pb/204Pb, respectively, and must be doubled in the case of leachate analyses.

207

Late Archean crustal growth, Ontario, Canada

151

tonic suites could also reflect progressive thickening of the crust through volcanic and tectonic processes. For example, pre-tectonic tonalites intruded into a thin crust would not form large magma chambers, producing only limited amounts of highly fractionated derivatives. In contrast, post-tectonic dioritic melts intruded into a thickened crust may develop larger magma chambers allowing for greater fractionation and more abundant REE-rich granitoids. In addition, assimilation and/or anatexis of thickened crustal segments would also result in a greater petrological diversity of granitoid suites. 5. DISCUSSION

5.1. Mixing Relationships Fig. 4. eNdt vs. 147Sm/144Nd. The distribution of the data may reflect mixing between depleted and enriched reservoirs as illustrated by the lines drawn on the diagram. The depleted mantle endmember (DM) corresponds to materials ultimately derived from a depleted mantle with eNdt 5 13 and variable 147Sm/144Nd ratios defined by basalts (b), intermediate to felsic rocks (g) or diorites to granodiorites (d). The intersection of the mixing lines is consistent with a single crustal endmember (CM) having eNdt ; 22 and 147Sm/144Nd ; 0.08. Such a low Sm/Nd ratio may result from partial melting of the CM endmember prior to mixing with mantle-derived magmas. Note that, at 2.7 Ga the CM endmember has a eNd value slightly lower than that calculated from the 3.0 Ga terranes (samples from Central Wabigoon region). This could indicate recycling of crustal material slightly older than 3.0 Ga. Symbols as Fig. 2.

values. Finally, and as was the case for Quetico dykes, a pegmatite (No. 54) intruded in the Smirch Lake diorite has a more radiogenic isotopic composition than its host lithology, with an eNdt value of 12.4 and a 207Pb/204Pb ratio of 14.58. Overall, with the exception of one dioritic body near Thunder Bay and some minor dykes and sills, the post-tectonic rocks of sanukitoid affinity yielded homogeneous neodymium and lead isotopic compositions of about eNdt 11 and 207Pb/204Pb 5 14.65, respectively (Fig. 3a,b). These values could correspond to mixtures between depleted and enriched mantle reservoirs produced in a subarc mantle through metasomatism of the mantle wedge (Shirey and Hanson, 1984; Stern and Hanson, 1991). Alternatively, the values could be produced by the intrusion of a depleted mantle-derived magma into an enriched crustal reservoir and subsequent assimilation of this crust. Finally, the 3.0 Ga old gneisses from the Marmion Lake batholith (Nos. 55-57) and a felsic tuff from the Lumby Lake greenstone belt (No. 58) yielded eNd3Ga values between 13.2 and 12.4, which are consistent with these rocks being direct mantle derivatives at their time of formation. However, at the time of the main crustal growth episode in the Wabigoon Sp, these rocks would have had eNd2.7Ga values in the range of 21.3 to 10.1. These values will be used as a first-order approximation for an old crustal reservoir potentially present over the whole terrane. In comparison with the evolution curves of both depleted and enriched crustal reservoirs (Fig. 3), the data from this study show that there is no unique isotopic evolution trend for the 2.74-2.66 Ga old terranes in the different subprovinces. The dispersion of the data could be due to mixtures of different reservoirs such as depleted mantle, enriched mantle, and continental crust. Differences in Nd concentrations and the Nd-Pb isotopic compositions between the pre-tectonic and post-tec-

