Earth’s Oldest Rocks Edited by Martin J. van Kranendonk, R. Hugh Smithies and Vickie C. Bennett Developments in Precambrian , Vol. 15 (K.C. Condie, Series Editor) ©2007 Elsevier B.V. All rights reserved.

THE GEOLOGY OF THE 3.8 GA NUVVUAGITTUQ (PORPOISE COVE) , NORTHEASTERN SUPERIOR PROVINCE, .

Jonathan O’Neil1, Charles Maurice2, Ross K. Stevenson3,4, Jeff Larocque5, Christophe Cloquet6, Jean David3 & Don Francis1

1 Earth & Planetary Sciences, McGill University and GÉOTOP-UQÀM-McGill, 3450 University St. Montreal, QC, Canada, H3A 2A7 ([email protected]) 2 Bureau de l’exploration géologique du Québec, Ministère des Ressources naturelles et de la Faune, 400 boul. Lamaque Val d’Or, QC, J9P 3L4 3 GÉOTOP-UQÀM-McGill, Université du Québec à Montréal, C.P. 8888, succ. centre-ville, Montreal, QC, Canada H3C 3P8 4 Département des Sciences de la Terre et de l’Atmosphère, Université du Québec à Montréal, C.P. 8888, succ. centre-ville Montreal, QC, Canada H3C 3P8 5 School of Earth and Oceanic Sciences, University of Victoria, P.O. Box 3055 STN CSC, Victoria, BC, Canada, V8W 3P6 6 INW-UGent, Department of analytical chemistry, Proeftuinstraat 86, 9000 GENT, Belgium

Abstract

The Nuvvuagittuq greenstone belt is a 3.8 Ga supracrustal succession preserved as a raft in remobilised tonalities along the eastern of Hudson Bay. The dominant of the belt is a -ribboned grey composed of variable proportions of cummingtonite, and . Although the have compositions, the presence of cummingtonite rather than hornblende in these rocks reflects their low Ca contents, which may result from the alteration and of mafic pyroclastic rocks. Two types of ultramafic sills are present in the western limb of the belt. Type-1 sills are characterised by low Al and Cr contents, but high Fe, and have amphibolitic margins and internal layers that have high normative clinopyroxene contents. Type-2 sills are richer in Al and Cr, but poorer in Fe, and are characterised by amphibolitic margins and internal layers that have high normative orthopyroxene contents. The calculated parental for both types of sills are . The estimated parental of the Type-1 sills is equivalent to an Al-depleted (ADK), while that of Type-2 sills is an Al-undepleted komatiite (AUK). sills have flat to slightly depleted REE profiles, indicating a lack of interaction with a pre-existing crust. The Nd isotopic compositions of the Nuvvuagittuq’s rocks (εNd = -0.18 to +3.4), however, indicate derivation from a source that had already experienced long-term trace element depletion.

A prominent silica-formation composed almost entirely of quartz can be continuously traced along the entire eastern limb of the belt and appears to grade into a banded formation (BIF) consisting of finely laminated quartz, magnetite, and grunerite. Samples of the BIF are characterised by concave-up LREE profiles with positive Eu and Y anomalies and exhibit heavy Fe isotopic enrichment (FFe=0.25- 0.48 ‰/amu) compared to the adjacent and amphibolites, consistent with an origin as a chemical precipitate origin and possibly indicative of the action of biological activity at 3.8 Ga. 1. Introduction

Our knowledge of the first billion years of the Earth’s evolution is limited and early magmatic processes, such as mantle differentiation and crustal formation, remain poorly understood. from the Jack Hills conglomerates (Wilde et al., 2001) suggest the existence of as old as 4.4 Ga. These early ages, however, are obtained on detrital zircons from much younger rocks, whose has long since been destroyed or reworked. Other than timing, such occurrences provide little information about the chemistry of the Earth’s early mantle. Presently, rare preserved relicts of mantle-derived crust provide the best compositional and isotopic constraints on early crust-mantle differentiation of the Earth. The 3.6-3.85 Ga Itsaq complex (West ), comprising the , is the most extensive early terrain preserved. The Nd isotopic compositions for these mantle- derived rocks indicate that their mantle source was already strongly depleted at 3.8 Ga (Bennett et al. 1993; Blichert-Toft et al. 1999; Frei et al. 2004), implying that significant volumes of continental crust had already formed during the . Such remnants of Eoarchean mantle-derived rocks are, however, rare, and models for the evolution of the mantle are poorly constrained for the first billion years of Earth’s history.

In this paper, we report the first detailed description of the Nuvvuagittuq (originally named Porpoise Cove) greenstone belt, dated at 3.8 Ga (David et al. 2002). As one of the world’s oldest known mantle-derived suite of rocks, the Nuvvuagittuq greenstone belt offers an extraordinary opportunity to further our understanding of the early Earth. Preliminary results for this newly discovered Eoarchean supracrustal assemblage indicate that both aluminum-depleted (ADK) and aluminum-undepleted (AUK) komatiitic magmas existed at 3.8 Ga and that the mantle had already experienced a long-term depletion at that time. Furthermore, a prominent , which serves as a stratigraphic marker horizon within the belt, displays Fe isotopic compositions that are systematically heavier than their enclosing igneous rocks, similar to results obtained at Isua. Although it has yet to be demonstrated that such isotopic fractionation requires an organic origin, the possibility that the formation of such Archean Algoma-type banded Fe-formations involves biological activity has major implications for the timing of the appearance of life on Earth.

2. Geological framework

The Nuvvuagittuq greenstone belt is located on the eastern coast of Hudson Bay, in the Northeastern Superior Province (NESP) of Canada (Figure 1). Early work on this portion of the Superior Province suggested that it was composed mostly of granulite-grade granitoids (Stevenson, 1968; Herd, 1978; Card and Ciesielski,

2 Canada

Ungava Orogen Huds on S Cape Smith Belt trait

Arnaud Ungava Bay

Churchill Nuvvagittuq Southeast Belt

Hudson Ne Bay w Qu eb ec Orogen

Hudson Bay Terrane

Labrador

0 50 100 Kilometres

Figure 1: Location map of the Nuvvuagittuq greenstone belt in the Northeastern Superior Province. Isotopic from Boily et al. (2006), Leclair (2005) and Leclair et al. (2006). 1986; Percival et al., 1992). More recent work has shown, however, that it is dominantly comprised of Neoarchean plutonic suites in which amphibolite- to granulite-grade greenstone belts occur as relatively thin keels (1-10 km) that can be traced continuously for up to 150 km along strike (Percival et al., 1994; Percival et al., 1995; Percival et al., 1996; Percival et al., 1997a; Leclair, 2005). The magmatic and metamorphic evolution of the NESP spans nearly 2 billion years of the Earth’s history (3.8 – 1.9 Ga), as determined by ~220 U-Pb ages acquired by governmental surveys (Leclair et al., 2006 and references therein). On a regional scale, distinct lithological assemblages appear as large linear positive and negative aeromagnetic anomalies, which have led to the partitioning of the NESP into lithotectonic domains (Percival et al., 1992; Percival et al., 1997b). These domains have subsequently been modified following further field mapping and the acquisition of more isotopic data (Leclair et al. 2006; Boily et al., 2006), and the NESP is now separated into two isotopically distinct terranes (Boily et al., 2006). To the East, the Arnaud River Terrane group rocks that are younger than ca. 2.88 Ga and characterized by juvenile isotopic signatures (Nd TDM < 3.0 Ga). To the West, rocks of the Hudson Bay Terrane, which includes the Nuvvuagittuq greenstone belt, represent a reworked Meso- to Eoarchean , with zircon inheritance ages and Nd depleted-mantle model ages (TDM) as old as 3.8 Ga (Stevenson et al., 2006)

3. Geology of the Nuvvuagittuq Belt

Lee (1965) first mapped the Nuvvuagittuq greenstone belt and small portions of it have subsequently been mapped in more detail by Nadeau (2003). We have now mapped the entire Nuvvuagittuq belt at a scale of 1:20 000, and the western limb of the belt at a more detailed scale of 1:2000 (Figure 2). The Nuvvuagittuq belt is a -sedimentary succession that occurs as a tight to isoclinal synform refolded into a more open south-plunging synform (David et al., 2002), with bedding largely parallel to the main steeply-dipping schistosity. The supracrustal assemblage of the belt is essentially composed of three major lithological units: 1) cummingtonite-amphibolite that is the predominant lithology of the belt, 2) ultramafic and mafic sills that intrude the amphibolites, and 3) chemical sedimentary rocks that comprise a banded iron formation and a silica-formation. The Nuvvuagittuq belt is surrounded by a 3.6 Ga , itself surrounded by a younger 2.75 Ga tonalite (Stevenson and Bizzarro, 2006; David et al., 2002; Simard et al., 2003).

