Dual sources of ensimatic magmas, Hearne domain, Western Churchill Province, Nunavut, Canada: Neorchean “infant arc” processes?

Precambrian Research 134 (2004) 169–188

Dual sources of ensimatic magmas, Hearne domain, Western Churchill Province, Nunavut, Canada: Neorchean “infant arc” processes? Brian L. Cousensa,∗ , Lawrence B. Asplerb , Jeffrey R. Chiarenzellic a

c

Department of Earth Sciences, Carleton University, Ottawa, Ont., Canada K1S 5B6 b 23 Newton Street, Ottawa, Ont., Canada K1S 2S6 Department of Geology, State University of New York at Potsdam, Potsdam, NY 13676, USA Received 19 June 2003; accepted 4 June 2004

Abstract In the Henik segment of the Central Hearne supracrustal belt, Neoarchean supracrustal rocks accumulated in deep-water depositional systems that were well removed from a continental influence. Repeated in time and space, these systems consisted of mafic lava plain, slope, and apron subenvironments, with local emergent to near-emergent felsic volcanic centres. Volcanic and plutonic rocks are subalkaline, and range in composition from basalt to rhyolite. Consistent with field relationships, the volcanic rocks are juvenile and, with predominantly positive εTNd values that are independent of compositional variation, display little evidence of contamination by continental crust. However, despite primary interlayering on scales of 10s of metres, mafic and felsic rocks display markedly different rare earth element (REE) characteristics. The mafic volcanic rocks are MORB-like and display a tholeiitic magma evolution trend, flat REE patterns, and positive ε2695 Nd values, suggesting that they were derived from partial melting of depleted mantle. In marked contrast, the felsic rocks display steeply sloping REE patterns with significantly lower heavy REE abundances, negative Nb anomalies, and positive Zr and Hf anomalies suggesting derivation from partial melting of slightly older basaltic crust. This duality of magma sources supports a generalized model for the Central Hearne supracrustal belt that draws analogies from Eocene intraoceanic “infant arc” processes described from the southwestern Pacific Ocean, rather than from those normally associated with fully developed Phanerozoic subduction. © 2004 Elsevier B.V. All rights reserved. Keywords: Geochemistry; Nd isotopes; Neoarchean sedimentation and volcanism; Western Churchill Province

1. Introduction

∗ Corresponding author. Tel.: +1 613 520 3515; fax: +1 613 520 2569. E-mail address: brian [email protected] (B.L. Cousens).

0301-9268/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2004.06.001

Archean supracrustal and allied plutonic rocks are extensively exposed in the Rae and Hearne domains of the Western Churchill Province in northern Canada (Fig. 1). Relative to more accessible parts of the Cana-

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Fig. 1. Regional context of the Hearne domain, Western Churchill Province (modified after Aspler and Chiarenzelli, 1997b).

dian Shield, these rocks have not previously been examined in detail. Over the last several years however, multi-disciplinary studies of the Western Churchill NATMAP Program (e.g., Hanmer et al., 2003, and references therein) have worked toward establishing the region’s paleogeography, tectonic history, and min-

eral endowment. Based largely on reconnaissance field and geochronologic data, Aspler and Chiarenzelli (1996a) suggested that predominantly ensialic supracrustal successions in the Rae domain were deposited during extension of a continental basement block called “Nunavutia” (Schau and Ashton, 1988),

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and that this extension overlapped with ensimatic volcanism and sedimentation in the Hearne domain. In this paper, we further document the ensimatic character of the Hearne domain, complementing studies by Davis et al. (2004), Hanmer et al. (2004), and Sandeman et al. (2004). Neoarchean supracrustal rocks in the Hearne domain, historically referred to as the Ennadai–Rankin greenstone belt (Wright, 1967), extend from northern Saskatchewan to Rankin Inlet (Fig. 2). Hanmer et al. (2004) subdivide the Hearne domain into “Northwestern” and “Central” subdomains, referring to a central, northeast- and east-trending corridor of greenschistgrade supracrustal rocks and related intrusions as the “Central Hearne supracrustal belt” (Fig. 2). They further subdivide this corridor into three geographic segments (Henik, Kaminak, and Tavani, Fig. 2). The present paper focuses on the Henik segment, the southernmost of the three subdivisions. Detailed descriptions

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and maps summarizing the stratigraphy, sedimentology, physical volcanology, and structure of the Henik segment are presented elsewhere (Aspler and Chiarenzelli, 1997a; Aspler et al., 1999a, 2000). Because geochemical and isotopic data have become indispensable for addressing the origin and tectonic significance of Archean granite-greenstone terranes, here we present major and trace element analyses and Sm–Nd isotopic results. Drawing comparisons between the Neorchean evolution of the Central Hearne domain and the Eocene history of the southwestern Pacific Ocean (as described by Stern and Bloomer, 1992 and Bloomer et al., 1995), Hanmer et al. (2004) and Sandeman et al. (2004) propose that the Hearne domain bears similarities to the earliest stages of intraoceanic arc growth in the southwestern Pacific, rather than to mature Phanerozoicstyle plate tectonics. Our data indicate that the Henik segment consists of juvenile oceanic crust, and that

Fig. 2. Location of study area, generalized geology of the Hearne domain (modified after Aspler and Chiarenzelli, 1996a; Northwest and Central Hearne subdomains, and Henik, Kaminak, and Tavani segments after Hanmer et al., 2004).

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beds of MORB-like rocks (derived from depleted upper mantle) interfinger with intermediate to felsic rocks (derived from partial melting of slightly older, juvenile mafic crust). This duality of magma sources supports the infant arc model proposed by Hanmer et al. (2004) and Sandeman et al. (2004).

2. Geological setting: Henik segment, Central Hearne supracrustal belt Samples for this study were collected from between Sealhole Lake and Noomut River in the Henik segment

of the Central Hearne supracrustal belt (Figs. 2 and 3). Originally mapped as the “Henik Group” during reconnaissance work in the 1960s (Eade, 1974), Neorchean supracrustal rocks in the Henik segment were later assigned four informal units, “A1 to A4” (Aspler and Chiarenzelli, 1996a). In Archean volcano-sedimentary terranes, where subenvironments related to magmatic centres produce a complex array of interfingering facies, iron formation units may serve as marker horizons (e.g., Goodwin and Ridler, 1970; Heather et al., 1996). Fig. 3 is a stratigraphic chart that illustrates the correlation of subunits in the Henik segment using a relatively continuous (strike length >100 km) iron formation in-

Fig. 3. Sample locations with respect to generalized stratigraphy of the Henik segment, Central Hearne supracrustal belt. Based on 1:50,000-scale mapping in the Sealhole Lake (Aspler et al., 2000), Ducker Lake (Aspler et al., 1994); Montomery Lake (Aspler et al., 1992); the Henik lakes (Aspler and Chiarenzelli, 1997a) and Noomut River (Aspler et al., 1999a) areas (revised after Aspler and Chiarenzelli, 1996a). U-Pb zircon age from granitic pluton near Magnet Bay is from Davis et al. (2004).