The neodymium and lead isotope data strongly implicate the interaction between depleted and enriched reservoirs in the formation of the late Archean Western Superior Province. The identity of these reservoirs is further investigated with the aid of the Sm-Nd and Pb-Pb isochron diagrams in Figs. 4 and 5, respectively. In Fig. 4, the Nd data are plotted in a eNdt vs. 147Sm/144Nd diagram to clarify the mixing relationships between ca. 2.7 Ga juvenile component(s) derived from a depleted mantle and older crustal endmember(s). The juvenile endmember encompasses the depleted mantle as well as any recent crustal additions derived from it. Thus it comprises rock units having eNdt values .12.5, but highly variable 147Sm/144Nd ratios: for example, ;0.19 for basalts and mafic rocks extracted directly from the depleted mantle and between 0.13-0.11 for granitic dykes and intermediate to felsic volcanics formed either by partial melting of a basaltic precursor, by fractional crystallization of mantle-derived magma, or both. The 147Sm/144Nd ratio of this juvenile component may even be as low as 0.09, as exemplified by a ca. 2.67 Ga diorite from the eastern Wawa Sp

Fig. 5. 207Pb/204Pb vs. 206Pb/204Pb. The distribution of the lead isotope ratios suggests at least two endmembers corresponding to depleted (DM) and enriched (CM) reservoirs with m values lower and higher than that the Bulk Silicate Earth (BSE, m ; 9), respectively. Therefore, the mixing array (gray array on the figure) corresponds to the equation BSE 5 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 explained by an older crust isolated from the depleted mantle at ca. 3.2 Ga and evolving with an average m value of about 12. Only, two rocks from Shebandowan greenstone belt have high 206Pb/204Pb ratios explained by in situ decay of U, in agreement with a 2.7 Ga isochron. The Pb-Pb evolution lines are defined in text. Symbols as Fig.2.

152

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

(No. 23, Table 1) which yielded an eNdt value of 13.1 suggesting a depleted mantle source. A granodiorite from the White Otter Lake batholith (No. 48) with a high eNdt value of 13.6 calculated from an even lower measured 147Sm/144Nd ratio of 0.0614 may be similarly derived through fractionation from a mafic precursor. The data from the plutonic samples, irrespective of their actual location in the Western Superior Province, fan towards a common endmember (Fig. 4) having an eNd2.7Ga value of ; 22 and a 147Sm/144Nd of ; 0.08. These values were arbitrarily chosen to represent the crustal endmenber. At 2.7 Ga, the isotopic composition of that endmember would be slightly lower than that of the 3.0 Ga Marmion Lake gneisses and the Lumby Lake felsic tuff, albeit its Sm/Nd ratio is clearly lower. The lower Sm/Nd ratio of the crustal component could result from the partial melting of rocks similar in composition to the Marmion and Lumby Lakes samples, producing magmas with steeper LREE-enrichment profiles. This does not mean that Marmion Lake materials were incorporated into the Wawa Sp, but it implies the presence of materials with similar age and composition. Older crustal contributions to the Wawa Sp may actually 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- and post-tectonic samples have interacted with older crustal materials including ancient terranes isolated from the depleted mantle at 3.0-3.2 Ga. The lead isotopic compositions of K-feldspar residues from the plutonic samples are shown in a conventional 207Pb/204Pb vs. 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 evidence of in situ growth of radiogenic Pb from U decay or to post-crystallization reequilibration. For the other analyses, although in situ growth of radiogenic Pb may be responsible some very slight scatter in the data, the preponderant trend defined by the majority of the data points clearly has a steeper slope than a 2.7 Ga isochron (Fig. 5). The steep array likely represents, 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 2.7 Ga geochron can only reflect the participation of a depleted endmember (DM) which evolved with a time-integrated 238U/ 204 Pb (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 defined for the Mulcahy mafic-ultramafic intrusion in the Wabigoon Sp (Carignan et al., 1995) as well as those determined for the late Archean depleted mantle underneath the Superior Province (Dupre´ et al., 1984; Tilton and Kwon, 1990). The most radiogenic samples can only reflect the involvement of crustal endmembers (CM) with time-integrated m values superior to that of the Bulk Silicate Earth. Most of the samples lie at intermediate positions between DM and CM (Fig. 5) suggesting, as was the case for the neodymium isotopic results, that most samples are mixtures of both depleted and enriched reservoirs. Within the single-stage model illustrated on Fig. 5, these samples lie close to a reference growth curve with a m 5 9, a value that is close to that estimated for the Bulk Silicate Earth (e.g., Galer and Goldstein, 1996). Assuming that the age of the Bulk