The Nuvvuagittuq belt contains rare felsic bands 15 to 50 cm in width (Figure 3a) that have been interpreted by Simard et al. (2003) to be a felsic . U-Pb ages obtained on zircons from one of these felsic bands, a plagioclase-quartz-biotite suggest an age of emplacement possibly as old as 3825 ± 16 Ma (David et al., 2002). Subsequent high-resolution geochronology work done by Cates and Mojzsis (2007) confirm a minimum age of emplacement for the Nuvvuagittuq sequence of 3751 ± 10

4 Top right corner map N Tonalite

Faux-amphibolite 6465000mN BIF GRT GRT Silica-formation Out In

Greenstone Bottom map Ultramafic & Gabbro sill Gabbro sill 6464000mN 3 4 0 m E Ultramafic sill 3 4 1 0 m E Boundary where becomes ubiquitous Pegmatite

Synform axial trace Attitude of contact Attitude of schistosity

6463000mN 300 m. Hudson Bay

60

GRT GRT Out In

85

78 75

84 100 m.

78 86 86

Figure 2: Geological map of the Nuvvuagittuq greenstone belt. Coordinates in UTM NAD27, Zone 18 Ma based on 206Pb/207Pb zircon ages. Although these felsic bands are geochemically similar to the surrounding tonalite and are rare in the Nuvvuagittuq belt, a U-Pb age of 3659 ± 2.5 Ma on zircons (David et al., 2002) from the surrounding makes it unlikely that these felsic bands represent remobilized tonalite.

3.1 Cummingtonite-amphibolite

Cummingtonite-amphibolites are the predominant of the Nuvvuagittuq greenstone belt. These peculiar amphibolites are dominated by cummingtonite, which gives this lithology a light grey to beige color, rather than the dark green to black color characteristic of hornblende-dominated amphibolites typical of the Superior Province. Because of the unusual color of the amphibolites in this region, they were referred to as “faux-amphibolite” in the field, a term which we will use for the rest of this contribution.

The faux-amphibolite is a heterogeneous gneiss consisting of cummingtonite + quartz + biotite + plagioclase ± anthophyllite ± garnet, with the majority of the biotite having been replaced by retrograde chlorite. It is generally characterised by a finely laminated defined by the alternation of biotite-rich and cummingtonite-rich laminations which is enhanced by ubiquitous mm to cm scale quartz ribboning that generally follows the main schistosity (Figures 3b and 3c). Variations in the proportion of cummingtonite and biotite also occur on a metre scale with large bands dominated by cummingtonite + quartz + plagioclase, within more biotite- rich faux-amphibolite.

One of the striking features of the faux-amphibolite is the variation in garnet content. The faux-amphibolite in the western limb of the belt rarely contains garnet, whereas cm-sized are ubiquitous in the eastern limb (Figure 2), although its proportion varies substantially, with alternating garnet-poor and garnet-rich layers (Figure 3c). In addition, there is a gradational transition in the southwestern corner of the belt from faux-amphibolite gneiss to massive aphanitic greenstones that are interpreted to be massive volcanic flows (Figure 2). In contrast to the typical faux-amphibolite, these rocks are characterized by a assemblage of chlorite + epidote + quartz + plagioclase ± ± .

The abundance of garnet in the faux-amphibolites of the eastern limb of the belt, along with the compositional layering, to their first being mapped as paragneiss (Simard et al., 2003). However, although the faux-amphibolites are compositionally somewhat variable, they are generally basaltic in composition and are significantly more mafic than Archean shales, with higher MgO and lower SiO2 (40-56 wt% SiO2, 4-16 wt% MgO) (Table 1) contents, similar to those of the Nuvvuagittuq’s gabbro sills and greenstones. The garnet-bearing faux- amphibolites are compositionally similar to the biotite and cummingtonite-rich

6 a b

Felsic band

{ cummingtonite-rich biotite-rich {

c d

Ultramafic Amphibolitic sill margin

Garnet-poor Garnet-rich

e f

sill sill Faux-amphibolite Gabbro Faux-amphiboliGteabbro

Figure 3: Photos of Nuvvuagittuq's rocks. a) Felsic band from which the 3.825 Ga U-Pb zircon age has been obtained (David et al., 2002). b) Garnet-bearing faux-amphibolite. c) Garnet-poor and garnet-rich layers within the faux-amphibolite. d) Ultramafic sill with gabbroic top. e) Gabbro sills intruding the faux- amphibolite. f) Banded iron formation with alternating quartz-rich and magnetite-rich laminations. Table 1. Major (wt.%) and trace (ppm) element data for Nuvvuagittuq rocks Major elements

Sample SiO2 TiO2 Al2O3 MgO FeO MnO CaO Na2O K2O P2O5 LOI UTM Easting UTM Northing

Cummingtonite-rich Faux-Amphibolite PC-54 54.27 0.41 18.60 6.88 7.22 0.16 7.20 1.40 1.86 0.06 1.54 339865 6464451 PC-58 54.53 0.79 14.57 6.98 9.81 0.19 8.78 1.28 0.62 0.10 1.40 339959 6464353 PC-131 49.95 0.34 16.34 11.44 8.78 0.18 5.14 2.08 1.92 0.03 2.91 339574 6464197 PC-149 52.67 0.51 16.07 9.58 8.46 0.17 6.77 1.25 1.47 0.04 2.18 339686 6463935 PC-151 50.12 0.37 17.36 9.34 9.20 0.20 7.40 1.17 1.76 0.03 2.02 339727 6463946 PC-159 54.47 0.47 16.09 6.81 10.04 0.26 5.10 2.05 1.38 0.04 2.22 339920 6463917 PC-162 52.07 0.45 15.42 9.43 9.02 0.21 7.55 1.23 1.02 0.05 2.46 339961 6463911 PC-171 48.86 1.08 14.70 8.31 14.54 0.24 4.53 1.12 3.07 0.17 1.46 340127 6463551 PC-173A 45.59 0.73 15.22 15.61 12.17 0.18 1.02 1.07 2.78 0.08 4.47 340111 6463566 PC-173B 47.34 0.66 14.20 15.42 12.40 0.19 2.04 1.25 1.56 0.07 3.81 340111 6463566

Biotite-rich Faux-Amphibolite PC-129 52.01 0.76 14.12 10.70 10.70 0.09 3.89 1.96 0.88 0.07 3.91 339550 6464197 PC-132 51.19 0.89 14.42 8.68 10.54 0.21 8.20 1.92 1.08 0.08 1.83 339582 6464222 PC-135 48.22 0.35 16.38 13.62 11.65 0.13 0.45 0.06 1.80 0.03 6.37 339632 6464229 PC-150 54.53 0.51 16.13 10.28 8.23 0.18 4.94 1.80 0.10 0.06 2.57 339689 6463919 PC-152 48.52 0.61 16.18 15.79 9.90 0.19 0.28 0.36 0.43 0.04 6.79 339740 6463925

Garnet-bearing Faux-Amphibolite PC-157 52.67 0.60 17.39 6.33 10.11 0.37 2.85 1.98 2.72 0.07 3.50 339912 6463958 PC-160 55.07 0.61 17.53 6.77 12.38 0.10 0.15 0.19 1.79 0.06 4.20 339933 6463921 PC-161 53.83 0.62 17.42 3.99 9.31 0.24 7.28 1.12 2.56 0.08 2.68 339955 6463924 PC-163 52.61 0.42 19.16 6.96 9.17 0.28 2.28 2.32 1.73 0.03 4.00 339977 6463823 PC-164 55.30 0.38 18.64 4.95 7.42 0.16 4.92 1.88 1.72 0.05 3.63 339989 6463855 PC-176 49.76 0.92 13.87 8.71 21.16 0.12 0.51 0.35 0.86 0.11 1.74 340079 6463584