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terval as a datum. Herein we retain the informal stratigraphic subdivisions outlined in Aspler and Chiarenzelli (1996a), although new data necessitate revisions to previous correlations (Aspler et al., 1999a, 2000). Unit A1 consists predominantly of immature sandstone and sandstone to mudstone in fining-upward sequences. It also contains bimodal mafic-felsic volcanic tongues with thickly bedded pillowed mafic flows (±gabbro sills) and local massive felsic flows 1–10 m thick. Unit A2 is a mixed sedimentary-volcanic assemblage consisting of: (1) massive, pillowed and sheetlike mafic volcanic rocks that locally contain pillow breccia and variolitic horizons, and have rare lenses of sulphide-facies iron formation; (2) felsic to intermediate volcanic rocks including massive volcanic breccias, and local interbeds of framework-intact monomictic conglomerate (with well-rounded felsic clasts) and wavy-bedded, carbonate-cemented sandstone; (3) interfingering turbiditic sandstone-mudstone sets, felsic to intermediate tuffs, mafic flows, and intraformational conglomerate; and (4) iron formation in which magnetite occurs either with turbiditic rocks (concentrated in the pelitic parts of decimetre-scale fining-upward sequences and millimetre-scale sandstone to mudstone rhythmites) or banded with hematite and varicoloured chert in entirely chemogenic beds. Unit A3 is a mafic volcanic-gabbro sill-dyke complex in which voluminous mafic volcanic flows (commonly pillowed, locally variolitic) and sparse volcaniclastic beds are cut by an extensive three dimensional network of gabbroic dykes and sills. Unit A4 consists entirely of sandstone to mudstone in fining-upward sequences. Aspler and Chiarenzelli (1996a) envisaged a deep oceanic depositional setting for these supracrustal rocks. Vertical stacking of subunits was thought to represent relatively simple progradation of an extensive mafic volcanic plain (containing local emergent to nearemergent felsic volcanic centres) over slope and basinal environments, and later burial by sediment gravity flow deposits. Because of recently appreciated stratigraphic complexities (Fig. 3; Aspler et al., 1999a), we now consider that these rocks record numerous interacting mafic lava plain—slope—apron depositional systems. These systems appear to have repeated in space and time owing to variations in the locus of volcanism (related to changes in the magma plumbing system), magma supply, sea level, and isostatic sinking of the mafic buildups. Deep-water (below storm-wave

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base) deposition, well removed from a continental influence, is indicated by: paucity of amygdaloidal or pyroclastic mafic flows, preservation of delicate lamination in pelitic and iron formation units; absence of features implying subaerial exposure, oscillatory flow or tidal currents (with the exception of the felsic-clast conglomerates and wavy-bedded sandstones in unit A2 that formed aprons around felsic volcanic centres); and lack of interfingering rocks that could represent laterally adjacent shelf, coastal or continental environments (Aspler et al., 1996a, 1999a). The supracrustal successions are perforated by isolated synteconic plutons consisting of granite, granodiorite and diorite. A small pluton west of Magnet Bay (South Henik Lake, Figs. 2 and 3) has a U-Pb zircon age of 2681 ± 1 Ma (Davis et al., 2004). Many of the facies in the Henik segment are remarkably similar to those in the Kaminak segment to the northeast (Aspler et al., 1999a; cf. Hanmer et al., 2003). Although supracrustal rocks continue between the two segments without a significant structural break, precise stratigraphic relationships remain unclear. A UPb zircon age of 2695 Ma (Mortensen and Roscoe, p. comm. 1994) from felsic volcanic rocks in the northeastern part of the Henik segment is within the time span of volcanism in the Kaminak and Tavani segments (∼2700–2680 Ma; Davis et al., 2004; Hanmer et al., 2004). In the Montgomery Lake and Sealhole Lake areas, steeply dipping Henik Group strata are unconformably overlain by the Montgomery Group, a local coarse continental siliciclastic sequence of uncertain age and tectonic significance (Aspler and Chiarenzelli, 1996b; Aspler et al., 2000; Rainbird et al., 2002). Regionally, the Henik Group is unconformably overlain by Paleoproterozoic (<2.45 to <1.91 Ga) continental and marine intracratonic basin deposits of the Hurwitz Group (Aspler et al., 2001) and continental deposits of the Kiyuk Group (<1.90 to >1.82 Ga, Aspler et al., 2002).

3. Major, trace, and rare earth element geochemistry, and Sm–Nd isotopic compositions 3.1. Analytical methods Only fresh, homogeneous, unaltered, and vein-free hand specimens were collected for analysis. All sam-

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Table 1 Major and trace element abundances Sample Area UTM easting UTM northing Type

91-48-10 Montg. 573400 6817670 Flow

98-7-4 Noomut 620300 6846300 Dyke

98-10-20 Noomut 623900 6831800 Flow

98-14-1 Noomut 602600 6844000 Flow

SiO2 wt.% TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total CO2 H2 O Nb (ppm) Zr Y Sr Rb Ba Cr Co Cu Ni Sc V Zn La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th U Hf Ta Nb ICPMS Pb

51.96 1.15 14.77 10.38 0.20 5.57 8.54 3.58 0.19 0.08 3.60 100.03 1.07 2.53 5 64 20 144 2 318 329 52 146 129 41 306 76 6.00 10.70 1.94 9.45 2.80 0.84 3.60 0.60 3.94 0.89 2.48 0.38 2.43 0.37 0.28 0.08 1.88 0.44 3.11 0.44

62.94 0.39 15.88 2.97 0.04 1.68 4.17 4.47 1.84 0.11 5.20 99.69 2.81 2.39 6 106 7 235 58 562 43 6 33 28 6 48 55 15.34 30.72 3.74 13.92 2.58 0.71 1.76 0.21 0.99 0.19 0.49 0.07 0.40 0.06 2.72 0.83 2.82 0.47 3.60 7.09

48.53 1.08 14.34 13.15 0.23 7.56 11.42 2.06 0.05 0.09 3.50 102.01 0.17 3.33 5 72 22 125 bdl 30 390 53 82 132 43 298 91 3.79 10.62 1.73 8.40 2.93 1.00 3.50 0.63 4.07 0.91 2.60 0.39 2.58 0.38 0.36 0.10 2.11 0.45 3.29 0.58