Fig. 6. Neodymium and lead isotopes vs. Nd concentrations. These mixing diagrams demonstrate that for pre-tectonic TTG suites having [Nd] , 30 ppm, mixing between the depleted mantle (DM) and a 3.2 Ga old felsic crust (CM) adequately models our data. This mixing is consistent with a concentration ratio of r 5 10 (see text) as shown by trend (1) in b. In the case of the Nd-rich post-tectonic plutons, there are large variations in Nd concentrations for rocks having similar neodymium or lead isotopic compositions (trends 2). This can be due to a mixing with a Nd-rich endmember (r ; 1) or to Nd enrichment resulting from processes such as partial melting, fractional crystallization, or metasomatism. But, regardless of the real process, three distinct isotopic compositions can be identified. One, labeled C1 having (eNdt, 207 Pb/204Pb ) values of (13, 14.53) in agreement with depleted mantle at 2.67 Ga. The two other crustal endmembers, labeled C2 (11.8, 14.60) and C3 (10.6, 14.68) have neodymium isotopic compositions consistent with those of metasedimentary rocks deposited in Wawa and Quetico Sp, respectively.

Silicate Earth is 4.48 Ga and that the endmember labeled CM on Fig. 5 represents the minimum lead isotopic composition of an ancient crustal reservoir derived in a simple two-stage process from a depleted mantle with U-Pb isotopic characteristics comparable to that of the DM endmember (m 5 8.5), one can estimate from the 207Pb/204Pb vs. 206Pb/204Pb systematics a minimum extraction age of ca. 3.2 Ga for that crustal endmember. This agrees with the estimate determined above from the Sm-Nd isotopes. The mixing relationships are further investigated in terms of the pre- and post-tectonic plutonic series with the aid of Fig. 6 which plots the neodymium and lead isotopic compositions of the suites as a function of their Nd concentrations. Two trends are apparent: (1) the neodymium and lead isotopic compositions of the pre-tectonic rocks with less than 30 ppm of Nd are consistent with simple, binary mixing (trend 1 on Fig. 6a,b) between a juvenile endmember, for example an oceanic crust derived from a depleted mantle, and older crustal segments formed at 3.2 Ga; (2) in contrast, the post-tectonic plutons are

Late Archean crustal growth, Ontario, Canada

characterized by increasing Nd contents with relatively constant neodymium and lead isotopic compositions. These trends branch off from the main data array (trend 2 on Figs. 6a,b) to form three parallel trends labeled C1, C2, and C3 which have eNdt: 207Pb/204Pb isotopic compositions of 13.0: 14.53, 11.8: 14.60, and 10.6: 14.68, respectively. These trends suggest the existence of parental magmas with variable initial isotopic compositions that were affected by fractionation processes, thus explaining the large ranges of Nd concentrations (30-110 ppm, Fig. 6). Conversely, the C1 to C3 compositions may also be interpreted as reflecting mixtures of juvenile components derived from a depleted mantle-like with several Nd-rich endmembers. These Nd-rich endmembers could correspond to the post-tectonic Archean sanukitoid suites whose REE-rich character is thought to be derived from an enriched or metasomatized mantle (Shirey and Hanson, 1984; Stern et al., 1989; Stern and Hanson, 1991). The 207Pb/204Pb vs. eNdt diagrams of Fig. 7 discriminate between the different possibilities outlined above. Figure 7a shows the isotopic results obtained on all pre-tectonic intrusives with superimposed mixing curves between ca. 2.73 Ga juvenile materials derived from a depleted mantle (DM) and a 3.2 Ga old crustal endmember (CM). The Nd and Pb data of the pre-tectonic TTG suites do not lie along a linear trend between the crustal and mantle reservoirs. The curvature of the mixing curves between CM and DM is controlled by the ratio r 5 CDM/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 r 51 (Fig. 7a) can be drawn to represent mixtures of crusts or sediments of juvenile and ancient origin, which have similar Nd and Pb concentrations. However, when 1-10% of a crustal component such as the Quetico sediments (CCM ; [20]/[10] 5 2; Zindler and Hart, 1986; Fralick and Purdon, 1995; this study) is directly mixed with either a depleted mantle (CDM ; [1]/[0.01] 5 100; r 5 50; Zindler and Hart, 1986) or a basaltic component (CDM ;[10]/[0.5] 5 20; r 5 10), the result is a curve where the 207Pb/204Pb ratio of the mixture rapidly approaches that of the crustal contaminant, whereas the eNdt value is lowered by only a few epsilon units. This is particularly well illustrated in Fig. 7a by the three analyses from the Indian Lake batholith (IL on Fig. 7a) which have a 207Pb/206Pb ratio of about 14.75 but the eNdt values range from 12.1 to 21.9. This is because lead isotopes are much more sensitive to crustal contamination compared to neodymium isotopes, due to the high concentration of Pb in the crust. However, the range of lead and neodymium isotopic compositions for all the TTG suites cannot be represented by a single mixing curve between the DM and CM endmembers. The distribution of the data require at least three crustal endmembers (CM, Q, and T on Fig. 7a) which produce different curves when mixed with depleted mantle-like material. The fact that the TTG suites lie along curves constructed from such mixtures could reflect the contamination of their source areas. TTG suites are considered to form in an arc environment by the subduction and melting of oceanic crust (Martin, 1987). The contamination of these suites could result from the subduction of sediments along with the oceanic crust and mixing during melting. However, the isotopic compositions of these sediments (Pb in particular)