Greenstone PC-177 64.05 0.58 11.00 3.76 6.94 0.13 2.96 2.77 2.49 0.07 4.16 339436 6463037 PC-178 56.01 0.91 13.61 4.87 12.20 0.10 1.83 0.13 3.95 0.08 4.89 339464 6463078 PC-179 39.62 0.70 16.32 7.16 26.63 0.10 0.02 0.07 1.41 0.01 5.52 339472 6463067 PC-180 44.52 0.77 13.88 8.60 10.61 0.18 6.29 1.16 4.12 0.06 8.21 339489 6463045 PC-181 44.69 1.03 17.89 10.07 10.29 0.14 2.73 2.47 2.54 0.10 6.96 339514 6463050 PC-182 51.00 0.74 13.80 9.05 11.33 0.12 2.74 1.69 2.18 0.07 6.04 339526 6463050 PC-183 40.65 1.39 15.79 11.39 11.33 0.17 5.21 1.52 1.55 0.12 9.53 339540 6463086 PC-184 43.26 0.40 19.25 7.34 7.22 0.14 5.98 1.78 4.36 0.05 9.25 339555 6463096 PC-185 43.13 0.45 15.36 11.06 9.67 0.18 5.97 2.01 1.09 0.05 9.96 339575 6463114 PC-186 51.45 0.56 15.57 6.43 8.48 0.17 4.99 0.45 2.83 0.06 7.84 339589 6463114 PC-189 47.15 0.66 15.20 9.06 8.92 0.15 5.56 1.73 1.44 0.06 9.04 339694 6462839

Gabbro PC-81 49.01 0.74 15.80 8.74 9.90 0.21 10.82 2.27 0.70 0.06 1.08 339669 6464278 PC-82 49.20 0.76 15.64 7.91 9.93 0.19 11.37 1.58 0.68 0.06 1.68 339669 6464278 PC-83 48.26 0.75 15.57 8.62 10.37 0.20 11.24 1.83 0.75 0.06 1.41 339669 6464278 PC-85 49.03 1.07 14.89 7.94 12.68 0.19 9.72 3.09 0.34 0.08 0.50 339669 6464278 PC-86 49.40 1.20 14.67 7.55 12.88 0.24 9.41 2.98 0.36 0.09 0.58 339669 6464278 PC-87 50.62 1.16 13.09 9.08 13.52 0.28 7.96 2.54 0.33 0.07 0.48 339669 6464278 PC-118 48.03 0.97 15.25 7.62 12.01 0.22 9.87 1.55 1.89 0.07 1.17 339534 6464230 PC-119 48.80 1.03 15.24 7.17 12.00 0.23 8.53 2.00 2.22 0.09 1.20 339537 6464234 PC-121 48.09 1.12 14.91 7.43 12.53 0.24 8.10 1.54 2.99 0.09 1.40 339536 6464220 PC-147 52.26 0.84 9.48 10.36 13.02 0.25 10.08 0.95 0.47 0.08 0.79 339649 6463952 PC-187 50.02 0.74 11.49 8.95 12.98 0.20 10.73 1.53 0.91 0.05 0.74 339830 6462843 PC-188 47.54 1.00 13.89 5.92 17.91 0.22 8.78 1.69 0.40 0.09 0.69 339772 6462769 PC-191 47.09 1.23 8.57 5.84 22.29 0.17 8.30 0.67 0.17 0.13 2.81 339789 6462840

Table 1 (continued) Major elements

Sample SiO2 TiO2 Al2O3 MgO FeO MnO CaO Na2O K2O P2O5 LOI UTM Easting UTM Northing

Ultramafic Type-1 PC-25 43.90 0.46 3.71 26.55 11.99 0.18 5.79 0.13 0.05 0.02 5.99 339633 6464857 PC-26 41.65 0.32 2.32 31.36 11.52 0.19 2.64 0.09 0.04 0.02 8.73 339633 6464857 PC-27 45.42 0.54 4.59 24.05 11.56 0.18 8.33 0.17 0.05 0.02 4.21 339633 6464857 PC-28 45.42 0.52 4.45 25.12 10.75 0.18 7.33 0.18 0.05 0.02 4.98 339633 6464857 PC-29 43.90 0.46 3.71 26.55 11.99 0.18 5.79 0.13 0.05 0.02 5.99 339633 6464857 PC-113 42.64 0.42 3.53 27.98 12.05 0.19 4.43 0.20 0.05 0.02 6.99 339513 6464278 PC-114 39.96 0.15 1.15 34.67 10.50 0.19 1.30 0.01 0.02 0.02 11.09 339524 6464261 PC-115 38.62 0.27 2.18 33.96 9.27 0.19 2.38 0.06 0.03 0.02 11.62 339519 6464268 PC-116 39.88 0.25 2.02 35.27 9.34 0.19 0.54 0.00 0.40 0.02 11.13 339529 6464245 PC-125 40.52 0.27 2.22 30.86 12.08 0.19 2.73 0.05 0.02 0.02 9.89 339520 6464113 27525A 40.28 0.41 3.19 31.03 11.40 0.18 3.94 0.19 0.04 0.06 7.80 27521A 38.55 0.35 2.58 31.62 11.57 0.19 3.61 0.21 0.39 0.09 9.30 19095A 42.70 0.24 4.60 25.70 13.77 0.09 4.44 0.29 0.06 0.01 7.00

Type-1 Amphibolitic chill margin M and layer L PC-15M 46.77 0.64 6.15 18.14 14.58 0.26 10.69 0.25 0.06 0.03 1.18 339633 6464857 PC-33M 49.44 0.65 5.51 17.79 11.80 0.29 11.34 0.48 0.13 0.03 1.45 339608 6464731 PC-110M 50.87 0.5 4.41 19.87 9.02 0.17 12.16 0.44 0.10 0.02 1.36 339505 6464264 L PC-20 43.64 0.59 5.65 22.69 13.47 0.18 8.02 0.22 0.05 0.02 3.82 339633 6464857 PC-21L 46.1 0.61 5.80 20.79 13.57 0.18 9.40 0.24 0.06 0.03 2.21 339633 6464857

Ultramafic Type-2 PC-72 42.66 0.18 5.04 29.88 7.81 0.13 4.38 0.23 0.06 0.02 8.51 339738 6464332 PC-73 40.56 0.19 5.28 31.59 8.08 0.13 2.90 0.11 0.04 0.02 9.95 339738 6464332 PC-74 40.73 0.16 4.27 31.71 8.37 0.12 3.16 0.11 0.04 0.02 9.98 339738 6464332 PC-75 38.40 0.16 4.32 31.99 9.00 0.14 2.99 0.06 0.04 0.02 11.33 339738 6464332 PC-76 40.26 0.20 5.12 31.11 8.36 0.13 3.59 0.10 0.04 0.02 9.96 339738 6464332 PC-91 42.27 0.11 4.71 29.48 7.49 0.15 5.18 0.23 0.05 0.01 8.94 339669 6464278 PC-92 40.82 0.08 3.85 33.03 8.19 0.13 2.13 0.07 0.03 0.01 10.67 339669 6464278 PC-93 39.68 0.08 3.86 32.59 8.40 0.15 2.81 0.13 0.03 0.01 10.81 339669 6464278 PC-94 40.37 0.11 5.21 31.67 8.06 0.14 3.05 0.14 0.04 0.01 9.85 339669 6464278 PC-138 42.17 0.24 6.27 28.22 8.58 0.13 4.80 0.10 0.03 0.03 8.05 339623 6463969 PC-141 40.28 0.17 4.56 31.23 8.87 0.14 3.16 0.05 0.03 0.02 9.98 339614 6463947

Type-2 Amphibolitic chill margin M and layer L M PC-71 46.51 0.39 11.28 20.89 10.51 0.17 6.56 0.66 0.12 0.04 2.42 339738 6464332 PC-142M 44.69 0.46 11.73 22.27 9.99 0.12 5.49 0.30 0.12 0.05 3.49 339619 6463967 PC-88L 47.34 1.14 14.21 16.48 12.41 0.29 1.77 0.64 1.91 0.09 2.80 339669 6464278

BIF PC-192 51.88 0.02 0.29 2.93 39.65 0.38 1.39 0.04 0.02 0.04 0.00 339782 6462834 PC-193 80.57 0.03 0.75 0.75 15.77 0.19 0.39 0.04 0.03 0.02 0.00 339777 6462849 PC-197 46.32 0.03 0.28 3.18 45.98 0.20 0.12 0.03 0.11 0.06 0.08 339506 6463764 PC-198B 34.04 0.02 0.25 1.90 59.06 0.15 0.37 0.02 0.05 0.08 0.00 339512 6463744 PC-199 59.88 0.03 12.31 1.51 20.34 0.58 2.67 0.09 0.37 0.02 0.25 339515 6463764 PC-200 32.20 0.02 0.27 3.49 59.14 0.22 0.09 0.02 0.15 0.06 0.00 339517 6463770