64.25 0.45 16.23 3.12 0.05 1.54 3.24 3.82 2.22 0.12 3.40 98.43 1.72 1.68 9 173 12 270 44 469 19 12 28 26 7 57 84 18.83 37.12 4.29 15.65 2.86 0.83 2.26 0.30 1.65 0.34 0.84 0.12 0.83 0.13 3.00 0.76 4.41 0.80 6.34 5.64

98-15-15 Noomut 592500 6846900 Flow 48.88 1.04 14.08 13.49 0.21 7.33 10.90 2.24 0.13 0.09 1.20 99.57 0.11 1.09 4 65 18 95 bdl bdl 167 49 78 85 39 285 94 3.63 10.28 1.64 8.35 2.66 0.87 3.34 0.58 3.79 0.86 2.52 0.36 2.38 0.37 0.36 0.09 1.94 0.44 3.35 0.91

98-19-10 Noomut 594400 6846550 Flow 49.57 0.98 14.80 12.75 0.21 5.84 10.99 3.04 0.23 0.08 1.10 99.59 0.14 0.96 4 61 19 158 1 67 190 50 268 99 41 288 105 4.04 10.29 1.67 7.98 2.53 0.93 3.22 0.56 3.73 0.83 2.41 0.36 2.26 0.37 0.32 0.10 1.85 0.41 2.98 1.72

98-19-6 Noomut 600700 6846350 Flow 60.02 0.63 15.10 5.46 0.09 4.34 5.90 5.36 0.90 0.25 1.00 99.05 0.14 0.86 8 146 12 899 16 347 143 20 6 77 13 103 86 29.69 70.19 9.38 36.96 5.92 1.60 3.58 0.42 2.01 0.38 1.01 0.14 0.83 0.13 2.75 0.65 3.32 0.49 6.32 6.91

98-20-6 Noomut 621600 6844500 Flow 72.62 0.31 12.53 1.54 0.04 0.67 1.75 5.60 0.71 0.10 2.50 98.36 1.83 0.67 5 149 9 195 22 91 10 4 18 8 4 32 37 11.32 22.76 2.67 9.64 1.68 0.48 1.23 0.18 0.97 0.18 0.46 0.06 0.43 0.07 1.73 0.56 3.83 0.55 3.79 8.91

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Table 1 (Continued ) Sample Area UTM easting UTM northing Type

99-4-30b Sealhole 523200 6742800 Flow

99-4-32a Sealhole 523000 6742500 Flow

99-4-32b Sealhole 523000 6742500 Flow

99-30-1 Sealhole 526121 6743947 Flow

SiO2 (wt.%) TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total CO2 H2 O Nb (ppm) Zr Y Sr Rb Ba Cr Co Cu Ni Sc V Zn La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th U Hf Ta Nb (ICP) Pb

70.58 0.26 14.62 2.11 0.03 0.77 3.01 5.25 0.76 0.07 1.10 98.57 0.57 0.53 3 122 6 202 37 341 19 5 7 7 3 34 30 17.19 30.72 3.29 11.16 1.85 0.42 1.18 0.14 0.71 0.14 0.36 0.05 0.27 0.05 5.80 1.29 3.13 0.55 3.40 3.14

45.66 0.78 16.69 13.09 0.24 9.43 8.32 2.55 0.13 0.05 3.20 100.14 0.10 3.10 3 44 14 155 <0.0 <10 384 53 34 176 46 278 124 2.73 7.13 1.10 5.45 1.69 0.58 2.26 0.41 2.82 0.65 1.80 0.27 1.75 0.28 0.20 0.05 1.36 0.36 2.01 6.67

72.37 0.24 14.54 1.54 0.02 0.83 1.34 5.89 1.19 0.07 0.60 98.63 0.06 0.54 4 127 7 129 48 373 11 bdl 8 4 2 24 14 10.80 23.50 2.81 10.10 1.81 0.44 1.27 0.15 0.72 0.14 0.34 0.04 0.26 0.04 8.70 2.74 3.61 0.66 3.79 4.85

66.25 0.31 14.66 2.51 0.10 0.67 5.12 4.15 2.03 0.08 2.70 98.57 2.21 0.49 4 114 6 126 42 590 20 7 15 11 4 44 29 10.57 19.65 2.30 8.42 1.51 0.40 1.05 0.14 0.66 0.13 0.34 0.05 0.30 0.05 1.63 0.68 3.09 0.47 2.58 3.57

91-38-4a Henik 563800 6820500 Gabbro 44.21 0.45 13.45 11.48 0.17 13.14 6.30 3.11 0.01 0.08 6.00 98.40 1.44 4.56 2 43 11 337 5 80 677 68 40 447 17 125 90 6.80 15.54 2.17 9.29 1.93 0.69 1.73 0.26 1.47 0.31 0.89 0.13 0.85 0.13 0.64 0.14 0.90 0.09 1.30

91-46-17 Henik 558300 6821350 Dyke 55.95 0.68 15.02 6.04 0.09 3.45 6.56 3.38 1.47 0.19 6.49 99.32 3.69 2.80 5 131 13 557 26 718 93 19 50 66 10 105 82 25.61 56.67 7.31 27.99 4.96 1.36 3.57 0.46 2.18 0.40 1.08 0.15 0.96 0.15 2.74 0.63 3.00 0.21 3.00

93-52-14a Henik 561600 6824500 Flow 64.17 0.45 16.18 4.03 0.06 3.00 4.88 2.78 1.14 0.12 2.00 98.81 0.11 1.89 6 141 14 184 25 237 63 20 18 54 11 81 27 12.78 24.96 2.94 11.14 2.30 0.85 2.26 0.34 2.08 0.44 1.23 0.18 1.15 0.18 1.81 0.44 3.47 0.61 4.68 2.35

96-22-1 Henik 573600 6826800 Flow 53.36 0.70 13.93 7.69 0.09 5.36 5.57 5.00 0.33 0.31 7.00 99.35 4.63 2.37 6 129 16 696 7 186 152 31 32 66 18 137 79 31.08 67.92 9.30 38.69 7.66 1.81 5.45 0.65 3.20 0.57 1.39 0.19 1.18 0.18 3.78 0.86 3.08 0.49 4.83 9.00

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Table 1 (Continued ) Sample Area UTM easting UTM northing Type

99-4-30b Sealhole 523200 6742800 Flow

99-4-32a Sealhole 523000 6742500 Flow

99-4-32b Sealhole 523000 6742500 Flow

99-30-1 Sealhole 526121 6743947 Flow

SiO2 wt.% TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total CO2 H2 O Nb (ppm) Zr Y Sr Rb Ba Cr Co Cu Ni Sc V Zn La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th U Hf Ta Nb (ICP) Pb