153

Fig. 7. 207Pb/204Pb vs. eNdt. (a) Nd-Pb mixing model for pre-tectonic TTG suites. Because lead isotopes are very sensitive to crustal contamination, the data dispersion found in pre-tectonic plutons is believed to represent mixtures between depleted (DM) and different enriched reservoirs (CM, Q, and T) having distinct 207Pb/204Pb ratios represented by the mixing curves calculated with r 5 10 (see text). The endmembers Q and T represent Quetico and Timiskaming-type sediments having measured neodymium and estimated lead isotopic compositions on a sedimentary mixing line (r 5 1, see text) between CM and DM. The estimated isotopic composition of the Quetico sediments is in agreement with the eNdt and 207Pb/204Pb values of S-type granites from the Quetico Sp (Q-g). In this model Q and T represent mixtures of 50:50 and 75:25, respectively, between DM and CM. Thus, the neodymium and lead isotopic compositions of the TTG suites can be explained by the partial melting of an amphibolite with some subducted sediments (1-10%) whose the isotopic composition varies as a function of the proportion of neighbouring young:old eroded terranes. (notation as in Fig. 2). (b) Nd-Pb mixing model for the post-tectonic plutons. At 2.67 Ga, the neodymium and lead isotopes of post-tectonic plutons suggest mixing between normal depleted mantle (DM 5 C1) and crustal compositions (CM, C2, or C3) where r ; 1 (see text). The Wawa post-tectonic intrusions have, on average, more juvenile signatures than those from the Quetico and Wabigoon Sp. A mixing array where r 5 1, and the fact that C2 and C3 compositions are similar to the compositions of Timiskaming-type (T) and Quetico (Q) sediments, respectively, strongly imply that the neodymium and lead isotopic compositions of the post-tectonic suites reflect crustal contamination of the dioritic magmas during intrusion into the Wawa, Quetico or Wabigoon crusts (see discussion in text).

may change from one area to another, producing a family of mixing curves as a result of greater contributions from juvenile orogens for example. Thus, Fig. 7a shows three mixing curves between amphibolite-like material with depleted mantle compositions (207Pb/204Pb 5 14.45; eNdt 5 13) and different crustal endmembers (207Pb/204Pb 5 14.80, 14.65, and 14.55 and eNdt 5 22.0, 10.6, and 11.8, respectively; r 5 10) similar to those estimated for a ca. 3.2 Ga old