Si-Formation PC-194 75.69 0.07 3.14 2.43 9.11 0.78 2.33 0.03 0.11 0.02 5.08 339701 6462834

Grt Si-rich unit PC-165 65.53 0.40 15.92 2.18 9.12 0.16 0.21 0.34 2.86 0.05 2.29 339977 6463773

Table 1 (continued) Trace elements Sample Rb Sr Zr Nb Y Ni Cr V Co La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Cummingtonite-rich Faux-Amphibolite PC-54 59 62 39 1.6 15 96 121 152 35 PC-58 22 51 161 6.7 27 61 88 208 35 PC-131 65 123 23 1.0 13 188 410 205 50 PC-149 34 44 40 1.7 13 87 233 219 51 PC-151 44 53 25 0.7 14 126 193 228 50 PC-159 38 58 63 3.1 21 90 127 199 40 PC-162 29 55 50 2.5 15 137 349 174 47 PC-171 66 50 91 2.6 44 115 266 274 48 PC-173A 81 60 39 1.7 20 172 314 221 55 PC-173B 47 42 37 1.5 24 176 326 245 53

Biotite-rich Faux-Amphibolite PC-129 29 47 56 2.2 17 92 171 249 41 PC-132 32 74 57 2.3 18 53 130 267 38 PC-135 58 8 23 1.0 9 186 416 203 42 PC-150 2 24 40 1.6 13 80 233 201 41 PC-152 10 4 67 3.1 16 93 142 213 54

Garnet-bearing Faux-Amphibolite PC-157 84 37 70 3.6 18 90 155 218 40 PC-160 60 16 69 3.9 15 140 177 224 50 PC-161 85 52 67 3.5 20 71 159 230 54 PC-163 53 51 28 1.1 16 104 247 245 41 PC-164 54 53 31 0.9 14 99 200 195 48 PC-176 37 5 52 2.2 18 415 1321 252 84

Greenstone PC-177 62 98 42 1.7 10 107 323 87 29 PC-178 131 63 47 1.8 15 86 242 77 32 PC-179 58 7 36 1.2 6 266 367 210 68 PC-180 293 124 38 1.4 17 138 333 252 40 PC-181 73 71 106 5.3 13 96 63 171 53 PC-182 49 59 76 4.2 11 88 51 95 52 PC-183 60 87 73 2.8 12 109 255 314 50 PC-184 174 97 28 0.9 11 123 289 237 42 PC-185 43 96 51 2.6 15 130 171 150 48 PC-186 74 66 61 3.1 15 85 131 188 40 PC-189 49 93 49 2.4 18 92 279 201 41

Gabbro PC-81 16 94 39 1.4 18 159 350 236 43 2.2 0.8 5.7 4.7 1.6 0.7 2.2 0.4 2.7 0.6 1.9 0.3 1.8 0.3 PC-82 21 94 43 1.5 17 120 324 232 41 PC-83 18 92 40 1.7 16 166 324 227 43 2.3 0.9 5.8 4.6 1.6 0.6 2.2 0.4 2.6 0.6 1.8 0.3 1.8 0.3 PC-85 4 78 57 2.1 21 132 180 257 52 3.1 1.2 8.1 6.5 2.2 0.9 2.9 0.5 3.4 0.7 2.3 0.3 2.1 0.3 PC-86 10 96 62 2.4 26 75 214 301 42 PC-87 5 77 62 2.4 25 82 192 291 49 3.2 1.5 9.5 8.1 2.7 0.8 3.5 0.6 4.0 0.9 2.7 0.4 2.6 0.4 PC-118 73 138 53 2.1 21 122 294 260 53 PC-119 76 112 58 2.2 23 116 228 252 56 PC-121 114 62 60 2.2 23 113 224 281 53 PC-147 8 46 80 4.4 18 119 547 192 63 PC-187 26 104 49 1.9 18 199 870 218 60 PC-188 5 65 66 2.6 27 56 155 298 64 PC-191 1 3 51 3.1 15 501 2782 214 157 1.9 0.9 5.5 5.5 2.1 0.9 3.0 0.5 2.9 0.6 1.7 0.3 1.6 0.2

Table 1 (continued) Trace elements Sample Rb Sr Zr Nb Y Ni Cr V Co La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Type-1 ultramafic PC-25 0 12 22 1.1 8 1237 2422 116 92 PC-26 7 10 17 0.8 4 1854 1769 64 124 PC-27 0 14 30 1.2 10 900 2181 129 83 PC-28 6 15 29 1.2 9 1096 1886 113 85 PC-29 6 12 22 1.1 8 1237 2424 115 92 PC-113 0 7 21 0.9 7 1295 2023 106 96 PC-114 0 5 6 0.4 4 1921 959 50 126 PC-115 0 9 14 1.0 4 2046 1891 54 110 PC-116 28 4 11 1.1 2 2053 1616 51 117 PC-125 2 14 14 0.8 3 1757 1717 66 128 27525A00000 0 0 0 0 27521A00000 0 0 0 0 19095A00000 024000 0

Type-1 Amphibolitic chill margin M and layer L PC-15M 0 8 31.3 1.5 11.7 284 1596 166 80 3.65 9.8 1.33 6.66 1.87 1.0 2.12 0.37 2.22 0.44 1.34 0.19 1.16 0.18 PC-33M 0 15.4 33.8 1.6 10 193 2051 175 60 2.28 5.32 0.73 4.0 1.46 0.85 1.8 0.32 2.0 0.41 1.22 0.18 1.13 0.16 PC-110M 0 10.3 26.9 1.4 6.8 712 2049 114 75 2.77 6.07 0.74 3.53 1.0 0.49 1.23 0.22 1.32 0.27 0.8 0.12 0.77 0.11 PC-20L 0 11 30.9 1.1 9.4 647 2520 154 66 1.0 3.05 0.49 3.14 1.2 0.43 1.48 0.26 1.67 0.35 1.06 0.15 0.94 0.14 PC-21L 0 13.7 32.7 1.3 11.1 457 1832 161 82 2.6 6.2 0.83 4.48 1.54 0.61 1.88 0.33 2.0 0.42 1.21 0.17 1.06 0.15

Type-2 ultramafic PC-72 4 12 18 0.6 7 1687 5851 76 85 PC-73 3 14 19 0.8 6 1935 5722 77 99 PC-74 4 16 17 0.8 6 2141 5257 62 121 PC-75 4 15 18 0.9 6 2270 5580 63 118 PC-76 4 19 22 1.0 7 1941 4551 67 113 PC-91 0 12 6 0.3 3 1559 5175 76 72 PC-92 38403162146725579 PC-93 09302184869055878 PC-94 475031795579076109 PC-138 0 16 27 1.2 10 1611 4190 90 108 PC-141 0 15 19 0.9 7 1935 4855 63 103

Type-2 Amphibolitic chill margin M and layer L PC-71M 0 43.7 44.7 2.1 18.1 616 1892 150 81 4.71 11.2 1.39 6.4 1.8 0.72 2.24 0.43 2.84 0.62 2.0 0.3 1.93 0.29 PC-142M 2 34.6 52.5 2.4 17.5 649 2137 161 80 5.34 11.8 1.53 7.0 2.0 1.1 2.4 0.4 2.9 0.6 2.0 0.3 2.0 0.3 PC-88L 34.3 82.5 62.8 2.3 18.8 102 241.5 277 63 2.67 6.24 0.91 5.0 1.9 0.4 2.5 0.5 3.3 0.7 2.3 0.4 2.4 0.4

BIF PC-192 0 3 3 0.5 12 56 8 11 0 1.9 3.7 0.5 2.1 0.6 0.3 1.0 0.2 1.2 0.3 0.9 0.1 0.9 0.1 PC-193 0 1 7 0.0 3 44 6 9 8 1.9 3.3 0.4 1.4 0.3 0.2 0.3 0.1 0.4 0.1 0.3 0.0 0.2 0.0 PC-197 6 2 1 0.4 8 78 23 11 4 2.2 4.4 0.5 2.1 0.5 0.3 0.8 0.2 1.0 0.2 0.8 0.1 0.8 0.1 PC-198B 3 2 3 0.5 9 67 8 9 0 3.2 5.7 0.6 2.4 0.5 0.3 0.7 0.1 0.9 0.2 0.8 0.1 0.8 0.1 PC-199 18 6 48 0.5 3 5 1 9 0 1.8 3.1 0.4 3.2 1.9 4.6 1.4 0.2 0.6 0.1 0.2 0.0 0.2 0.0 PC-200 9 2 1 0.3 13 77 9 11 0 2.3 4.6 0.6 2.6 0.8 0.4 1.2 0.2 1.5 0.4 1.2 0.2 1.2 0.2

