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’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 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 = -0.18 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 (FFe=0.25- 0.48 ‰/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. 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 3.6-3.85 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 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 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 Canada Ungava Orogen Huds on S Cape Smith Belt trait Arnaud River Ungava Terrane 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 terranes 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 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. 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 craton, 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 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 Top right corner map N Tonalite Faux-amphibolite 6465000mN BIF GRT GRT Silica-formation Out In Greenstone Bottom map Ultramafic & Gabbro sill E Gabbro sill E 6464000mN m m 0 0 0 0 0 0 0 1 4 4 3 Ultramafic sill 3 Boundary where garnet becomes ubiquitous Pegmatite Synform axial trace Attitude of contact Fault Attitude of schistosity 6463000mN 300 m.