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felsic crust (CM), Quetico sediments (Q), and Timiskamingtype sediments from the Shebandowan greenstone belt (T). Conversely, the range of neodymium and lead isotope compositions of the Nd-rich (.30 ppm), post-tectonic plutonic rocks is much more limited than that of the pre-tectonic TTG. The data do not conform to any of the mixing trends defined by the TTG but rather define a linear trend (i.e., r ;1) between DM and CM (Fig. 7b) which overlaps with the neodymium and lead 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) range from compositions similar to depleted mantle (DM) at 2.67 Ga, to values estimated for the Timiskaming-type sediments from the Wawa Sp (T) and for sediments and two-mica granites from the Quetico Sp (Q), respectively. In addition, there is a clear geographical distribution, with the Archean sanukitoid rocks from the Wawa Sp yielding eNd2.67Ga values, on average, more radiogenic than those from the Quetico and the Wabigoon Sp (Fig. 7b). This geographical distribution mirrors that observed for the Timiskaming-type sediments in the Wawa Sp which have neodymium isotopic compositions more radiogenic than those of the Quetico sediments. This suggests that the neodymium isotopic compositions and the estimated lead isotopic compositions of the sediments provide reasonable estimates of the isotopic compositions of the different crustal endmembers for the mixing trends in Figs. 6 and 7 for both pre- and post-tectonic plutons. 5.2. Evolution of the Western Superior Crust and Sedimentary Basins The 207Pb/204Pb signatures of late Archean sediments used above can only be indirectly estimated from the isotopic compositions of S-type Quetico granites produced by the partial melting of the sediments. Two samples of late-tectonic, twomica granites (Nos. 36 and 38) from large intrusive bodies within the Quetico sediments, have eNdt isotopic compositions (10.4) almost identical to that of the sediments (10.6). Assuming that these granites were entirely derived from melting of intracrustal materials, one could postulate their lead isotopic composition (207Pb/204Pb ratio of ;14.65) reflects that of the crust/sediments from which they were derived. On Fig. 7a, such an endmember plots halfway between the CM and DM and would correspond to a mixture with an r value of ;1. This r value is comparable to values determined for modern marine sediments (e.g., Abouchami and Goldstein, 1995). In principle, the Sm-Nd isotopic signatures of fine clastic sediments record, overall, the mean age of their crustal source areas (e.g., Alle`gre and Rousseau, 1984; Taylor and McLennan, 1985). However, during intensive episodes of crustal growth such as that which occurred during the 2.76-2.70 Ga accretion of the Superior Province, the composition of the crust may be significantly modified and its mean age considerably reduced. This phenomenon is likely recorded in the neodymium isotopic signatures of the Quetico and the Timiskaming-type sediments, which have different mean eNdt values (10.6 and 11.8, respectively) intermediate between that of the Marmion Lake gneisses (21) and the depleted mantle (13), but similar Nd concentrations (;20 ppm) and 147Sm/144Nd ratios (;0.11) that are typical of relatively mature sediments (e.g., Jahn and Condie