Si-Formation PC-194 8 10 16 1.1 18 46 55 23 8 20.4 33.0 3.4 11.2 1.6 0.5 1.6 0.3 2.2 0.5 1.7 0.3 1.5 0.2

Grt Si-rich unit PC-165 99 17 36 1.6 13 114 110 206 59 3.8 7.8 0.9 4.1 1.1 0.3 1.5 0.3 2.6 0.6 2.1 0.3 2.1 0.3

Alteration-free samples were crushed in a steel jaw crusher and ground in an alumina shatter box. Major and trace elements were analyzed by X-ray fluorescence (XRF) by the McGill Geochemical Laboratories, using a Philips PW2400 4kW automated XRF spectrometer system. Major elements, Ba, Co, Cr, Cu and V were analyzed using 32 mm diameter fused beads, while Rb, Sr, Zr, Nb and Y were analyzed using 40 mm diameter pressed pellets. The accuracy for silica is within 0.5% and within 1% for other major and trace elements. REE concentrations were determined by Activation Laboratories, using a Perkin Elmer SCIEX ELAN 6000 coupled-plasma mass-spectrometer (ICP-MS) using a lithium metaborate/tetraborate fusion technique for digestion. Coordinates are in UTM NAD27, Zone 18. facies, but have lower Mg numbers and higher Al2O3 contents. Both the garnet- bearing and garnet-free faux-amphibolite are, however, Ca-poor relative to the gabbro sills that intrude them. Although similar in composition, the greenstones on the southwestern limb of the belt tend to have slightly lower SiO2 contents than the faux-amphibolites at similar MgO contents. The relatively high loss on ignition (LOI) (4 – 10 wt%) and K2O contents (up to 4 wt%) of the greenstones suggest that they may have been extensively altered.

3.2 Gabbro and Ultramafic Sills

The striking feature of the western limb of the Nuvvuagittuq belt is the presence of numerous ultramafic and gabbroic conformable bodies within the faux- amphibolite (Figures 3d and 3e). These bodies are interpreted to be sills because of the absence of any volcanic features as well as the lack of asymmetry of the upper and lower margins typical of flows. The ultramafic sills range from 5 to 30 metres in width and consist of brown weathering serpentine-rich interiors with thin grey to dark green -rich margins. The ultramafic interiors of the sills consist mainly of serpentine and talc, with lesser tremolite, hornblende and chromite, but also contain amphibole-rich layers 10 to 20 cm in thickness. The amphibolitic margins of the ultramafic sills are composed dominantly of hornblende and talc and are interpreted to be chilled margins, while the amphibole-rich layers within the sill interiors2 are thought to have been cumulate horizons. Locally, the presence of gabbroic tops suggests the separation of a residual liquid.

Two types of ultramafic sills can be recognized in the western limb of the Nuvvuagittuq belt, with those on the western side of the BIF being compositionally distinct from those on the eastern side of the BIF. The sills on the western side of the BIF (Type-1) are relatively poor in Al and rich in Fe, whereas those on the eastern side on the BIF (Type-2) are relatively rich in Al and poorer in Fe (Figures 4a and 4c). The serpentine-rich rocks of these two types of ultramafic sills fall along distinct control lines in a Pearce-type plot (Figure 5), suggesting that they are both olivine cumulates. Most strikingly, Cr increases with decreasing MgO within the olivine cumulates of Type-1 sills, but decreases with MgO in the Type- 2 sills (Figure 4b). The calculated CIPW-normative mineralogy of the amphibolite layers and margins also support the existence of two types of ultramafic sills in that normative clinopyroxene is abundant in Type-1 sills, whereas orthopyroxene predominates in Type-2 sills (Figure 6). Moreover, metamorphic orthopyroxene is observed in the amphibolites of Type-2 sills, but not in Type-1 sills. The amphibolitic margins of both sill types exhibit slightly fractionated light rare earth element (LREE) profiles, but have different heavy rare earth element (HREE) profiles, with the chill margins of Type-2 sills displaying relatively flat HREE, while those of the Type-1 sills having slightly fractionated HREE profiles (Figure 7a).

12 20 7000

a 6000 b 15 5000

4000 Al2O3 10 Cr 3000

2000 5

1000

0 0 0 10 20 30 40 0 10 20 30 40 MgO MgO

20 20

c d 15 15

FeO 10 CaO 10

5 5

0 0 0 10 20 30 40 0 10 20 30 40 MgO MgO Figure 4: MgO vs. selected major and trace elements for the gabbro and ultramafic sills. Symbols: black circles = Type-1 ultramafic; black triangles = Type-1 amphibolitic layer; black inverted triangles = Type-1 amphibolitic chill margin; open circles = Type-2 ultramafic; open triangles = Type-2 amphibolitic layer; open inverted triangles = Type-2 amphibolitic chill margin; black plusses = gabbro. 25

e 20 n Opx Olivi A l / 15 Type-1 M g ] e +

[ F 10 Cpx Type-2

5

0 0 5 10 15 20 25 Si /Al Figure 5: Pearce-type plot of [Mg+Fe]/Al vs. Si/Al. Symbols as in Figure 4.

Ol

Opx Cpx

Figure 6: Normative mineralogy for the ultramafic sills, the amphibolitic layers and the amphibolitic chilled margins. Symbols as in Figure 4. NESP granitoids a 100 e t i r d n o h C

10 S a m p l e /

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

NESP granitoids b 100 e t i r d n o h C 10 S a m p l e /

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 7: Chondrite-normalized REE profiles. a) Amphibolitic chill margin of the ultramafic sills. b) Gabbro sills. Symbols: black inverted triangles = Type-1 amphibolitic chill margin; open inverted triangles = Type-2 amphibolitic chill margin; black plusses = gabbro. Data for NESP granitoids recovered from the SIGEOM database (available at www.mrnf.gouv.qc.ca/english/products-services/mines.jsp). Values for chondrite are from Sun and McDonough (1989). Type-2 ultramafic sills disappear approximately 75 metres east of the BIF in the western limb of the Nuvvuagittuq belt, but are replaced by numerous gabbro sills. The gabbro sills are typically metres to tens of metres in width, are distinctly darker than their cummingtonite-amphibolite host, and also lack the latter’s ubiquitous quartz ribboning. The gabbros consist of coarse to medium grained hornblende - plagioclase - quartz ± orthopyroxene ± cummingtonite. They are commonly characterized by a fine-scale gneissosity defined by plagioclase-rich and amphibole-rich bands such that they were referred to as the “black and white” gneiss in the field. Despite their metamorphic mineral assemblage, the igneous term “gabbro” will be used to distinguish these dark hornblende-rich units from the dominant lighter coloured cummingtonite-rich faux-amphibolites of the belt. The gabbros are relatively uniform in terms of major and trace elements, with SiO2 ranging from 46 to 52 wt% and MgO from 10 to 5 wt%. (Figure 4 and Table 1). The gabbros have CaO contents (7.5 to 11.5 wt%) that are systematically higher than the faux- amphibolite at equivalent MgO contents. All of the gabbroic sills have flat to slightly depleted LREE profiles (Figure 7b).

The ratio of 147Sm/144Nd in the gabbros and of one of the Type-2 ultramafic sills range from 0.1519 to 0.2175, with calculated εNd values at 3.8 Ga ranging from -1.8 to +3.4 (Table 2), with the majority being positive. The gabbros and ultramafics define a coherent array that is dispersed along a calculated 3.8 Ga isochron in a plot of 147Sm/144Nd versus 143Nd/144Nd (Figure 8).

3.3 Banded iron formation and silica-formation

A banded iron formation (BIF) 5 to 30 meters in width can be traced continuously along the western limb of the belt and discontinuously along the eastern limb. The BIF is essentially a finely laminated quartz + magnetite + grunerite with thin alternating quartz-rich and magnetite-rich laminations of 0.1 to 1 cm in width (Figure 3f). Grunerite is preferentially associated with the oxide-rich laminae, although it also commonly occurs as disseminated grains in the quartz-rich laminae. Actinolite is also found and locally both have been replaced by minnesotaite. Garnetiferous quartz-rich horizons occur locally within and adjacent to the BIF and the silica-formation. In the southwestern corner of the belt, the iron formation transgresses from the lower grade greenstones, where the quartz-rich laminae have a distinct jasper-like colour, into the cummingtonite-bearing faux-amphibolites.

