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

Transcription

1 Earth s Oldest Rocks Edited by Martin J. van Kranendonk, R. Hugh Smithies and Vickie C. Bennett Developments in Precambrian Geology, Vol. 15 (K.C. Condie, Series Editor) 2007 Elsevier B.V. All rights reserved. THE GEOLOGY OF THE 3.8 GA NUVVUAGITTUQ (PORPOISE COVE) GREENSTONE BELT, NORTHEASTERN SUPERIOR PROVINCE, CANADA. Jonathan O Neil 1, Charles Maurice 2, Ross K. Stevenson 3,4, Jeff Larocque 5, Christophe Cloquet 6, Jean David 3 & Don Francis 1 1 Earth & Planetary Sciences, McGill University and GÉOTOP-UQÀM-McGill, 3450 University St. Montreal, QC, Canada, H3A 2A7 (oneil_jo@eps.mcgill.ca) 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 coast of Hudson Bay. The dominant lithology of the belt is a quartz-ribboned grey amphibolite composed of variable proportions of cummingtonite, biotite and plagioclase. Although the amphibolites have mafic compositions, the presence of cummingtonite rather than hornblende in these rocks reflects their low Ca contents, which may result from the alteration and metamorphism 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 magmas for both types of sills are komatiites. The estimated parental magma of the Type-1 sills is equivalent to an Al-depleted komatiite (ADK), while that of Type-2 sills is an Al-undepleted komatiite (AUK). Gabbro sills have flat to slightly depleted REE profiles, indicating a lack of interaction with a pre-existing felsic crust. The Nd isotopic compositions of the Nuvvuagittuq s rocks (εnd = to +3.4), however, indicate derivation from a mantle 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 iron 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 (F Fe = /amu) compared to the adjacent gabbros and amphibolites, consistent with an origin as a chemical precipitate origin and possibly indicative of the action of biological activity at 3.8 Ga.

2 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. Zircons from the Jack Hills conglomerates (Wilde et al., 2001) suggest the existence of continental crust as old as 4.4 Ga. These early ages, however, are obtained on detrital zircons from much younger rocks, whose protolith 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 Eoarchean mantle-derived crust provide the best compositional and isotopic constraints on early crust-mantle differentiation of the Earth. The Ga Itsaq gneiss complex (West Greenland), comprising the Isua greenstone belt, is the most extensive early Archean terrain preserved. The Nd isotopic compositions for these mantlederived 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 Hadean. 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 banded iron formation, 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

3 Canada Ungava Orogen Cape Smith Belt Hudson Strait Arnaud River Terrane Ungava Bay Hudson Bay Nuvvagittuq Belt New Quebec Orogen Churchill Southeast Hudson Bay Terrane Labrador Kilometres Figure 1: Location map of the Nuvvuagittuq greenstone belt in the Northeastern Superior Province. Isotopic terranes from Boily et al. (2006), Leclair (2005) and Leclair et al. (2006).

4 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 ( Ga), as determined by ~220 U-Pb zircon 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 Ga and characterized by juvenile isotopic signatures (Nd T DM < 3.0 Ga). To the West, rocks of the Hudson Bay Terrane, which includes the Nuvvuagittuq greenstone belt, represent a reworked Meso- to Eoarchean craton, with zircon inheritance ages and Nd depleted-mantle model ages (T DM ) 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 volcano-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 tonalite, 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 tuff. U-Pb ages obtained on zircons from one of these felsic bands, a plagioclase-quartz-biotite schist 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

5 Top right corner map Tonalite Faux-amphibolite N mN BIF Silica-formation GRT Out GRT In Greenstone Ultramafic & Gabbro sill Boundary where garnet becomes ubiquitous Bottom map Gabbro sill Ultramafic sill Pegmatite mE mE mN Synform axial trace Fault Attitude of contact Attitude of schistosity Hudson Bay 300 m mN 60 GRT Out GRT In m Figure 2: Geological map of the Nuvvuagittuq greenstone belt. Coordinates in UTM NAD27, Zone 18