1995). If this model is correct, depending on their geographical locations and on their neighbouring crustal segments, sedimentary basins would have received in various proportions detritus from pre-existing crustal segments which may be as old as 3.2 Ga and detritus from young volcanic arcs and TTG magmatism where younger juvenile terranes were initiated. In this scenario, the Quetico sediments record a contribution of ;50% from new crust and the Timiskaming-type sediments ;75% (Fig. 7a). In addition, sedimentary deposits with contrasting isotopic compositions provide suitable endmembers to explain the apparent scattered isotopic compositions of the TTG suites. This strongly suggests that the parental magmas of all pre-tectonic tonalites contained, in some proportion, sediments that were recycled back into the mantle. Thus, the Indian Lake batholith would record the largest contribution from older crustal materials whereas the Revell pluton would record higher proportions of detritus derived from juvenile crust. The Shebandowan Lake and Atikwa batholiths would essentially record the participation of juvenile, ca. 2.7 Ga old terranes, an interpretation which is in agreement with those of Edwards and Davis (1984) and Davis and Edwards (1985). The model invoking sediment subduction to explain the Nd-Pb isotopic systematics is preferred to alternative explanations calling upon intracrustal melting and/or crustal contamination of the parental TTG magmas. This is because in the case of the latter, the [Nd]/[Pb] ratios of the parental TTG magmas are expected to be quite close to those characterizing the crustal endmember which should produce mixing trends around a mean r value of 1. However, two samples, one from the Revell and one from the Indian Lake batholiths (Nos. 44 and 47), have 207 Pb/204Pb ratios comparable to other samples from the same plutons, but significantly lower eNdt values, and plot close a mixing line with r 5 1 (Fig. 7a). In this situation, crustal contamination/intracrustal melting may be responsible for their neodymium isotopic signatures. All the post-tectonic plutons exhibit isotopic compositions lying along a trend with r ;1 (Fig. 7b). This suggests that crustal contamination could be important in rocks having sanukitoid affinities. In order to reconcile their high Mg# and their LILE-enriched nature, Stern et al. (1989) suggested that Archean sanukitoid rocks were derived from the melting of a metasomatized mantle wedge above a subduction zone. In such a model, the geographical distribution observed in Fig. 7b could mean that the metasomatized mantle wedge beneath the Wawa Sp was more radiogenic than that of the Quetico and Wabigoon Sp. However in both cases, this requires that the metasomatic fluids and/or the metasomatized endmember were characterized by [Nd]/[Pb] ratios comparable to those of normal depleted mantle in order to account for a mixing trend with r values close to unity. Thus, it is unlikely that the isotopic differences between the post-tectonic intrusions reflect differences in the mantle isotopic compositions. Alternatively, the geographical differences in the Nd and Pb compositions of the post-tectonic plutons intruded in the Wawa, Quetico, and Wabigoon crusts, may be imparted to the magmas by interaction with crustal materials. These variations are clearly less marked than in the pre-tectonic TTG intrusives, because at the time of Archean sanukitoid rock formation, the different crustal segments were dominated by the presence of

Late Archean crustal growth, Ontario, Canada

juvenile components. The more juvenile isotopic compositions of Archean sanukitoid suites in the Wawa Sp may result from differences in the type of crust assimilated or differences in the initial magmas concentrations. Because dioritic to granodioritic plutons found in the Wawa Sp have Nd contents higher than those found in Quetico and Wabigoon Sp, their neodymium isotopic compositions would be less modified by crustal contamination. Another assumption could be that the plutons are less contaminated in the Wawa Sp because the younger Wawa crust might be thinner than that of the Wabigoon Sp, and thus lead to lesser assimilation. Finally, and more likely, because the isotopic compositions of the plutons mirror the differences recorded by neodymium isotopes of metasedimentary rocks from Wawa and Quetico Sp (T and Q on Fig. 7b, respectively), the differences between the Archean sanukitoid plutons from Wawa, Quetico, and Wabigoon Sp reflect the differences between the average isotopic compositions of each crust; the Wawa crust being, on average, more juvenile than those of Quetico and Wabigoon Sp. 6. CONCLUSIONS AND IMPLICATIONS FOR LATE ARCHEAN CRUSTAL GROWTH