70.58 0.26 14.62 2.11 0.03 0.77 3.01 5.25 0.76 0.07 1.10 98.57 0.57 0.53 3 122 6 202 37 341 19 5 7 7 3 34 30 17.19 30.72 3.29 11.16 1.85 0.42 1.18 0.14 0.71 0.14 0.36 0.05 0.27 0.05 5.80 1.29 3.13 0.55 3.40 3.14

45.66 0.78 16.69 13.09 0.24 9.43 8.32 2.55 0.13 0.05 3.20 100.14 0.10 3.10 3 44 14 155 bdl bdl 384 53 34 176 46 278 124 2.73 7.13 1.10 5.45 1.69 0.58 2.26 0.41 2.82 0.65 1.80 0.27 1.75 0.28 0.20 0.05 1.36 0.36 2.01 6.67

72.37 0.24 14.54 1.54 0.02 0.83 1.34 5.89 1.19 0.07 0.60 98.63 0.06 0.54 4 127 7 129 48 373 11 bdl 8 4 2 24 14 10.80 23.50 2.81 10.10 1.81 0.44 1.27 0.15 0.72 0.14 0.34 0.04 0.26 0.04 8.70 2.74 3.61 0.66 3.79 4.85

66.25 0.31 14.66 2.51 0.10 0.67 5.12 4.15 2.03 0.08 2.70 98.57 2.21 0.49 4 114 6 126 42 590 20 7 15 11 4 44 29 10.57 19.65 2.30 8.42 1.51 0.40 1.05 0.14 0.66 0.13 0.34 0.05 0.30 0.05 1.63 0.68 3.09 0.47 2.58 3.57

91-38-4a Henik 563800 6820500 Gabbro 44.21 0.45 13.45 11.48 0.17 13.14 6.30 3.11 0.01 0.08 6.00 98.40 1.44 4.56 2 43 11 337 5 80 677 68 40 447 17 125 90 6.80 15.54 2.17 9.29 1.93 0.69 1.73 0.26 1.47 0.31 0.89 0.13 0.85 0.13 0.64 0.14 0.90 0.09 1.30

91-46-17 Henik 558300 6821350 Dyke 55.95 0.68 15.02 6.04 0.09 3.45 6.56 3.38 1.47 0.19 6.49 99.32 3.69 2.80 5 131 13 557 26 718 93 19 50 66 10 105 82 25.61 56.67 7.31 27.99 4.96 1.36 3.57 0.46 2.18 0.40 1.08 0.15 0.96 0.15 2.74 0.63 3.00 0.21 3.00

93-52-14a Henik 561600 6824500 Flow 64.17 0.45 16.18 4.03 0.06 3.00 4.88 2.78 1.14 0.12 2.00 98.81 0.11 1.89 6 141 14 184 25 237 63 20 18 54 11 81 27 12.78 24.96 2.94 11.14 2.30 0.85 2.26 0.34 2.08 0.44 1.23 0.18 1.15 0.18 1.81 0.44 3.47 0.61 4.68 2.35

96-22-1 Henik 573600 6826800 Flow 53.36 0.70 13.93 7.69 0.09 5.36 5.57 5.00 0.33 0.31 7.00 99.35 4.63 2.37 6 129 16 696 7 186 152 31 32 66 18 137 79 31.08 67.92 9.30 38.69 7.66 1.81 5.45 0.65 3.20 0.57 1.39 0.19 1.18 0.18 3.78 0.86 3.08 0.49 4.83 9.00

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Table 1 (Continued ) Sample Area UTM easting UTM northing Type

96-24-12 Henik 554900 6827500 Flow

96-26-15 Henik 559200 6824650 Flow

96-26-24 Henik 556850 6821750 Gabbro

96-26-6 Henik 557850 6824500 Flow

SiO2 (wt.%) TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P 2 O5 LOI Total CO2 H2 O Nb (ppm) Zr Y Sr Rb Ba Cr Co Cu Ni Sc V Zn La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th U Hf Ta Nb (ICP) Pb

55.71 0.84 18.78 7.51 0.08 4.58 1.36 5.23 1.49 0.22 3.70 99.49 0.50 3.20 9 168 16 246 37 611 70 25 19 60 16 139 89 22.82 51.65 6.70 25.88 4.74 1.13 3.37 0.47 2.72 0.56 1.51 0.23 1.39 0.21 3.84 0.80 4.36 0.64 5.69 3.57

60.86 0.51 18.48 3.87 0.09 2.14 2.48 4.36 2.17 0.11 4.00 99.05 1.41 2.59 6 123 9 105 54 401 128 18 55 49 14 104 48 11.20 22.06 2.64 9.64 2.04 0.78 1.63 0.24 1.25 0.26 0.70 0.10 0.56 0.10 1.58 0.25 3.18 0.60 4.52 1.07

51.83 0.78 13.26 8.88 0.14 8.57 7.18 3.08 2.21 0.26 3.00 99.18 0.53 2.47 6 113 12 564 59 704 456 39 49 225 21 165 79 20.71 45.41 6.17 25.23 4.88 1.33 3.58 0.48 2.50 0.50 1.35 0.19 1.15 0.19 2.28 0.51 2.67 0.53 5.24 6.00

50.60 1.03 14.14 9.94 0.18 5.98 6.08 3.20 0.10 0.08 8.60 99.92 4.65 3.95 4 61 20 65 bdl bdl 258 44 86 95 38 280 89 3.43 8.86 1.40 7.23 2.35 0.85 2.92 0.54 3.47 0.78 2.10 0.33 2.06 0.32 0.28 0.07 1.81 0.42 2.92 0.76

96-11-4a Henik 567300 6830800 Granitic pluton 67.78 0.25 15.68 1.92 0.03 1.41 1.77 4.51 2.91 0.04 2.68 98.98 1.30 1.38 4 80 6 390 83 1247 16

10 2 20 49 20.00 41.00 4.70 16.00 2.30 0.90 1.50 0.15 0.60 0.08 0.26 0.02 0.29 0.04 3.30 0.70 1.10 0.20 2.40

96-19-5 Henik 569300 6830000 Granitic pluton 60.62 0.66 15.57 5.41 0.06 3.77 5.58 4.17 1.88 0.19 0.98 98.89 0.11 0.87 5 126 11 771 60 532 118 20 40 88 9 94 72 38.00 82.00 10.00 39.00 6.40 1.70 4.60 0.53 2.30 0.36 0.99 0.11 0.92 0.13 6.30 1.50 1.70 0.43 6.90

96-27-4 Henik 566100 6826800 Granitic pluton 66.41 0.44 15.71 3.58 0.04 2.04 3.62 4.54 2.32 0.11 1.21 100.02 0.14 1.07 6 132 10 573 73 596 53 10 20 40 5 54 53 31.00 66.00 8.10 31.00 4.80 1.20 3.40 0.39 1.70 0.27 0.75 0.08 0.72 0.11 7.80 1.00 1.70 1.10 6.90

