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Isua Supracrustal Belt (Greenland)—A Vestige of a 3.8 Ga Suprasubduction Zone Ophiolite, and the Implications for Archean Geology

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Isua Supracrustal Belt (Greenland)—A Vestige of a 3.8 Ga Suprasubduction Zone Ophiolite, and the Implications for Archean Geology Lithos 113 (2009) 115–132 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Isua supracrustal belt (Greenland)—A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology Harald Furnes a,⁎, Minik Rosing b, Yildirim Dilek c, Maarten de Wit d a Department of Earth Science & Centre for Geobiology, University of Bergen, Norway b Nordic Centre for Earth Evolution, and Geological Museum, Univ. of Copenhagen, Denmark c Department of Geology, Miami University, Oxford, OH, USA d AEON & Department of Geological Sciences, University of Cape Town, South Africa article info abstract Article history: The mafic–ultramafic units of the ∼3.8 Ga Isua supracrustal belt (ISB) in Greenland occur in a two-armed Received 15 May 2008 arcuate zone (eastern and western arms) and are grouped into two major tectonostratigraphic units based Accepted 20 March 2009 on their lithological and geochemical characteristics: (1) Undifferentiated amphibolites (UA), and Available online 6 April 2009 (2) Garbenschiefer amphibolites (GA). The UA contains all major lithological units of a typical Penrose- type complete ophiolite sequence. The GA is composed dominantly of volcaniclastic and volcanic rocks, Keywords: commonly found in immature island arcs. The available geochemical data from UA and GA show distinct Isua supracrustal belt Ophiolites differences between the two units. Compared with the geochemical evolution of some of the well known Suprasubduction zone magmatism Phanerozoic ophiolites, the pillow lavas and associated dikes of the UA show a compositional range that is Archean oceanic crust and tectonics similar to typical MORB-type Ligurian ophiolites in the Western Alps–Apennines and those displayed by LIP- type Caribbean ophiolites. The GA is characterized by island arc tholeiite (IAT) to boninite-like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone (SSZ) ophiolites in the Mediterranean region. Our proposed geodynamic model for the ISB suggests that the UA was built by primary to differentiated, mantle-generated melts during seafloor spreading, little to moderately affected by subduction processes, and that the IAT to boninitic-like rocks of the GA formed at a later stage by melting from a strongly subduction-affected, depleted and hydrated mantle. Our interpretation of the ISB is that the UA and GA represent early and late stages, respectively, in the formation of a SSZ ophiolite. This implies that Phanerozoic-type plate tectonic processes, such as seafloor spreading and subduction, were operating by 3.8 Ga in the Palaeoarchean. © 2009 Elsevier B.V. All rights reserved. 1. Introduction established vary by more than 1 billion years, e.g. at 3.2 Ga (van Kranendonck, 2007); at 3.6 Ga (Nutman et al., 2007); by 3.8 Ga Archean supracrustal lithological associations, commonly referred (Furnes et al., 2007a,b; Dilek and Polat, 2008); at 4.0 Ga (de Wit, to as greenstone belts, generally contain submarine mafic–ultramafic 1998); by 4.2 Ga (Cavosie et al., 2007). Whereas some would argue lavas and intrusions that vary compositionally from low- to high-MgO that plate tectonic processes operated throughout the Precambrian, basalts to basaltic komatiites and komatiites (e.g. Arndt and Nesbitt, others suggest that Phanerozoic-like plate tectonic processes did not 1982; de Wit and Ashwal, 1995, 1997; de Wit, 2004; Sproule et al., commence until the Neoproterozoic (Hamilton, 1998, 2003; Stern, 2002). There is a general agreement among geologists working on 2005; Brown, 2006). Some of the alternative interpretations envisage Archean geology that a plate tectonic-like Earth, with divergent and that Archean tectonics was instead controlled by vertically controlled convergent plate boundaries, is consistent with field and geochemical plume activities and associated crustal delamination. data from the late Archean greenstone belts (b3.0 Ga; e.g. Goodwin One of the most critical criteria for modern plate tectonics is the and Ridler, 1970; Tarney et al., 1976; Condie, 1981; Windley, 1993; de subduction-driven horizontal motion of lithospheric plates, resulting in Wit, 1998; Kusky and Polat, 1999; Kerrich and Polat, 2006). However, changes in their spatial relationship over time (e.g. Cawood et al., 2006). estimates on precisely when this mode of horizontal tectonics was Palaeomagnetic studies now exist that demonstrate that large-scale horizontal plate motions similar to those known from the Phanerozoic Eon may have occurred in the Archean. Both examples are from the Pilbara craton: the first is one from a late Archean supracrustal sequence ∼ ⁎ Corresponding author. Department of Earth Science, University of Bergen, Norway. ( 2.7 Ga by Strik et al., 2003) and the second from an early Archean E-mail address: [email protected] (H. Furnes). supracrustal sequence (3.4 Ga, by Suganuma et al., 2006). These results, 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.03.043 