This neodymium and lead isotopic study of crustal growth in Western Superior Craton reveals that: (1) Mafic sequences in 2.73-2.70 Ga greenstone belts of the southern portion of the Western Superior Province have isotopic compositions consistent with their formation from a depleted mantle for which the neodymium isotopic compositions correspond closely to those in the depleted mantle models of DePaolo (1980) and Jacobsen (1988). At 2.7 Ga, this depleted mantle had an eNd value of about 13 and lead isotope ratios (207Pb/204Pb of 14.3-14.5) in good agreement with previous estimates 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; Stevenson, 1995). (2) All pre- to post-tectonic sequences record the participation of enriched reservoirs mixed with depleted mantle-like materials. The types of enriched reservoirs which have interacted to form Archean crust is often controversial, ranging from crustal to enriched mantle reservoirs. For example, a negative correlation between eNdt and Pb-Pb ratios (Fig. 7) in Archean units has also been remarked upon by Vervoort et al. (1994) who, although they did not discard crustal contamination as a cause, suggested the role of an enriched source formed by an ancient differentiation event in the mantle. The mixing relationships detailed above demonstrate that the neodymium and lead isotopic compositions of such enriched reservoirs endmembers mirror those measured (Nd) and estimated (Pb) for metasedimentary rocks deposited in Quetico or in Wawa Sp. Therefore, crustal contamination processes are thought to explain better the enriched signatures of late Archean rocks. Neodymium and lead model ages also indicate that the recycled crustal materials include an older component that was isolated from the depleted mantle at ca. 3.2 Ga. (3) Lead and neodymium mixing models indicate that pretectonic tonalites (Nd , 30 ppm, 147Sm/144Nd ;0.10) with ages between 2.74 to 2.69 Ga were likely generated by partial melting of a garnet-bearing amphibolite source, as suggested by

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Martin (1987), which was contaminated by the subduction and incorporation of sediments (;1-10%). This contamination varied from place to place because the isotopic compositions of the sedimentary basins varied as a result of differences in the isotopic compositions of the eroded sources due to the erosion of 2.73 to 2.69 Ga juvenile terranes. (4) Post-tectonic magmatism, dominated by Nd(LREE)-enriched plutonic rocks with sanukitoid affinities, yielded neodymium and lead isotope data which overlap with those of the Quetico and Timiskaming-type sedimentary units. Furthermore, a geographical isotopic distribution is observed with the more mantle-like compositions found in the Wawa Sp and the more crustal-like compositions found in the Wabigoon Sp. Although these magmas are interpreted to be derived from a depleted mantle that was metasomatized before melting (Stern et al., 1989), these observations are thought to reflect the importance of crustal assimilation with concurrent fractionation in the evolution of the diorite-monzodiorite-granodiorite-granite series of the Archean sanukitoid suite. The Wawa Archean sanukitoid suites reflect the involvement of younger crust, less crust, or the same amount of crust if the initial magmas were more enriched in Nd and Pb and thus less susceptible to contamination. Geochemical and volcanological work on present-day oceanic plateaus have led to the suggestion that mantle plumes may be important sources of heat for the production and reworking of Archean crust (e.g., Storey et al., 1991; Vervoort et al., 1993; Abbott, 1996; Patchett, 1996). Mantle plumes are commonly associated with an enriched mantle reservoir and, in the Archean, komatiites are generally regarded as being of plume origin (e.g., Campbell et al., 1989). However, almost all late Archean komatiites have neodymium and lead isotopic signatures consistent with an origin from a depleted mantle (e.g., Campbell et al., 1989; Stevenson, 1995; Lahaye and Arndt, 1996). If there were enriched reservoirs associated with plume generated komatiites, they were not old enough to have generated isotopic signatures distinct from those of the depleted mantle; i.e., the enriched reservoirs were more like those hypothesized for the formation of Archean sanukitoid suites (Stern and Hanson, 1991). While plume-tectonics may have been an important catalyst in initiating intra-oceanic subduction (Boher et al., 1992; Abouchami et al., 1990), oceanic plateau-like terranes are volumetrically inferior to the volcanic arc-related suites which dominate the Western Superior Province (e.g., Thurston and Chivers, 1990; Desrochers et al., 1993). The role of plumeplateau volcanism in the creation of felsic crusts in the Archean also remains controversial in view of the strong relationship between TTG genesis and volcanic arc environments (Martin, 1987, 1994). In this respect, the isotopic data presented here are consistent with crust formation in arc environments where extensive interaction could occur with crust from preceeding volcanic cycles and/or with subducted sediments (Fig. 7). These data also indicate crustal recycling was an important process in the overall construction of the Western Superior Province, but both zircon ages and isotopic Nd and Pb tracer studies indicate that most of the crust that was recycled was not much older than 3.2 Ga; i.e., not excessively older than the crust presently exposed in the southern Western Superior Province and in agreement with the oldest 3.17 Ga U-Pb zircon ages