96-27-25 Henik 568800 6829600 Granitic pluton 62.04 0.57 15.97 4.72 0.05 3.04 4.45 4.39 2.17 0.15 1.48 99.03 0.24 1.24 5 152 9 715 72 635 90 10 40 65 7 77 62 39.00 84.00 10.00 38.00 6.10 1.60 4.20 0.49 2.00 0.34 0.91 0.10 0.85 0.12 6.80 1.40 1.60 0.38 6.90

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Table 1 (Continued ) Sample Area UTM easting UTM northing Type SiO2 wt.% TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total CO2 H2 O Nb (ppm) Zr Y Sr Rb Ba Cr Co Cu Ni Sc V Zn La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th U Hf Ta Nb (ICP) Pb

98-6-12a Noomut 612400 6824100 Granitic pluton 61.46 0.50 16.17 4.68 0.07 2.94 4.32 4.62 2.69 0.30 0.80 98.58 0.17 0.63 9 231 18 985 112 1711 98 15 43 56 8 82 69 66.32 128.09 15.44 55.51 8.50 1.88 4.72 0.53 2.48 0.45 1.07 0.15 0.92 0.14 8.62 1.12 4.71 0.64 7.82 18.15

98-16-16 Noomut 589670 6847900 Granitic pluton 44.16 1.20 7.07 11.72 0.17 13.88 14.95 1.00 0.46 0.92 3.30 98.82 1.64 1.66 9 114 24 687 1 122 505 56 9 227 38 238 95 45.08 135.15 22.90 >100.00 21.00 >5.00 13.42 1.46 5.94 0.99 2.13 0.26 1.52 0.21 0.76 0.14 3.39 0.56 9.54 2.49

98-19-8 Noomut 597950 6843850 Granitic pluton

99-19-5 Sealhole 507650 6739700 Granitic pluton

64.44 0.39 15.57 3.43 0.05 1.85 3.85 4.50 2.20 0.12 2.30 98.70 1.16 1.14 6 125 9 482 69 681 47 11 22 32 6 49 53 23.22 43.48 4.94 17.49 2.77 0.82 1.81 0.22 1.14 0.22 0.57 0.08 0.48 0.08 5.37 1.78 3.13 0.54 4.10 8.66

63.51 0.44 15.80 4.47 0.08 3.04 2.69 4.80 2.04 0.16 1.80 98.82 0.22 1.58 7 159 10 658 77 468 73 14 6 44 9 75 87 28.62 47.99 5.35 17.90 2.87 0.80 2.02 0.27 1.31 0.26 0.71 0.11 0.69 0.11 4.80 1.97 4.02 0.46 4.17 8.37

99-20-1 Sealhole 525650 6740450 Granitic pluton 71.65 0.19 14.93 1.27 0.02 0.69 1.21 5.00 3.09 0.06 0.40 98.52 0.10 0.30 4 91 6 393 89 728 15 1 6 9 3 19 29 3.96 14.06 1.13 4.32 1.00 0.33 0.86 0.12 0.70 0.13 0.36 0.05 0.40 0.06 3.76 0.91 2.79 0.61 3.61 14.04

Precision (%)

0.7 2.7 3.4 1.3 6.5 0.8 1.3 7.4 6.5 14.2 5.8 12.0 22.0 9.5 1.6 4.5 3.4 9.0 7.0 7.0 3.9 16.5 5.3 1.0 1.8 7.5 4.8 3.9 5.6 3.0 5.2 11.8 10.3 9.3 5.6 1.8 6.7 6.1 6.1 8.9 4.9 1.0 4.5 5.0 3.0 5.0

Notes: Precision is average percent difference between duplicate analyses. Major elements in weight percent oxides, trace elements in weight parts per million. See text for analytical details. Bdl: below detection limit. Blank: no data.

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ples were cut into thin slabs, from which any weathered rims were trimmed and discarded. The remaining material was wrapped in plastic and broken into 1 cm chips with a rock hammer. The chips were further reduced to granule size in a Bico jaw crusher, then ground to a fine powder in an agate ring mill. An aliquot of each ground sample was retained for analysis of Nd and Sm concentration and 143 Nd/144 Nd isotopic composition. The remainder of the powder was sent to the University of Ottawa X-Ray Spectrometry Facility for major and trace element analysis, then to the Ontario Geological Survey (OGS) Geochemical Laboratories for further trace element analysis. S and CO2 were determined by infrared combustion (LECO furnace) at the OGS lab. Major element oxides, Nb, Zr, Y, Sr, Rb, Ba, Cr, Co, Cu, Ni, Sc, V, and Zn were analyzed by fused-disc X-ray fluorescence (XRF) spectrometry. Rare earth element (REE), Ta, Th, U, Hf and Nb determinations were by acid-dissolution ICP-mass spectrometry at the OGS lab. The precision of analyses (Table 1 ) is based on analyses of blind duplicates. All Sm–Nd isotopic analyses were performed at Carleton University, Ottawa (for procedures, see Cousens, 1996). Whole-rock powders were spiked with a 148 Nd–149 Sm mixture prior to dissolution. The uncertainties in Sm and Nd concentrations are ±1–2%, but 147 Sm/144 Nd ratios are reproducible to better than 1%. Eighty-three runs of the La Jolla standard aver-

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aged 143 Nd/144 Nd = 0.511876 ± 18 (1␴, September 1992–March 2001). Epsilon Nd (εTNd ) values (DePaolo and Wasserburg, 1976) were calculated relative to a modern Chondrite Uniform Reservoir (CHUR) value of 0.512638 and 147 Sm/144 Nd = 0.1967, using measured and estimated ages. The precision of the εTNd values are ±0.8 epsilon units, based on duplicate analyses of geochemical standards and other rock samples. Depleted mantle model (TDM ) ages were calculated assuming a 147 Sm/144 Nd of 0.2140 and 143 Nd/144 Nd of 0.513151 for modern depleted mantle (see Faure, 1986). 3.2. Results 3.2.1. Major and trace elements Volcanic and plutonic rocks in the Henik segment are subalkaline, and range in composition from basalt to rhyolite (Table 1; Fig. 4A). In the Noomut River and Sealhole Lake areas, most of the mafic rocks are basalts, whereas in the South Henik Lake area, mafic rocks include andesites. In addition, felsic volcanic rocks in the Noomut and Sealhole areas include dacites and rhyolites, but those at South Henik Lake consist exclusively of dacite. The range of compositions from the Henik segment corresponds to those in the Kaminak and Tavani segments (cf. Sandeman et al., 2004). The mafic to intermediate rocks follow a tholeiitic magma evolution

Fig. 4. (A) Neoarchean volcanic and plutonic rocks from the Henik segment plotted on alkalis–silica classification diagram of le Bas et al. (1986). Kaminak segment field from Sandeman et al. (2004). (B) TiO2 vs. FeOt /MgO plot. Kaminak segment field from Sandeman et al. (2004), Angikuni Lake field from Aspler et al. (1999b).