6 Ma based on 206 Pb/ 207 Pb 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 tonalites makes it unlikely that these felsic bands represent remobilized tonalite. 3.1 Cummingtonite-amphibolite Cummingtonite-amphibolites are the predominant lithologies 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 texture 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 biotiterich 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 garnets 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 greenschist facies mineral assemblage of chlorite + epidote + quartz + plagioclase ± actinolite ± carbonate. The abundance of garnet in the faux-amphibolites of the eastern limb of the belt, along with the compositional layering, lead 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 SiO 2 (40-56 wt% SiO 2, 4-16 wt% MgO) (Table 1) contents, similar to those of the Nuvvuagittuq s gabbro sills and greenstones. The garnet-bearing fauxamphibolites are compositionally similar to the biotite and cummingtonite-rich 6

7 b a Felsic band biotite-rich { { cummingtonite-rich d c Amphibolitic margin Garnet-poor Garnet-rich ib Ga b sil bro l am xfa u Ga b sil bro l ph ph m -a f ib ol ol ite ite e Fa ux Ultramafic sill Figure 3: Photos of Nuvvuagittuq's rocks. a) Felsic band from which the 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 fauxamphibolite. f) Banded iron formation with alternating quartz-rich and magnetite-rich laminations.

8 Table 1. Major (wt.%) and trace (ppm) element data for Nuvvuagittuq rocks Major elements Sample SiO 2 TiO 2 Al 2 O 3 MgO FeO MnO CaO Na 2 O K 2 O P 2 O 5 LOI UTM Easting UTM Northing Cummingtonite-rich Faux-Amphibolite PC PC PC PC PC PC PC PC PC-173A PC-173B Biotite-rich Faux-Amphibolite PC PC PC PC PC Garnet-bearing Faux-Amphibolite PC PC PC PC PC PC Greenstone PC PC PC PC PC PC PC PC PC PC PC Gabbro PC PC PC PC PC PC PC PC PC PC PC PC PC

9 Table 1 (continued) Major elements Sample SiO 2 TiO 2 Al 2 O 3 MgO FeO MnO CaO Na 2 O K 2 O P 2 O 5 LOI UTM Easting UTM Northing Ultramafic Type-1 PC PC PC PC PC PC PC PC PC PC A A A Type-1 Amphibolitic chill margin M and layer L PC-15 M PC-33 M PC-110 M PC-20 L PC-21 L Ultramafic Type-2 PC PC PC PC PC PC PC PC PC PC PC Type-2 Amphibolitic chill margin M and layer L PC-71 M PC-142 M PC-88 L BIF PC PC PC PC-198B PC PC Si-Formation PC Grt Si-rich unit PC

10 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 PC PC PC PC PC PC PC PC-173A PC-173B Biotite-rich Faux-Amphibolite PC PC PC PC PC Garnet-bearing Faux-Amphibolite PC PC PC PC PC PC Greenstone PC PC PC PC PC PC PC PC PC PC PC Gabbro PC PC PC PC PC PC PC PC PC PC PC PC PC

11 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 PC PC PC PC PC PC PC PC PC A A A Type-1 Amphibolitic chill margin M and layer L PC-15 M PC-33 M PC-110 M PC-20 L PC-21 L Type-2 ultramafic PC PC PC PC PC PC PC PC PC PC PC Type-2 Amphibolitic chill margin M and layer L PC-71 M PC-142 M PC-88 L BIF PC PC PC PC-198B PC PC Si-Formation PC Grt Si-rich unit PC 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.

12 facies, but have lower Mg numbers and higher Al 2 O 3 contents. Both the garnetbearing 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 SiO 2 contents than the faux-amphibolites at similar MgO contents. The relatively high loss on ignition (LOI) (4 10 wt%) and K 2 O 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 fauxamphibolite (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 lava 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 amphibole-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 interiors 2 are thought to have been pyroxene 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 olivine 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

13 a 6000 b Al 2 O Cr MgO MgO c 15 d FeO 10 CaO 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.

14 25 20 Olivine Opx Mg] Al / 15 Type-1 [Fe + 10 Type-2 Cpx 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.

15 NESP granitoids a 100 Sample/Chondrite 10 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu NESP granitoids b 100 Sample/Chondrite 10 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 Values for chondrite are from Sun and McDonough (1989).