A large silica-formation occurs in the eastern limb of the belt that reaches 100 meters in width. It is composed almost entirely of massive recrystallized quartz with minor disseminated pyrite. At the southern-most edge of this limb, the silica-formation grades into BIF, suggesting that it may be a silica-rich facies of the BIF. Such interpretation is supported by the local presence of a thinner silica-rich unit (1 to 15 meters in width) adjacent to the BIF in the western limb of the belt.

16 Table 2. Sm-Nd isotopic data for Nuvvuagittuq’s gabbro and ultramafic sills. 147 144 143 144 Sample Rock Type Nd (ppm) Sm (ppm) Sm/ Nd Nd/ Nd 2σ error εNd(3.8 Ga)

por 21 Ultramafic 2.5 0.8 0.1963 0.512716 0.000009 1.75 por 25 Ultramafic 3.3 1.1 0.2069 0.512886 0.000010 -0.18 WP78 Gabbro 5.1 1.4 0.1637 0.511759 0.000010 -0.98 WP62b Gabbro 5.8 2.0 0.2125 0.513012 0.000010 -0.48 WP43 Gabbro 7.2 2.4 0.2020 0.512792 0.000009 0.43 WP42a Gabbro 7.5 2.5 0.2011 0.512807 0.000010 1.12 WP42c Gabbro 7.0 1.8 0.1519 0.511682 0.000010 3.38 WP47a Gabbro 5.4 1.8 0.2046 0.512838 0.000001 0.00 WP47b Gabbro 4.7 1.7 0.2175 0.513070 0.000001 -1.77 PC-81 Gabbro 5.2 1.8 0.2047 0.512872 0.000001 0.63 PC-83 Gabbro 5.0 1.7 0.2006 0.512797 0.000001 1.22 PC-85 Gabbro 7.2 2.3 0.1956 0.512722 0.000001 2.21 PC-89 Chill margin 0.9 0.3 0.2068 0.512997 0.000017 2.05 PC89-D Chill margin 0.8 0.3 0.2065 0.512985 0.000010 1.98 PC-93 Ultramafic 0.3 0.1 0.2154 0.513138 0.000031 0.57 PC-94 Ultramafic 0.4 0.2 0.2058 0.512899 0.000019 0.62

Samples for Nd analysis were crushed to powder form and dissolved with a HF-HNO3 mixture in high-pressure Teflon containers. A 149Sm-150Nd tracer was added to determine the Nd and Sm concentrations. The REE were concentrated by cation exchange chromatography and the Sm and Nd were extracted using an orthophosphoric acid-coated Teflon powder after Richard et al (1976). Sm and Nd isotopic ratios were measured on a VG SECTOR-54 mass spectrometer using triple filament assembly, in the GEOTOP laboratories at the Université du Québec à Montréal. Repeated measurements of LaJolla Nd standard yielded a value of 143Nd/144Nd = 0.511849 ± 12 (n=21). The total combined blank for Sm and Nd is less than 150 pg. The reported Sm and Nd concentrations and the 147Sm/144Nd ratios have accuracies of 0.5%, corresponding to an average error of 0.5 εNd unit for the initial Nd isotopic composition. 146Nd/144Nd was normalized to 0.7219 for mass fractionation corrections. 143 144 147 144 The reference value for Nd/ NdCHUR was taken to be 0.512638, while that for Sm/ NdCHUR was taken to be 0.1967, and the decay constant  for 147Sm was assumed to be 6.54 x 10-12 a-1.

0.5145

0.5140

3.8 Ga calculated isochron 0.5135

0.5130

4 4 1 2.7 Ga calculated

isochron

d N / d

N 0.5125

3 4 1

0.5120

0.5115

0.5110 0.100 0.150 0.200 0.250 147Sm / 1 4 4 Nd Figure 8: 147Sm/144Nd vs. 143Nd/144Nd for gabbro and ultramafic sills. Also shown are a 3.8 Ga and a 2.7 Ga reference isochrons

10

1 S A A

.1 S a m p l e / P

.01

.001 La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu Figure 9: Post-Archean Australian Shale (PAAS)-normalized REE+Y profiles for Nuvvuagittuq's BIFs. Symbols: open diamonds = PC-165; grey circles = PC-192; grey squares = PC-193; black circles = PC-194; grey plus = PC-197; grey crosses = PC- 198b; black plusses = PC-199; grey diamonds = PC-200; small light grey plusses = BIF from SW Greenland (Bolhar et al., 2004). The Nuvvuagittuq BIFs display concave-up depleted LREE profiles with relatively flat HREE profiles when normalized to post-Archean shales in a REE+Y plot. Their trace element profiles are characterized by strong positive Eu anomalies and weak positive Y anomalies (Figure 9), which are thought to be common features of Archean BIFs precipitated from seawater (Bolhar, 2004). Although the HREE profile of the silica-formation (sample PC-194) closely parallels those of the Fe-formation samples, exhibiting similar positive Eu and Y anomalies, their LREE are relatively unfractionated relative to PAAS.

Fe isotopic compositions were determined for both BIF samples and samples of the enclosing faux-amphibolite and gabbro (Table 3). All Nuvvuagittuq’s BIFs have heavier Fe isotopic compositions (FFe=0.25-0.48 ‰/amu) than the adjacent magmatic lithologies, whose Fe isotopic compositions range from 0 to 0.2 ‰/amu (Figure 10) (O’Neil et al., 2006). Sample PC-133, an amphibolite at the contact with the BIF gave an FFe value intermediate between those of the BIF and the enclosing mafic lithologies. These values are similar to those measured by Dauphas et al. (2007).

4. Discussion

4.1 Protolith of the cummingtonite-amphibolites

The dominance of amphiboles like hornblende and cummingtonite in most lithologies, along with the occurrence of metamorphic orthopyroxene and the abundance of garnet, suggest that the metamorphic conditions in the Nuvvuagittuq belt reached at least upper amphibolite facies. Most biotite is, however, altered to chlorite, indicating the existence of extensive retrograde metamorphic effects. The progression from the chlorite-epidote greenstones to garnet-free amphibolites and then to garnet-bearing amphibolites (Figure 2) suggests the presence of a map-scale metamorphic gradient from upper greenschist in the West to upper amphibolite facies in the East. This interpretation is supported by the observation that the BIF in the western limb of the Nuvvuagittuq belt cuts across this gradient. The highest temperatures obtained using the garnet-biotite geothermometer on the least altered biotite ranges from 550 to 600°C.

The compositions of the faux-amphibolites are similar to the gabbros and cluster along the gabbroic cotectic, as defined by MORB glasses (Figure 11). Moreover, the faux-amphibolites are compositionally different from Archean shales and thus they are interpreted to be meta-igneous rocks rather than metamorphosed shales. Although the faux-amphibolites are compositionally similar to the Nuvvuagittuq gabbro sills, the dominance of cummingtonite over hornblende in the faux-amphibolite appears to reflect their lower CaO contents (Figure 12). The abundance of garnet in the faux-amphibolites of the eastern limb of the Nuvvuagittuq belt may reflect

19 Table 3. Fe isotopic data for BIFs and igneous rocks from Nuvvuagittuq. 56 57 Sample δ Fe (‰) 2sd δ Fe (‰) 2sd FFe (‰/amu) 2sd