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trend of increasing TiO2 with increasing FeOt /MgO or SiO2 (Fig. 4B). The Kaminak and Tavani segments include both tholeiitic and calc-alkaline rocks, although tholeiitic rocks predominate (Sandeman et al., 2004). Henik mafic rocks have Cr (750–150 ppm) and Ni (500–75 ppm) abundances that are generally too low to represent primary melts of the mantle, and have likely

undergone some fractional crystallization. Compatible elements such as Ni and V display an inverse relationship with SiO2 from basalt to rhyolite, consistent with fractionation of phases such as olivine, clinopyroxene, and Fe–Ti oxides (Fig. 5), although TiO2 abundances increase from basalt through andesite (Fig. 4). Similarly, with the exception of Th, which does not vary

Fig. 5. Variation of incompatible (Zr, Ce, Y), compatible (Ni, V) elements, and La/Sm (normalized to Primitive Mantle) with SiO2 .

B.L. Cousens et al. / Precambrian Research 134 (2004) 169–188

systematically with SiO2 , incompatible trace elements such as Zr, Ce and Dy decrease with increasing SiO2 , at values of SiO2 greater than 55% (Fig. 5). Such decreases are inconsistent with simple fractional crystallization from a basaltic parent. Also inconsistent with significant fractional crystallization, the La/Sm ratio increases by a factor of six from basalt to rhyolite (Fig. 5). Basalts and basaltic andesites display different magma evolution trends compared to andesites, dacites and rhyolites, as indicated by changes in chemical characteristics at ∼55% SiO2 (Fig. 5). 3.2.2. Rare earth elements Incompatible element patterns for the volcanic rocks, normalized to Primitive Mantle (Sun and McDonough, 1989), are presented in Fig. 6. The basaltic rocks (Fig. 6A) have uniformly parallel, flat patterns typical of mafic magmas derived by melting of depleted upper mantle, with the exception of a gabbro from South Henik Lake. This anomalous gabbro has a pattern comparable to those displayed by the basaltic andesites and andesites (Fig. 6B). The intermediate rocks are light rare earth element-enriched (La/Smpmn = 2.0–4.0; pmn: normalized to Primitive Mantle), and generally display negative Nb and Ti anomalies and heavy REE depletion (3.0–1.8 times Primitive Mantle) relative to the basaltic rocks (Fig. 6B). The dacites and rhyolites (Fig. 6C) have lower abundances of all the incompatible elements compared to the intermediate rocks. Neither the felsic nor the intermediate rocks have negative Eu anomalies, indicating that they did not crystallize significant amounts of plagioclase feldspar during their evolution. Also similar to the intermediate rocks, the felsic rocks have negative Nb and Ti anomalies. However, unlike the intermediate rocks, the felsic samples display large positive Zr and Hf anomalies, and at SiO2 >55%, the size of the Zr anomaly increases with increasing SiO2 (Fig. 6C, upper inset). In addition, the felsic rocks are generally even more depleted in the heavy REE than the intermediate rocks: Yb abundances range from 2.5 to 0.5 times Primitive Mantle, and heavy REE abundances decrease with increasing SiO2 content (Fig. 6C, lower inset). Basaltic rocks from the Henik segment have incompatible element patterns identical to mafic Group 1 rocks (the most common mafic to intermediate rock type) from the Kaminak and Tavani segments, basaltic

181

andesite to andesite patterns resemble mafic Group 2, and dacite to rhyolite patterns range from felsic Type-1 to Type-3 (cf. Sandeman et al., 2004). A wider variety of rock types have been distinguished in the Kaminak and Tavani segments relative to the Henik sement, possibly due to more intensive sampling. Granitic rocks are medium-K, and become increasingly peraluminous with increasing SiO2 content. This is particularly well-displayed by the small pluton west of Magnet Bay (South Henik Lake, Figs. 2 and 3), where abundances of most incompatible elements decrease with increasing SiO2 (Fig. 7B ). The REE patterns of the plutonic rocks are similar to those displayed by the felsic volcanic rocks (compare Fig. 6C and Fig. 7). 3.2.3. Sm–Nd isotopes With the exception of an andesite from South Henik Lake (ε2695 Nd = 0.0) all volcanic rocks from the Noomut River, South Henik Lake, and Montgomery Lake areas have ε2695 Nd between +2 and +4 (Table 2; Figs. 7 and 8). Volcanic rocks from Sealhole Lake have ε2695 Nd values that are consistently lower at any SiO2 content, including a rhyolite with a negative value (ε2695 Nd −2.4). Nd isotopic values are not correlated with SiO2 (Fig. 8), indicating that compositional variation of the volcanic rocks is not related to contamination by continental crust. Granitic samples from the Noomut River and Henik areas display ε2681 Nd values (+2.2 to +3.3) in the same range as those from adjacent volcanic rocks, and are positively correlated with SiO2 (Figs. 7 and 8), ruling out crustal contamination. The one analyzed plutonic rocks from Sealhole Lake has a slightly lower ε2681 Nd value than other plutonic rocks from the study area, but is still positive.

4. Discussion 4.1. Sources of volcanic rocks in Henik segment We interpret that mafic rocks in the Henik segment were derived by partial melting of Archean depleted upper mantle because they display: (1) a tholeiitic magma evolution trend; (2) parallel, flat rare earth element patterns that are MORB-like and lack a “subduction zone”

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Fig. 6. Incompatible elements, normalized to Primitive Mantle (Sun and McDonough, 1989), for volcanic rocks and post-volcanic intrusions of the Henik segment. Upper inset: Zr* vs. SiO2 , where Zr* is the degree of Zr enrichment over “expected” Zr abundance by extrapolating between Sm and Eu (measured Zr/([Sm + Eu]/2) from the normalized plot. Lower inset: Yb vs. SiO2 .

geochemical signal (such as depletion in Nb relative to La and Th, enrichment in Ba and Th); and (3) ε2695 Nd values that are comparable to those (+2.5 to +3.0) assumed for depleted mantle at 2.7 Ga (Machado et al., 1986; Davidson, 1987).