116 H. Furnes et al. / Lithos 113 (2009) 115–132 however, are under some doubt as to their reliability because of the authors to conclude that ophiolites are not represented in the earliest absence of robust paleomagnetic field tests. Geological observations, on stages of the Earth's history (Bickle et al., 1994; Hamilton, 1998, 2003). the other hand, support the concept of early Archean horizontal crustal Thus, whether ophiolites as we know from the Phanerozoic rock motions both in terms of extension (e.g. de Vries et al., 2006)and record exist in the Archean greenstone belts remains a fundamental shortening (e.g. de Wit and Ashwal, 1997; Moyen et al., 2006), but none question in global tectonics. of these inferred paleo-motions have been properly quantified. On the The Paleoarchean Isua supracrustal (ISB) or greenstone belt (e.g. other hand, many geochemical studies of greenstone belts have Nutman et al., 1997) in southwestern Greenland (Fig. 1) contains one demonstrated similarities between Archean and modern maficmag- of the oldest intact submarine igneous sequences in the world, and matic rocks, which formed in different plate tectonic settings (e.g. hence it presents an excellent opportunity to address the “Archean Tarney et al., 1976; Lafleche et al., 1992; Kerrich et al., 1998; Kusky and ophiolite and plate tectonic conundrum”. In a series of studies, based Polat, 1999). One particularly debated question pertains to the on mapping and understanding of field relations, geochronological similarities between Archean komatiites and Phanerozoic boninites, and geochemical studies of the volcanic and volcanogenic rocks, some the latter being exclusively related to subduction zone environments researchers have proposed that plate tectonic-like processes must (e.g. Arndt, 2003; Arndt et al., 1998, 2008; Parman et al., 2001, 2003; have operated during the generation of the ISB and its surrounding Parman and Grove, 2004; Grove and Parman, 2004; Dilek and Polat, region in the early Archean (Maruyama et al., 1994; Rosing et al., 1996; 2008). Komiya et al., 1999; Hanmer and Greene, 2002; Hanmer et al., 2002; Identification of ophiolites in the rock record has become one of Polat et al., 2002; Polat and Hofmann, 2003; Polat and Frei, 2005). the geological keystones with which to unravel plate tectonic Furnes et al. (2007a,b) identified a well-preserved sheeted dike processes in the rock record, because the majority of ophiolitic rock complex within the suite of undifferentiated amphibolites and suites are now widely recognized as sections of ancient oceanic crust/ ultramafic rocks of the western arm of the ISB. This observation lithosphere formed in subduction rollback cycles (Dilek and Flower, implies that the ISB has a highly deformed ophiolite that might have 2003). Therefore, recognition of ophiolites in the Archean record is of formed as a result of plate tectonic-like processes around 3.7–3.8 Ga, utmost significance to establish the mode and tempo of plate tectonic although this interpretation has been disputed (Hamilton, 2007; processes in the early evolution of the Earth. The presence of Archean Nutman and Friend, 2007). In this paper, we present additional ophiolites, or ophiolite-like sequences, has been proposed by several geochemical data in support of our earlier interpretation that the ISB authors for a large number of Archean greenstone belts (e.g. de Wit contains ophiolite assemblages. We compare the geology and et al., 1987; Kerrich et al., 1998; Kusky and Polat, 1999; Kusky et al., geochemistry of the ISB to those of Phanerozoic ophiolites, which 2001; Polat et al., 2002; Polat and Hofmann, 2003; de Wit, 2004; Dilek formed in different tectonic environments at different stages of their and Polat, 2008). However, Archean greenstone belts are commonly evolution. We then discuss the significance of the occurrence of believed to lack one or more of the crustal components of the Penrose suprasubduction zone (SSZ) ophiolites in the Archean record for the pseudostratigraphy (Anonymous, 1972), and this has led some early stages of the Earth's evolution. Fig. 1. Geological map of Isua. The compilation is mainly based on the detailed, regional map of Nutman (1986). The numbers on the eastern part of the western arm of the undifferentiated amphibolites (UA) are after Furnes et al. (2007a). H. Furnes et al. / Lithos 113 (2009) 115–132 117 2. Geology of Isua sequences were interpreted as the deformed and disrupted fragments of a Paleoarchaean ophiolite complex (Furnes et al., 2007a). These 2.1. Rock types and tectonostratigraphic units deformed rock suites are in turn intruded by later undeformed Ameralik dikes (Nutman and Friend, 2007; Furnes et al., 2007b). The ISB (Nutman et al., 1984) and the Akilia Association (Mcgregor and Mason, 1977; Baadsgaard et al., 1984) of mainly supracrustal rocks 2.2. Age relations and deformation were formed in a similar time span, 3800–3500 Ma, named the Isuan Era by Harland et al. (1982). They are invariably older than the felsic The Isua supracrustal belt has been deformed and metamorphosed Amitsoq orthogneisses (Mcgregor, 1973) that form part of the Itsaq during several distinct events.
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