PC-101 0.00 0.02 0.03 0.02 0.00 0.01 PC-103 0.07 0.06 0.10 0.07 0.03 0.03 PC-110 0.07 0.03 0.09 0.02 0.03 0.01 PC-111 0.25 0.10 0.32 0.10 0.12 0.04 PC-114 0.09 0.05 0.12 0.05 0.04 0.02 PC-119 0.10 0.12 0.19 0.19 0.06 0.06 PC-119a 0.02 0.03 0.07 0.07 0.02 0.02 PC-123 0.03 0.01 0.06 0.01 0.02 0.01 PC-133 0.48 0.16 0.72 0.24 0.24 0.08 PC-133a 0.56 0.17 0.96 0.06 0.30 0.05 PC-140 0.36 0.21 0.58 0.34 0.18 0.11 PC-144 0.36 0.22 0.52 0.22 0.18 0.09 PC-146 0.19 0.06 0.33 0.06 0.10 0.03 PC-146a 0.38 0.10 0.54 0.21 0.18 0.06 PC-147 0.30 0.16 0.45 0.25 0.15 0.08 PC-149 -0.04 0.11 0.10 0.06 0.01 0.04 PC-150 0.37 0.09 0.67 0.13 0.20 0.05 PC-187A 0.10 0.14 0.14 0.10 0.05 0.05 PC-187B 0.12 0.07 0.19 0.05 0.06 0.03 PC-189 0.06 0.05 0.09 0.11 0.03 0.03 PC-190 0.22 0.10 0.32 0.09 0.11 0.04 PC-194 0.17 0.15 0.29 0.25 0.09 0.08 PC-195 0.26 0.30 0.35 0.54 0.12 0.17 PC-191 0.59 0.18 0.86 0.38 0.29 0.11 PC-192 0.71 0.10 0.97 0.16 0.34 0.05 PC-193 0.67 0.19 1.00 0.40 0.34 0.12 PC-197 0.97 0.10 1.42 0.16 0.48 0.05 PC-198B 0.57 0.16 0.83 0.21 0.28 0.07 PC-199 0.49 0.12 0.74 0.24 0.25 0.07 PC-200 0.62 0.13 0.90 0.17 0.30 0.06

Samples for Fe isotope analysis were digested using HNO3-HF and HNO3-HClO4 mixtures as well as HCl in Teflon containers. Fe was extracted using AGMP-1 anion exchange chemistry in HCl media following a procedure similar to that of Dauphas et al. (2004a). Fe were measured using a VG Multicollector-ICPMS Isoprobe at the GEOTOP laboratories. A standard-sample-standard analytical protocol was used to correct for mass bias using the IRMM014 international isotopic standard as reference. The reference materials used for precision and accuracy were BCR-1 (), AC-E (), and IF-G (iron formation from Isua, Greenland). FFe for reference materials gave: BCR-1 = 0.07±0.02 ‰/amu (N=2 measurements), AC-E = 0.17±0.09 ‰/amu (N=2 measurements) and IF-G = 0.31±0.04 ‰/amu (N=3 measurements). Standard values are similar to those previously published (Beard et al., 2003; Butler et al., 2005; Dauphas and Rouxel, i 2006; Poitrasson et al., 2004; Rouxel et al., 2005). FFe = δ j /(i-j), and j = 54 56 57 58 i i j i j Fe, i equals either Fe, Fe, or Fe, and δ j = ((Fe/ Fe)/( Fe/ Fe)standard -1) × 1000, with IRMM-014 as the reference standard. a: Duplicate.

BIF N u v a g i t q I n c r e a s i g d t f o m B F

Magmatic rocks BIF S W G r e n l a d

-0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60

FF E (‰/amu) Figure 10: Fe isotopic composition of the BIFs and surrounding lithologies expressed in FFe (‰/amu). Symbols : black circles = BIF; grey circle = garnet-bearing BIF; black plusses = gabbro; open circle = ultramafic; black asterisk = silica-formation; open triangles = amphibolitic sill margin and layer; open diamond = cummingtonite-rich faux-amphibolite; grey square = biotite-rich faux-amphibolite; open square with black plus = tonalite; open square with black cross = felsic band. SW Greenland data are from Dauphas et al. (2004b). 20

Gabbroic cotectic

MgO 10 B

AAS P NASC S

0 40 50 60 70 80 SiO2

Figure 11: SiO2 vs. MgO for Nuvvuagittuq's faux-amphibolite and gabbros. Symbols: black plusses = gabbro; open diamonds = cummingtonite-rich faux-amphibolite; grey squares = biotite-rich faux-amphibolite; black squares = garnet-bearing faux-amphibolite; open crosses = greenstone; small grey crosses = Archean shales from literature; B = average Archean basalt; S = average Archean shales. Data for shales are from Feng and Kerrich (1990), Hofmann et al. (2003), Bolhar et al. (2005), Fedo et al. (1996) and Wronkiewicz and Condie (1987). Data for average Archean shales and are from Condie (1993). The gabbroic cotectic is from MORB 22-25 North gasses (data from Bryan (1981)). CaO

Al2O3 MgO Figure 12: CaO - Al2O3 - MgO ternary diagram for the faux-amphibolites and gabbros. Symbols as in Figure 11.

25

Olivin

e 20

15 Fe Type-1 sill 10 ~ Fo 88

~ Fo 91 5 Type-2 sill

0 20 30 40 50 60 70 Mg Figure 13: Fe vs. Mg in cation units for Type-1 and Type-2 ultramafic. Symbols as in Figure 4. their systematically lower Mg number and higher Al content compared to the garnet-free faux-amphibolite. The abundance of quartz ribboning within the faux- amphibolite suggests that Si and probably other elements such as Ca were relatively mobile in the faux-amphibolite during metamorphism. These features combined with the fine compositional layering of the faux-amphibolite suggest that the faux-amphibolite may represent mafic pyroclastic deposits that were more susceptible to alteration and than the more massive gabbroic sills that intrude them. According to this interpretation, loss of Ca during alteration and/or metamorphism favoured the formation of cummingtonite in the faux-amphibolites over hornblende in the gabbros, with the more magmatically evolved faux-amphibolite compositions developing garnet because of their higher Al and Fe contents.

4.2 Significance of the ultramafic and gabbro sills for the mantle at 3.8 Ga

A comparison of the REE profiles of the Nuvvuagittuq gabbros with the LREE- enriched profiles of over 500 NESP granitoids argues against any significant interaction with surrounding felsic crust (Figure 7). The slightly fractionated LREE profiles of the chilled margin of the ultramafic sills indicate possible interaction with the host faux-amphibolites. The similarity between the HREE profiles of the gabbros and the chill margins of the Type-2 sills suggests that they are cogenetic, an interpretation supported by field evidence that gabbroic tops are best developed on Type-2 sills.

Although it is possible that the ultramafic and gabbro sills are somewhat younger than the faux-amphibolite, a number of observations argue that they are comagmatic feeders to the Nuvvuagittuq volcanic succession, despite their chemical differences due to alteration. First, there is a systematic progression in the western limb of the Nuvvuagittuq belt from west to east: tonalite, Type-1 ultramafic sills, BIF, Type-2 ultramafic sills and gabbro sills (Figure 2). This sequence is mirrored in the eastern limb of the belt, although with many fewer sills. The consistency of this sequence suggests that, despite the complexities of deformation, the original volcanic of the belt with its comagmatic sills, is preserved. Second, the felsic unit that yielded an age of 3.8 Ga occurs as a structurally conformable layer within a gabbro sill that can be traced around outcrop-scale folds for many meters. This felsic unit is either coeval with its gabbro host, or intrudes it.

The different compositions and mineralogy of the Type-1 and Type-2 ultramafic sills suggest that they were derived from distinct magmas. Since the cumulate rocks of both types of ultramafic sills define olivine control lines, it is possible to estimate the composition of the parental magmas of both sill types. The parental liquids for each sill type were calculated by mathematically extracting the olivine defined by their olivine control lines (~Fo88 for Type-1 sill and ~Fo91 for Type-2

24 sill) (Figure 13) until the remaining composition would be in equilibrium with the extracted olivine if it were a liquid, assuming an Fe/Mg KD ([Fe/Mg]ol / [Fe/Mg]liq) of 0.3. The calculated liquids range from komatiitic basalt to komatiite in composition with the average calculated liquid being komatiite for both sill types (Table 4). Although the compositions of the calculated liquids for both sill types have similar MgO contents (18-20 wt.%), they have distinctly different Al contents (Table 4, Figure 14). The estimated composition of the parental magma for the Type-1 sills is similar to that of aluminum-depleted komatiite (ADK; ~ 6 wt% Al2O3), while that of the Type-2 sills resembles that of aluminum-undepleted komatiite (AUK; > 10 wt% Al2O3). The presence of both ADK and AUK in the same 3.8 Ga volcanic sequence contradicts the commonly held view that there has been a secular evolution from ADK to AUK during the Archean (Figure 14; Francis, 2003).

