Lithos 113 (2009) 133–157 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Geochemistry and geodynamic origin of the Mesoarchean Ujarassuit and Ivisaartoq greenstone belts, SW Greenland J.C. Ordóñez-Calderón a,⁎, A. Polat a, B.J. Fryer a,b, P.W.U. Appel c, J.A.M. van Gool c, Y. Dilek d, J.E. Gagnon a,b a Department of Earth and Environmental Sciences, University of Windsor, Windsor, ON, Canada N9B 3P4 b Great Lakes Institute for Environmental Research, University of Windsor, Windsor, ON, Canada N9B 3P4 c Geological Survey of Denmark and Greenland, 1350 Copenhagen, Denmark d Department of Geology, Miami University, Oxford, OH 45056, USA article info abstract Article history: The Ivisaartoq (ca. 3075 Ma) and Ujarassuit (ca. 3070 Ma) greenstone belts are the largest Mesoarchean Received 23 May 2008 supracrustal lithotectonic assemblages in the Nuuk region, SW Greenland. Both greenstone belts underwent Accepted 5 November 2008 polyphase deformation and amphibolite facies metamorphism, and were in due course variously Available online 21 November 2008 dismembered. Pillow lavas, pillow breccia, magmatic layering, and relic sedimentary structures are well preserved in the Ivisaartoq belt. Volcanic rocks include basalts, and minor andesites and picrites. Boninite- Keywords: like rocks occur in the Ujarassuit belt. Metasedimentary rocks in the Ujarassuit belt occur as thin layers (0.5– Mesoarchean oceanic crust Greenstone belt 1.0 m) of biotite schists, and in the Ivisaartoq belt as ~500 m-thick biotite schists intercalated with minor Supra-subduction zone quartzitic gneisses. There is no field evidence indicating that the Ivisaartoq and Ujarassuit supracrustal rocks Amphibolite were deposited on older continental basement, and their volcanic rocks do not exhibit any geochemical Boninite trends indicating contamination by Archean upper continental crust. Picrite Four groups of amphibolites (metavolcanic rocks) were recognized in the Ujarassuit greenstone belt on the basis of their REE and HFSE characteristics: (1) Group 1 is characterized by near-flat REE patterns (La/Smcn=0.77–1.14; La/Ybcn=0.84–1.24) and moderate negative Nb anomalies (Nb/Nb⁎=0.60–0.79); (2) Group 2 displays LREE-depleted patterns (La/Smcn=0.53–1.02; La/Ybcn=0.32–0.61) and pronounced negative Nb anomalies (Nb/Nb⁎= 0.32–0.67); (3) Group 3 consists of LREE depleted patterns (La/Smcn =0.69–0.84; La/Ybcn =0.55–0.91) and absence of significant Nb anomalies (Nb/Nb⁎=0.92–1.15); and (4) Group 4 has concave-upward REE patterns (La/Smcn =1.64–2.42, Gd/Ybcn =0.57–1.01) and large negative Nb anomalies (Nb/Nb⁎=0.28–0.42). The depletion of LREE in Group 2 amphibolites appears to have resulted from mobility of these elements during metamorphism. Group 1, 3, and 4 amphibolites retain their near-primary geochemical signatures and their trace element patterns are comparable to those of Phanerozoic oceanic island arc tholeiites (IAT), normal-mid-ocean ridge basalts (N-MORB), and boninites, respectively. The trace element patterns of the least altered meta-ultramafic rocks (La/Ybcn=2.48–1.35; Nb/Nb⁎=0.31–0.60) are comparable to those of modern subduction-related picrites. Amphibolites with an andesitic composition show large negative Nb anomalies (Nb/Nb⁎=0.21–0.55), enriched LREE patterns (La/Ybcn=6.29–15.64), and fractionated HREE (Gd/Ybcn=2.61–3.12) implying deep melting in equilibrium with residual garnet in the source. Biotite schists and quartzitic gneisses have low chemical indexes of alteration values (CIA=46 to 62) and trace element characteristics indicating volcaniclastic sedimentary protoliths derived from poorly weathered felsic to mafic source rocks. Collectively, the geochemical features of metavolcanic and metasedimentary rocks suggest that the Ivisaartoq and Ujarassuit greenstone belts represent dismembered fragments of Mesoarchean supra- subduction zone oceanic crust formed in an arc-forearc-backarc tectonic setting. © 2008 Elsevier B.V. All rights reserved. 1. Introduction ultramafic volcanic rocks. The lithogeochemical diversity of Archean greenstone belts is the product of complex igneous and sedimentolo- Archean greenstone belts are composed dominantly of volcanic gical processes operating in a wide variety of tectonic settings either at rocks and include a significant fraction of siliciclastic, volcaniclastic, and intra-oceanic or intra-continental environments (Eriksson and Catu- chemical sedimentary rocks (Condie, 1994). Basaltic lava flows are neanu, 2004). Accordingly, it has been suggested that greenstone belts prevalent and are commonly associated with felsic, intermediate, and may represent the volcanic remnants of Archean intra-oceanic island arcs, active continental margins, mid-ocean ridges, large igneous ⁎ Corresponding author. Tel.: +1 519 253 3000x2486. provinces, and intra-continental rifts (Bickle et al., 1994; Ohta et al., E-mail address: [email protected] (J.C. Ordóñez-Calderón). 