Isotopically, the felsic rocks are similar to the basalts, indicating that both were derived from sources with a time-integrated Sm/Nd greater than chondritic or Bulk Earth values. However, despite primary interlayering on the scale of 10s of metres, the two have

B.L. Cousens et al. / Precambrian Research 134 (2004) 169–188

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Table 2 Nd isotopic compositions Sample

Area

Type

Age (Ma) Nd (ppm) Sm (ppm)

143 Nd/144 Nd

91-38-4a 91-46-17 91-48-10 93-52-14a 96-19-17c 96-22-1 96-24-12 96-26-15 96-26-24 96-26-6 96-11-4a

Henik Montgomery Montgomery Henik Henik Henik Henik Henik Henik Henik Henik

2695 2695 2695 2695 2695 2695 2695 2695 2695 2695 2681

10.42 26.13 9.08 12.59 6.37 34.95 18.22 8.46 29.23 5.73 13.99

2.14 4.58 2.77 2.62 1.29 7.06 3.52 1.68 5.78 1.77 1.83

96-19-5

Henik

2681

40.36

96-27-25

Henik

2681

96-27-4

Henik

98-10-20 98-14-1 98-15-15 98-16-16

Noomut Noomut Noomut Noomut

98-19-10 98-19-6 98-19-8

Noomut Noomut Noomut

98-20-6 98-5-14 98-6-12a

Noomut Noomut Noomut

98-6-2 98-6-9a 98-6-9b 98-7-4 99-19-5

Noomut Noomut Noomut Noomut Sealhole

99-2-5 99-4-27 99-4-29 99-4-30a 99-4-30b 99-4-32a 99-4-32b 99-30-1

Sealhole Sealhole Sealhole Sealhole Sealhole Sealhole Sealhole Sealhole

Gabbro Dyke Flow Flow Flow Flow Flow Flow Gabbro Flow Granitic pluton Granitic pluton Granitic pluton Granitic pluton Flow Flow Flow Granitic pluton Flow Flow Granitic pluton Flow Gabbro Granitic pluton Flow Flow Flow Dyke Granitic pluton Flow Flow Flow Flow Flow Flow Flow Flow

εTNd

147 Sm/144 Nd

143 Nd/144 Nd

0.511462 0.511191 0.512618 0.511487 0.511499 0.511465 0.511217 0.511398 0.511401 0.512656 0.510723

0.1243 0.1059 0.1848 0.1260 0.1225 0.1221 0.1168 0.1201 0.1195 0.1867 0.0790

0.509248 0.509305 0.509332 0.509246 0.509322 0.509294 0.509140 0.509262 0.509276 0.509337 0.509327

2.2 3.4 3.8 2.1 3.6 3.0 0.0 2.4 2.7 3.9 3.3

2792 2697 2582 2804 2675 2721 2957 2771 2749 2545 2685

6.57

0.511018

0.0984

0.509277

2.3

2749

38.88

6.13

0.510975

0.0954

0.509288

2.5

2734

2681

41.52

6.67

0.511026

0.0972

0.509308

2.9

2710

2695 2695 2695 2695

7.56 14.10 7.63 97.33

2.47 2.54 2.42 19.27

0.512742 0.511236 0.512642 0.511416

0.1974 0.1091 0.1917 0.1197

0.509232 0.509296 0.509233 0.509287

1.8 3.1 1.8 2.9

3393 2714 3210 2731

2695 2695 2681

7.35 39.55 17.97

2.33 6.58 2.82

0.512685 0.511060 0.510959

0.1921 0.1007 0.0949

0.509269 0.509270 0.509280

2.5 2.5 2.4

2971 2748 2744

2695 2695 2681

9.53 4.41 51.34

1.70 1.48 8.38

0.511182 0.512883 0.511019

0.1081 0.2024 0.0987

0.509260 0.509283 0.509273

2.4 2.8 2.3

2766 3037 2754

2695 2695 2695 2695 2695

9.98 7.04 9.85 13.22 19.60

3.05 2.29 1.51 2.28 3.15

0.512627 0.512819 0.511013 0.511201 0.510917

0.1848 0.1969 0.0928 0.1042 0.0972

0.509341 0.509318 0.509363 0.509348 0.509189

4.0 3.5 4.4 4.1 1.0

2533 2626 2629 2642 2851

2695 2695 2695 2695 2695 2695 2695 2695

5.78 9.83 5.29 7.08 10.74 5.16 8.33 8.58

1.93 2.59 1.71 2.36 1.78 1.65 1.59 1.57

0.512810 0.512010 0.512785 0.512729 0.510929 0.512668 0.511070 0.511130

0.2017 0.1594 0.1957 0.2015 0.1005 0.1933 0.1153 0.1106

0.509225 0.509176 0.509305 0.509146 0.509143 0.509231 0.509019 0.509164

1.7 0.7 3.2 0.1 0.0 1.8 −2.4 0.5

3728 3063 2735 4662 2916 3263 3137 2907

m

i

T(DM) **

Note: 143 Nd/144 Ndm: measured present-day ratio, 143 Nd/144 Ndi: initial ratio assuming crystallization age T. *Epsilon Nd value: εTNd = (((143 Nd/144 Ndi )/(143 Nd/144 NdCHUR )) − 1) × 10,000; CHUR: chondritic meteorites at time T. T (DM) **: depleted mantle model age, assuming depleted mantle 147 Sm/144 Nd = 0.214 and present-day 143 Nd/144 Nd = 0.513115.

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Fig. 7. Incompatible elements, normalized to Primitive Mantle (Sun and McDonough, 1989), for granitic rocks of the Henik segment.

markedly different trace element characteristics. The felsic rocks display steeply sloping REE patterns with significantly lower heavy REE abundances, negative Nb anomalies, and positive Zr and Hf anomalies. These traits cannot result from fractional crystallization of olivine, plagioclase, and clinopyroxene from a basaltic parent. Such fractional crystallization would have produced felsic rocks with higher overall REE abundances than in the parental basalts, and negative Eu anoma-

lies resulting from feldspar fractionation (see Perfit et al., 1983; Green, 1994). Conceivably, fractionation of apatite (with a strong affinity for the REE) could be responsible for the lower total REE in the felsic rocks, but this seems unlikely because distribution coefficients for the light and heavy REE in apatite are very similar (Green, 1994), thus apatite fractionation would fail to account for the steeply sloping REE patterns in the Henik felsic samples.

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Fig. 8. Summary of ε2695 Nd vs. SiO2 for Henik segment rocks. Kaminak segment field from Sandeman et al. (2004).