16 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 SiO 2 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 fauxamphibolite at equivalent MgO contents. All of the gabbroic sills have flat to slightly depleted LREE profiles (Figure 7b). The ratio of 147 Sm/ 144 Nd in the gabbros and peridotites of one of the Type-2 ultramafic sills range from to , 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 147 Sm/ 144 Nd versus 143 Nd/ 144 Nd (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 rock 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 amphiboles 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

17 Table 2. Sm-Nd isotopic data for Nuvvuagittuq s gabbro and ultramafic sills. Sample Rock Type Nd (ppm) Sm (ppm) 147 Sm/ 144 Nd 143 Nd/ 144 Nd 2σ error εnd (3.8 Ga) por 21 Ultramafic por 25 Ultramafic WP78 Gabbro WP62b Gabbro WP43 Gabbro WP42a Gabbro WP42c Gabbro WP47a Gabbro WP47b Gabbro PC-81 Gabbro PC-83 Gabbro PC-85 Gabbro PC-89 Chill margin PC89-D Chill margin PC-93 Ultramafic PC-94 Ultramafic Samples for Nd isotope analysis were crushed to powder form and dissolved with a HF-HNO 3 mixture in high-pressure Teflon containers. A 149 Sm- 150 Nd 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 143 Nd/ 144 Nd = ± 12 (n=21). The total combined blank for Sm and Nd is less than 150 pg. The reported Sm and Nd concentrations and the 147 Sm/ 144 Nd ratios have accuracies of 0.5%, corresponding to an average error of 0.5 εnd unit for the initial Nd isotopic composition. 146 Nd/ 144 Nd was normalized to for mass fractionation corrections. The reference value for 143 Nd/ 144 Nd CHUR was taken to be , while that for 147 Sm/ 144 Nd CHUR was taken to be , and the decay constant for 147 Sm was assumed to be 6.54 x a -1.

18 Nd / Nd Ga calculated isochron Ga calculated isochron Sm / 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 144 Nd 10 1 Sample / PAAS 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).

19 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 (F Fe = /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 F Fe 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

20 Table 3. Fe isotopic data for BIFs and igneous rocks from Nuvvuagittuq. Sample δ 56 Fe ( ) 2sd δ 57 Fe ( ) 2sd F Fe ( /amu) 2sd PC PC PC PC PC PC PC-119 a PC PC PC-133 a PC PC PC PC-146 a PC PC PC PC-187A PC-187B PC PC PC PC PC PC PC PC PC-198B PC PC Samples for Fe isotope analysis were digested using HNO 3 -HF and HNO 3 -HClO 4 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 isotopes 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 (basalt), AC-E (granite), and IF-G (iron formation from Isua, Greenland). F Fe 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, 2006; Poitrasson et al., 2004; Rouxel et al., 2005). F Fe = δ i j /(i-j), and j = 54 Fe, i equals either 56 Fe, 57 Fe, or 58 Fe, and δ i j = (( i Fe/ j Fe)/( i Fe/ j Fe) standard -1) 1000, with IRMM-014 as the reference standard. a : Duplicate.

21 BIF Magmatic rocks BIF Increasing distance from BIF SW Greenland Nuvvuagittuq F FE ( /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).

22 20 Gabbroic cotectic MgO 10 B S PAAS NASC SiO 2 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 basalts are from Condie (1993). The gabbroic cotectic is from MORB North gasses (data from Bryan (1981)).

23 CaO Al 2 O 3 MgO Figure 12: CaO - Al2O3 - MgO ternary diagram for the faux-amphibolites and gabbros. Symbols as in Figure Olivine 15 Fe 10 5 Type-1 sill Type-2 sill ~ Fo 88 ~ Fo Mg Figure 13: Fe vs. Mg in cation units for Type-1 and Type-2 ultramafic. Symbols as in Figure 4.

24 their systematically lower Mg number and higher Al content compared to the garnet-free faux-amphibolite. The abundance of quartz ribboning within the fauxamphibolite 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 metasomatism 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 LREEenriched 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 stratigraphy 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 (~Fo 88 for Type-1 sill and ~Fo 91 for Type-2 24

25 sill) (Figure 13) until the remaining composition would be in equilibrium with the extracted olivine if it were a liquid, assuming an Fe/Mg K D ([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% Al 2 O 3 ), while that of the Type-2 sills resembles that of aluminum-undepleted komatiite (AUK; > 10 wt% Al 2 O 3 ). 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 Proterozoic Superior-type banded Fe-formations are believed to be formed by oxidation on shallow continental shelves of Fe 2+ rising from deep ocean basins. This scenario cannot, however, explain the Algoma-type Fe-formations that typify 25