Gondwana Research 11 (2007) 69–91 www.elsevier.com/locate/gr Field and geochemical characteristics of the Mesoarchean (∼3075Ma) Ivisaartoq greenstone belt, southern West Greenland: Evidence for seafloor hydrothermal alteration in supra-subduction oceanic crust ⁎ Ali Polat a, , Peter W.U. Appel b, Robert Frei c, Yuanming Pan d,Yıldırım Dilek e, Juan C. Ordóñez-Calderón a, Brian Fryer a,f, Julie A. Hollis b, Johann G. Raith g a Department of Earth Sciences, University of Windsor, Windsor, ON, Canada N9B 3P4 b Geological Survey of Denmark and Greenland, 1350 Copenhagen, Denmark c Geological Institute, University of Copenhagen, 1350 Copenhagen, Denmark d Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK., Canada S7N 5E2 e Department of Geology, Miami University, Oxford, OH 45056, USA f Great Lakes Institute for Environ. Res., University of Windsor, Windsor, ON, Canada N9B 3P4 g Department of Applied Geosciences and Geophysics, University of Leoben, Leoben, A-8700, Austria Received 12 December 2005; received in revised form 12 February 2006; accepted 14 February 2006 Abstract The Mesoarchean (ca. 3075Ma) Ivisaartoq greenstone belt in southern West Greenland includes variably deformed and metamorphosed pillow basalts, ultramafic flows (picrites), serpentinized ultramafic rocks, gabbros, sulphide-rich siliceous layers, and minor siliciclastic sedimentary rocks. Primary magmatic features such as concentric cooling-cracks and drainage cavities in pillows, volcanic breccia, ocelli interpreted as liquid immiscibility textures in pillows and gabbros, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic flows are well preserved in low-strain domains. The belt underwent at least two stages of calc-silicate metasomatic alteration and polyphase deformation between 2963 and 3075Ma. The stage I metasomatic assemblage is composed predominantly of epidote (now mostly diopside)+quartz+plagioclase±hornblende ±scapolite, and occurs mainly in pillow cores, pillow interstitials, and along pillow basalt-gabbro contacts. The origin of this metasomatic assemblage is attributed to seafloor hydrothermal alteration. On the basis of the common presence of epidote inclusions in diopside and the local occurrence of epidote-rich aggregates, the stage I metasomatic assemblage is interpreted as relict epidosite. The stage II metasomatic assemblage occurs as concordant discontinuous layered calc-silicate bodies to discordant calc-silicate veins commonly associated with shear zones. The stage II metasomatic assemblage consists mainly of diopside+garnet+amphibole+plagioclase+quartz±vesuvianite±scapolite±epidote±titanite ±calcite±scheelite. Given that the second stage of metasomatism is closely associated with shear zones and replaced rocks with an early metamorphic fabric, its origin is attributed to regional dynamothermal metamorphism. The least altered pillow basalts, picrites, gabbros, and diorites are characterized by LREE-enriched, near-flat HREE, and HFSE (especially Nb)-depleted trace element patterns, indicating a subduction zone geochemical signature. Ultramafic pillows and cumulates display large positive initial εNd values of +1.3 to +5.0, consistent with a strongly depleted mantle source. Given the geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, we suggest that the Ivisaartoq greenstone belt represents Mesoarchean supra-subduction zone oceanic crust. © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Greenstone belt; Archean; Pillow basalt; Picrite; Epidosite; Ocelli; Forearc; Ophiolite 1. Introduction Archean greenstone belts are the product of multiple ⁎ Corresponding author. geological processes such as tectonism, magmatism, sedimen- E-mail address: [email protected] (A. Polat). tation, and metamorphism, operating over different spatial and 1342-937X/$ - see front matter © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2006.02.004 70 A. Polat et al. / Gondwana Research 11 (2007) 69–91 temporal scales (Condie, 1981; de Wit, 1998; Polat et al., 1998; Hall et al., 1987; Chadwick, 1990; Appel, 1997). The presence van Kranendonk, 2004; Condie, 2005; Polat and Kerrich, 2006; of these primary structures provides a unique opportunity to Benn et al., 2006). Geochemical data accumulated from studies study the characteristics of hydrothermal alteration, physical of Archean greenstone belts over the last three decades are volcanology, and geodynamic processes. This study has been consistent with diverse types of volcanic rocks on all major made possible by the detailed mapping and recognition of Archean cratons, reflecting diverse geodynamic processes pillow basalts in the Ivisaartoq greenstone belt in the 1970s and (Dostal