These trace element differences can be reconciled with the isotopic similarities if the dacites and rhyolites are partial melts of juvenile, garnet- or amphibolebearing mafic crust. First, partial melting of mafic crust is capable of producing felsic rocks; partial melts of amphibolite at temperatures and pressures in the lower crust (Beard and Lofgren, 1991; Rushmer, 1991; Tepper et al., 1993) or of eclogite at lower lithospheric depths (Yaxley and Green, 1998) for example, are siliceous compared to melts of mantle peridotite. Second, residual garnet in such sources retain heavy REE, and thus impart steeply sloping REE patterns and low heavy REE abundances to the melt, such as exhibited by the Henik felsic rocks. Third, studies that model the generation of modern dacites from partial melting of metabasalts (e.g., Defant and Drummond, 1990, 1993) show that, because Y and Sr are strongly fractionated, such melts have extremely high Sr/Y and low Y compared to melts of mantle peridotite. Felsic volcanic rocks from the Henik segment have these exact same characteristics (Fig. 9). Finally, melts of garnet-bearing metabasalt are characterized by high Zr/Sm, leading to positive Zr and Hf anomalies in normalized incompatible element diagrams (Defant and Drummond, 1990), again similar to the Henik samples. Intermediate rocks from the Henik segment have incompatible element patterns that are similar in shape to those of the dacites and rhyolites, except that incompatible element abundances are generally higher (Fig. 6). Some of the andesites also display subtle pos-

185

Fig. 9. Sr/Y vs. Y plot, Henik segment. Curve for partial melting of garnet-hornblende metabasalt and fractional crystallization of modern mid-ocean ridge basalt are after Defant and Drummond (1990, 1993).

itive Zr and Hf anomalies. Because of their distinct incompatible element patterns, the andesites are unlikely to have been derived from the basalts through fractional crystallization processes. The intermediate volcanic rocks may have originated as mixtures of basalt and dacite melts. Significant fractional crystallization of these mixtures may have raised incompatible element abundances to observed levels. Alternatively, the andesites too may record melting of juvenile mafic crust, but the felsic partial melts would have to had interacted with high-MgO rocks (with high Cr, Ni, V) to attain the andesite’s less evolved composition. 4.2. Comparison to the Kaminak segment and the infant arc model Geochemical and isotopic similarities between the Henik and Kaminak segments support the hypothesis, based on geological, structural and geochronological grounds (Aspler et al., 1999a; Hanmer et al., 2004), that rocks in the two areas are tectonostratigraphic equivalents. Like the Henik segment, most mafic volcanic rocks in the Kaminak segment have flat to slightly lightREE-enriched patterns, lack negative Nb anomalies, and display εTNd values between +2 and +3.5 (Sandeman et al., 2004). Furthermore, many felsic rocks in the Kaminak segment, like their Henik counterparts, display steep REE patterns, low heavy REE abundances,

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positive Zr (and Hf) anomalies, and positive εTNd values, but lack significant negative Eu anomalies (Sandeman et al., 2004). Both the Kaminak and Henik segments contain interlayered mafic and felsic volcanic rocks with similar isotopic compositions but markedly different REE compositions, suggesting derivation from different sources. The extensive deep-water basaltic rocks appear to be melts of depleted Archean upper mantle, whereas the dacitic to rhyolitic interbeds record partial melting of juvenile, garnet-hornblende-bearing mafic crust such as occurs in oceanic settings or near the base of the lower crust in thickened continental regions. In addition to geological evidence (Aspler and Chiarenzelli, 1996a; Hanmer et al., 2004), the predominantly positive εTNd values presented here and by Sandeman et al. (2004), indicate that older continental crust did not significantly influence magmas in the central Hearne domain. Volcanic rocks from Sealhole Lake, with consistently less positive εTNd values, are an exception. Likely recording contamination by older continental crust, these magmas may have been influenced by potential basement rocks indicated in reconnaissance geochronologic studies to the southeast near Edehon Lake (Fig. 2; Loveridge et al., 1988). Data presented from the Henik segment here support a model developed for the evolution of the central Hearne domain by Hanmer et al. (2004) and Sandeman et al. (2004). This model draws analogies to processes envisaged by Stern and Bloomer (1992) and Bloomer et al. (1995) for the early stages of intraoceanic arc growth in the southwestern Pacific Ocean, beginning in the middle Eocene. In what ultimately became the forearc region of the Izu-Bonin arc, fragments of differently aged oceanic lithosphere were juxtaposed along transform faults related to the separation of Australia and Asia. Before steep subduction and arc growth, the gravitational instability resulting from this juxtaposition led to vertical sinking of older lithosphere along the transforms (Stern and Bloomer, 1992; Bloomer et al., 1995). As adapted to the Hearne domain, Hanmer et al. (2004) and Sandeman et al. (2004) suggest that, due to reduced negative buoyancy of oceanic crust under elevated Archean geothermal gradients, this “infant arc” sinking never advanced to steep subduction, and that a longitudinally continuous arc - accretionary wedge pair failed to develop. In this setting, the mafic rocks may represent melts derived from depleted mantle dur-

ing decompression melting of upwelling athenosphere that was displaced by sinking of the old oceanic crust. It is unlikely that the new basaltic plate thus created ever attained the thickness required for garnet to be a stable phase, hence melting of the new plate could not produce felsic magmas with the heavy REE depletion seen in Henik segment dacites and rhyolites. More likely, the felsic volcanic rocks represent melts of the older oceanic crust that reached melting temperatures upon vertical sagging. The intermediate volcanic rocks may represent a hybrid in which a silica-rich melt component derived from the older crust reacted and fertilized melts derived from the upper mantle.

5. Conclusion Major, trace, and rare earth element geochemical, and Sm–Nd isotopic data suggest that interlayered mafic and felsic rocks in the Henik segment had fundamentally different sources. These data indicate that the mafic rocks record partial melting of depleted upper mantle, and that the felsic rocks represent melts of slightly older juvenile mafic crust. The intermediate rocks could represent fractionated mixtures from both these sources; alternatively they record partial melting of mafic crust followed by re-equilibration with mafic to ultramafic rocks. Similar geochemical and isotopic results from the Kaminak segment (Sandeman et al., 2004) support interpretations that supracrustal deposition in the Henik and Kaminak segments was physically continuous at 2700–2680 Ma in a series of semi-independent depocentres. Results from the Henik segment are thus consistent with a generalized model developed for the central Hearne domain (Hanmer et al., 2004; Sandeman et al., 2004) that draws analogies to intraoceanic processes such as described from the southwestern Pacific Ocean (Stern and Bloomer, 1992; Bloomer et al., 1995) rather than to Phanerozoic-style plate tectonics.

Acknowledgments Analytical studies at Carleton University and field work in Nunavut were funded by the Geology Office, Indian and Northern Affairs Canada (Yellowknife), and the Geological Survey of Canada. Preparation of

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the manuscript was supported by the Canada-Nunavut Geoscience Office. We thank Sandy Barham and Comaplex Minerals Corp. for logistical support. Simon Hanmer and Hamish Sandeman provided preprints and ongoing discussions. Critical comments by P.C. Thurston and an anonymous reviewer helped to improve the paper. This is a contribution to the Western Churchill NATMAP Project and the Canada-Nunavut Geoscience Office.

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