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found in Winnipeg-River Sp (Corfu, 1988). Stevenson and Patchett (1990) also noted that there is seldom isotopic evidence for the recycling of material much older than that which is already exposed in the Superior Craton as a whole, as well as in the North Atlantic and Kaapvaal Cratons (see also Jahn and Condie, 1995). However, this contrasts with late Proterozoic and Phanerozoic orogens where there is often evidence for recycling of much older, isotopically evolved crust which underlines the difficulty in identifying crustal recycling of relatively less evolved crust in the Archean without a detailed geochronology database. Acknowledgments—The authors gratefully thank Fernando Corfu and Don Davis of the Royal Ontario Museum for stimulating discussions, generous help in obtaining samples and for their landmark contributions to the geochronology of the Western Superior Province without which this study would not have been possible. Jean Carignan is thanked for thought-provoking discussions and help in the Pb laboratory. L. Donahue, A. Hamel, E. Malka, R. Lapointe, Y. Larbi and F. Robert were of great help in sample preparation and mass spectrometer maintenance. The final manuscript benefited from encouraging reviews by P.J. Patchett and J. Kramers. This work was funded by LITHOPROBE, NSERC and FCAR grants to Garie´py and Stevenson. LITHOPROBE contribution No. 884. Editorial handling: K. Mezger REFERENCES Abbott D. H. (1996) Plumes and hotspots as sources of greenstone belts. Lithos 37, 113–127. Abouchami W. and Goldstein S. L. (1995) A lead isotopic study of Circum-Antartic manganese nodules. Geochim. Cosmochim. Acta 59, 1809 –1820. Abouchami W., Boher M., Michard A., and Albare`de F. (1990) A major 2.1 Ga event of mafic magmatism in West Africa: An early stage of crustal accretion. J. Geophys. Res. 95, 17605–17629. Alle`gre C. J. and Rousseau D. (1984) The growth of the continent through geological time studied by neodymium isotope analysis of shales. Earth Planet. Sci. Lett. 67, 19 –34. Alle`gre C. J., Dupre´ B., and Lewin E. (1996) Three time-scales for the mantle. In Earth Processes: Reading the isotopic record. (ed. A. Basu and S. Hart); Geophys. Monogr. 95, 99 –108. Barker F. and Arth J. G. (1976) Generation of trondhjemitic-tonalitic liquids and Archean bimodal trondhjemite-basalt suites. Geology 4, 596 – 600. Blackburn C. E. et al. (1985) Evolution of Archean volcanic-sedimentary sequences of the western Wabigoon Subprovince and its margins: a review. In Evolution of Archean Supracrustal Sequences (ed.); Geol. Assoc. Canada Spec. Pap. 28, 89 –116. Blackburn C. E., Johns G., Ayer J., and Davis D. W. (1991) Wabigoon Subprovince. In Geology of Ontario (ed. ); Ont. Geol. Surv. Spec. 4, 303–381. Boher M., Abouchami W., Michard A., Albare`de F., and Arndt N. (1992) Crustal growth in West Africa. J. Geophys. Res. 97, 345–369. Campbell I. H., Griffiths R. W., and Hill R.I. (1989) Melting in the Archean mantle: heads it’s basalts, tails it’s komatiites. Nature 339, 697– 699. Card K. D. and Ciesielski A. (1986) Subdivisions of the Superior Province of the Canadian Shield. Geosci. Canada 13, 5–14. Carignan C., Garie´py C., Machado N., and Rive N. (1993) Lead isotopic geochemistry of granitoids and gneisses from the late Archean Pontiac and Abitibi Subprovinces of Canada. Chem. Geol. 106, 299 –316. Carignan C., Machado N., and Garie´py C. (1995) Initial lead isotopic composition of silicate minerals from the Mulcahy layered intrusion: Implications for the nature of the Archean mantle and the evolution of greenstone belts in the Superior Province, Canada. Geochim. Cosmochim. Acta 59, 97–105. Corfu F. (1988) Differential response of U-Pb systems in co-existing

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