and Mueller, 1997; Polat et al., 1998; Kusky and 1980s (Friend and Hall, 1977; Friend et al., 1981; Brewer et al., Polat, 1999; Polat and Hofmann, 2003; Dostal and Mueller, 1984; Chadwick, 1985, 1986; Hall et al., 1987; Chadwick, 2004; Manikyamba et al., 2005; Smithies et al., 2005a,b; and 1990). The objectives of this study are two-fold: (1) to assess the references therein). These studies have documented two major geochemical and mineralogical effects of the post-magmatic types of volcanic rock associations: (1) an oceanic plateau hydrothermal alteration on pillow basalts, gabbros, diorites, and association composed of compositionally relatively uniform ultramafic pillows and cumulates; and (2) to understand the komatiites and Mg- to Fe-rich tholeiitic basalts erupted from geodynamic setting of the Ivisaartoq greenstone belt in which mantle plumes and (2) a compositionally diverse intra-oceanic the mafic-ultramafic rocks experienced hydrothermal alteration. island arc association, dominated by ‘normal’ tholeiitic to calc- alkaline basalts, andesites, dacites, and rhyolites (BADR). In 2. Regional geology and field characteristics addition, the latter association includes small volumes of boninites, picrites, low-Ti tholeiites (LOTI), adakites, high- The Ivisaartoq greenstone belt contains the largest magnesian andesites (HMA), and Nb-enriched basalts (NEB) Mesoarchean supracrustal assemblage in southern West Green- (Polat and Kerrich, 2004, 2006). land (Fig. 1; Hall and Friend, 1979; Brewer et al., 1984; Despite the two major phases of deformation, including Chadwick, 1985, 1986, 1990; Friend and Nutman, 2005a). It is isoclinal folding, and amphibolite-facies metamorphism, pillow located in the central part of the inner Godthåbsfjord of the structures, volcanic breccia, cumulate and liquid immiscibility northeastern Nuuk region (Fig. 1). The belt occurs within the textures, as well as magmatic layering have been well preserved recently recognized Mesoarchean (∼3075–2950Ma) Kapisilik in low-strain domains of the Mesoarchean Ivisaartoq greenstone tectonic terrane (Friend and Nutman, 2005a). The Kapisilik belt (Friend et al., 1981; Hall, 1981; Chadwick, 1985, 1986; terrane is tectonically bounded by the Paleoarchean Isukasia Fig. 1. A simplified geological map of the northeastern Nuuk region, showing the tectonic terranes of late to early Archean age and location of the Ivisaartoq belt. Modified after Friend and Nutman (2005a). A. Polat et al. / Gondwana Research 11 (2007) 69–91 71 terrane (3600–3800Ma) to the north, and by the Paleoarchean Færingehavn and the Neoarchean Tre Brødre terranes to the south to west, respectively (Fig. 1; Friend and Nutman, 2005a). The Isukasia and Færingehavn terranes may represent the fragments of the same Paleoarchean continent that rifted apart at about 3500Ma and onward, resulting in the opening of an oceanic basin. The subsequent closure of this ocean in the late Archean may have resulted in the amalgamation of the above tectonic terranes as oceanic island arcs and/or continental blocks. The Kapisilik and Isukasia terranes were juxtaposed (collided) and metamorphosed by 2950Ma. It appears that the collision between the southern Færingehavn and the Kapisilik terranes took place at about 2800Ma (Friend and Nutman, 2005a). Field relationships indicate that the Isukasia terrane is structurally overlain by the Kapisilik terrane to the south; and the Kapisilik terrane is in turn structurally overlain by the Færingehavn and Tre Brødre terranes to the south–southwest. The precise age of the Ivisaartoq greenstone belt is unknown. Siliceous volcano-sedimentary rocks have yielded an average U–Pb zircon age of 3075Ma (Friend and Nutman, 2005a), constraining the maximum age of the belt. The Ivisaartoq supracrustal rocks are intruded by weakly deformed 2961 ±12Ma granites to the north, constraining the minimum age of the belt (Chadwick, 1990; Friend and Nutman, 2005a). The Ivisaartoq rocks are truncated by an up to 2-m-thick mylonite zone to the south, separating the belt from an association of leucogabbros and anorthosites. The leucogabbro and anortho- site association is intruded by 2963±8Ma old tonalites and granodiorites (now gneisses). On the basis of field observations and zircon ages, Friend and Nutman (2005a) interpreted the Fig. 2. A simplified tectonostratigraphic column of the Ivisaartoq belt. Modified after Chadwick (1986, 1990). mylonite zone as a post-2960Ma structure deforming the Kapisilik terrane. The Ivisaartoq greenstone belt is composed mainly of The foliation is cut by 1- to 10-cm-thick deformed (folded) quartz metamorphosed mafic to ultramafic volcanic rocks, gabbros,