1996; Polat et al., 1998; Polat and Kerrich, 2001; Hartlaub et al., 2004; 0024-4937/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2008.11.005 134 J.C. Ordóñez-Calderón et al. / Lithos 113 (2009) 133–157 Fig. 1. Geological map of the Ivisaartoq and Ujarassuit greenstone belts with approximate sample locations (modified from Friend and Nutman, 2005 after Chadwick and Coe, 1988). The location of field photographs are also indicated (Figs. 2a–h and 3a–h). Structural observations and sampling of metavolcanic and metasedimentary rocks were conducted in the eastern and western flanks of the Ujarassuit greenstone belt. The lower metasedimentary unit of the Ivisaartoq greenstone belt was also sampled. Sandeman et al., 2004; Thurston and Ayres, 2004; Kusky, 2004). The geodynamic origin of the Archean oceanic crust. In addition, given that recognition of intra-oceanic Archean greenstone belts is of funda- oceanic volcanic rocks are less susceptible to crustal contamination mental importance because they provide invaluable information on the than greenstone belts deposited on continental crust, they also provide Table 1 Mineralogical characteristics and interpreted protoliths for the main rock types discussed in this study. Lithology Mineralogical composition Protolith Hornblende-rich amphibolites Hornblende+plagioclase+quartz+zircon+titanite+ Basaltic rocks. Groups 1–2 amphibolites represent island (Groups 1–3 amphibolites) apatite±magnetite±garnet±biotite±cummingtonite arc tholeiites (IAT). Group 3 amphibolites represent rare N-MORB-like tholeiites Cummingtonite-rich amphibolites Cummingtonite±anthophyllite±hornblende + Boninite-like rocks (Group 4 amphibolites) plagioclase+quartz±biotite±magnetite Plagioclase-rich amphibolites Hornblende+plagioclase+quartz+zircon+titanite+ Transitional to calc-alkaline basaltic andesites and andesites apatite±magnetite±biotite Serpentinites Serpentine+talc+actinolite+tremolite+magnetite± Olivine-rich pricritic cumulus cummingtonite±olivine ±clinopyroxene Actinolite–tremolite-rich amphibolites Actinolite+tremolite+magnetite±cummingtonite± Picritic volcanic rocks hornblende±clinopyroxene±magnetite Biotite schists Biotite+plagioclase+quartz+magnetite±garnet± Immature volcanoclastic graywackes derived from mafic epidote±titanite±muscovite±microcline±hornblende ± to felsic source rocks cummingtonite±anthophyllite±tourmaline Quartzitic gneisses Quartz+plagioclase +biotite +zircon+muscovite±garnet± Quartz-rich arkoses derived from felsic source rocks hornblende±magnetite±tourmaline J.C. Ordóñez-Calderón et al. / Lithos 113 (2009) 133–157 135 information on thermal and geochemical characteristics of the Archean However, the intra-oceanic origin of Archean greenstone belts is mantle (Bennett et al., 1993; Ohta et al., 1996; Pollack, 1997; Kerrich controversial because their original stratigraphic relationships and et al., 1999; Polat et al., 1999; Komiya et al., 2004; Condie, 2005a). primary geochemical signatures have been variably modified during Fig. 2. Field photographs of various rock types in the Ivisaartoq (a–c) and Ujarassuit (d–h) greenstone belts. (a) Pillow basalt from the Ivisaartoq belt. (b–c) Relict felsic cobbles in biotite schist of the Ivisaartoq greenstone belt. (d) Serpentinized olivine-rich meta-ultramafic rock with lens-shaped geometry. (e) Biotite schist with small-scale F3 isoclinal folds. (f–g) Migmatitic TTG–gneiss, close to the contact with supracrustal rocks, with large hornblende-rich amphibolite xenolith. (h) Migmatitic hornblende-rich amphibolite affected by recumbent F3 folding. 136 J.C. Ordóñez-Calderón et al. / Lithos 113 (2009) 133–157 hydrothermal alteration, regional metamorphism, polyphase deforma- in the Ivisaartoq and Ujarassuit greenstone belts to understand their tion, and plutonism (Fryer et al., 1979; Gruau et al., 1992; Lahaye et al., provenance and to provide additional constraints on the depositional 1995; Wilkins, 1997; Polat et al., 2003; Weiershäuser and Spooner, setting of these supracrustal belts. 2005). The Nuuk region in southern West Greenland comprises several 2. Geological setting and field characteristics early to late Archean (3850–2800 Ma) tectono-stratigraphic terranes assembled into a single block in the late Archean (Friend et al., 1988, Structural and U–Pb zircon geochronological studies have shown 1996; Nutman et al., 1989; McGregor et al., 1991; Friend and Nutman, that the Nuuk region is composed of several Eo- to Neoarchean (3850– 2005; Garde, 2007). The diachronous accretion of allochtonous terranes 2800 Ma)