A number of early Archean rocks from southern West Greenland, Labrador and South Africa have yielded positive initial εNd values (up to +4 at 3.8 Ga) requiring an early and rapid depletion of the Earth’s mantle (Collerson et al., 1991; Bennett et al., 1993; McCulloch and Bennett 1994; Brandl and de Wit, 1997; McCulloch and Bennett, 1998; Blichert-Toft et al., 1999). The Sm-Nd isotopic compositions of the Nuvvuagittuq rocks also support an early depletion of the mantle with gabbro and ultramafic sills yielding εNd values as high as +3 (Figure 15). Although the preliminary results indicate that the Nuvvuagittuq rocks are not as depleted as the 3.8 Ga rocks of SW Greenland (Bennett et al., 1993; Blichert-Toft et al., 1999), εNd values greater than +1.9 in a mantle source at 3.8 Ga require a significant earlier depletion equivalent to that seen in the present-day MORB source. The more positive εNd values commonly obtained in mantle-derived rocks of Eoarchean supracrustal sequences imply that the Earth’s mantle had already experienced an even more extensive trace element depletion well before 3.8 Ga. For example, an εNd of +4 at 3.8 Ga would require a trace element depletion factor more than twice the average value for the present-day depleted mantle. Such a depletion is not supported by the flat to slightly depleted REE profiles of the Nuvvuagittuq’s gabbros (Figure 7b) and the general lack of evidence for extensive trace element depletion in Eoarchean mantle-derived rocks is a continuing puzzle.

4.3 Significance of banded iron formation

Although iron formations are generally thought to represent chemical precipitates produced by marine exhalations (Graf, 1978; Uitterdijk Appel, 1983; Gross, 1983; Jacobsen and Pimentel-Klose, 1988; Olivarez and Owen, 1991), the causes of Fe precipitation throughout geologic time are less well understood. The Superior-type banded Fe-formations are believed to be formed by oxidation on shallow continental shelves of Fe2+ rising from deep ocean basins. This scenario cannot, however, explain the Algoma-type Fe-formations that typify

25 Table 4. Major (wt.%) and trace (ppm) elements for the average calculated liquids for the Nuvvuagittuq’s ultramafic sills.

Type-1 Sill Type-2 Sill average average calculated liquid calculated liquid

SiO2 49.85 50.55

TiO2 0.67 0.33

Al2O3 5.92 11.45 MgO 19.22 18.22 FeO 15.04 9.82 MnO 0.24 0.17 CaO 8.13 6.85

Na2O 0.24 0.27

K2O 0.07 0.09

P2O5 0.04 0.04

Rb 8.6 8.7 Sr 19 38 Zr 19 29 Nb 7.4 6.2 Y21 12 Ni 672 1236 Cr 3070 13330 V 168 164 Co 155 248 1.8-2.4 Ga Komatiites

15 10 5 20 20 0.09 Ga Composition change Komatiites through time?

15 AUK 2.7 Ga 10 Komatiites A l C a t i o n s

5 ADK 3.4 Ga Komatiites

35 40 45 50 55 Si Cations

Figure 14: Al vs. Si in cation units for Type-1 and Type-2 ultramafic and for Type-1 and Type-2 calculated liquid. Symbols: black circles = Type-1 ultramafic; black squares = Type-1 calculated liquid; open circles = Type-2 ultramafic; open squares = Type-2 calculated liquid. Grey grid indicates the pressures in kbar of experimental partial melts of primitive mantle Data are from Francis (2003) and Casey (1997). Early depleted mantle ? +5

DM εNd

0 CHUR

-5 3000 3500 4000 4500 Time (Ma) Figure 15: Nd vs. time diagram. Symbols: black circles = rocks from this study; grey plusses = Nuvvuagittuq's rocks from Stevenson and Bizzarro (2005); open diamonds = data from Jahn et al. (1982), Cloué-Long et al. (1984), Grau et al. (1987), Hamilton et al. (1987), Wilson and Carlson (1989), Collerson et al. (1991), Bennett et al. (1993), Vervoot et al. (1996) and Blichert-Toft et al. (1999). DM (depleted mantle) curve is modeled after DePaolo (1981). Chondritic ratios used for Nd are 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1966. earlier Archean greenstone belts, because they must have been precipitated under anoxic conditions. It has recently been discovered that Fe2+ can be directly oxidized by microbial activity under anaerobic conditions (Lovley et al., 1987; Widdel et al., 1993) Konhauser et al. (2002) have gone so far as to even proposed that Algoma-type banded Fe-formations were precipitated by the activity of anoxic bacteria in the Archean. The origin of life on Earth and its evolution over time has been highly debated in past years (Mojzsis et al., 1996; Fedo and Whitehouse, 2002a, b; Mojzsis and Harrisson, 2002; Lepland et al., 2005; Moorbath, 2005), and establishing the chemical sedimentary origin for a rock is a prerequisite to demonstrating potential biological activity and addressing the controversial question of the timing of the origin of life on Earth. The concave-up depleted LREE profile of the Nuvvuagittuq BIF, combined with its flat HREE profile and positive Eu and Y anomalies with respect to PAAS, are characteristic features of Archean Algoma-type banded Fe-formations of marine exhalite origin (Fryer, 1977; Graf Jr., 1978; Fryer et al., 1979; Jacobsen and Pimentel-Klose, 1988). Moreover, the REE+Y profiles for the Nuvvuagittuq’s BIFs are similar to BIFs from SW Greenland (Bolhar et al., 2004) that are interpreted to have a distinct seawater signature (Figure 9).

The Nuvvuagittuq’s BIFs have heavier Fe isotopic compositions than the surrounding igneous lithologies (Figure 10), a feature that is also consistent with an origin as a chemical precipitate. The Fe isotopic compositions of the Nuvvuagittuq’s BIF are similar to those in the Akilia BIF (SW Greenland), which are also enriched in heavy Fe isotopes (0.1 to 0.5‰/amu) relative to their surrounding igneous lithologies (Dauphas et al., 2004b), suggesting a common depositional process for both these example of Eoarchean BIF. Although the Fe isotopic enrichment observed in the amphibolites directly adjacent to the banded Fe-formation suggest some degree of local exchange, the fact that such diverse meta-igneous lithologies, including the faux-amphibolites that are interpreted to have been altered, the amphibolitic sill margins, the gabbro and ultramafic sills, as well as the tonalite and the felsic bands, all share the same Fe isotopic composition suggests that the heavy Fe isotopic enrichment displayed by the Nuvvuagittuq BIF is not a metasomatic or alteration feature. The REE+Y profiles and the Fe isotopic compositions of Nuvvuagittuq’s BIF confirm their origin as marine exhalites and, although the mechanism(s) responsible are not well understood, the Fe isotopic fractionation observed in the BIF of Nuvvuagittuq and Akilia raises the possibility that life was already established on Earth at 3.8 Ga.

29 5. Conclusion

The newly discovered 3.8 Ga Nuvvuagittuq greenstone belt represents one of the Earth’s oldest mafic mantle-derived supracrustal suites and constitutes an important constraint for modelling the evolution of the early Earth. The Nuvvuagittuq greenstone belt differs from other greenstone belts in the Northeastern Superior Province in that it is dominated by amphibolites composed mainly of cummingtonite, unlike typical Archean amphibolites that are dominated by hornblende. This feature is thought to reflect the loss of Ca in altered and metamorphosed mafic pyroclastic rocks. The chemical compositions of the calculated parental liquids for the ultramafic sills that intrude these rocks indicate that both aluminium-depleted komatiite (ADK) and aluminium-undepleted komatiite (AUK) magmas were present at 3.8 Ga, arguing against a temporal evolution from ADK to AUK during the Archean. Despite their relatively undepleted trace element profiles, most Nuvvuagittuq’s mafic and ultramafic rocks display positive εNd values, implying derivation from a depleted mantle source and supporting evidence from other >3.6 Ga supracrustal suites for early depletion of the Earth’s mantle. The concave-up LREE profiles, positive Eu and Y anomalies, and the heavy Fe isotopic enrichment in the Nuvvuagittuq BIF confirms its origin as a marine chemical precipitate and, along with similar rocks in SW Greenland, may indicate that life was already established on the Earth at 3.8 Ga.

Acknowledgements

This research was supported by National Science and Engineering Research Council of Canada (NSERC) Discovery Grants to Francis (RGPIN 7977-00). The whole rock XRF analyses were performed by Glenna Keating and Tariq Ahmedali, the thin sections were made by George Panagiotidis, and the Microprobe work for the garnet-biotite geothermometer was performed by Lang Shi at McGill University. We would like to thank the municipality of Inukjuak and the Pituvik Landholding Corporation for permission to work on their territory. We also thank the Inukjuak community and especially Mike Carroll, Valerie Inukpuk Morkill, Rebecca Kasudluak, and Johnny Williams for their hospitality and support. We thank Witold Ciolkiewicz and Alexandre Jean for assistance in the field and Shoshana Goldstein for late night discussions in the office. GEOTOP publication n° 2007-0006.

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37