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Home , Archean subduction, Boninite, Forearc, Isua Greenstone Belt, Ophiolite, Peridotite

Lithos 113 (2009) 115–132

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Lithos

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Isua supracrustal belt ()—A vestige of a 3.8 Ga suprasubduction zone , and the implications for

Harald Furnes a,⁎, Minik Rosing b, Yildirim Dilek c, Maarten de Wit d a Department of 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 (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 differences between the two units. Compared with the geochemical evolution of some of the well known Suprasubduction zone Phanerozoic ophiolites, the pillow and associated dikes of the UA show a compositional range that is Archean oceanic and tectonics similar to typical MORB-type Ligurian ophiolites in the Western –Apennines and those displayed by LIP- type Caribbean ophiolites. The GA is characterized by tholeiite (IAT) to -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, -generated melts during seafloor spreading, little to moderately affected by 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, to basaltic 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 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 : 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 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 (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 , 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 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 / ultramafic rocks of the western arm of the ISB. This observation 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., 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 The Isua supracrustal belt has been deformed and metamorphosed Amitsoq orthogneisses (Mcgregor, 1973) that form part of the Itsaq during several distinct events. Penetrative deformation and amphibo- Complex (Nutman et al., 1996). The exact age of the ISB and lite facies affected all supracrustal units and the Akilia rocks are unknown (Nutman et al., 1993; Moorbath et al., 1997; enveloping gneiss complex prior to the emplacement of the basaltic Whitehouse et al., 1999; Nutman et al., 2000, 2007). The ISB (Fig. 1)is Ameralik dike swarm ca. 3550 Ma ago (Gill and Bridgwater, 1979; composed of a broad suite of that can be divided into three Nutman et al., 2004). These deformed amphibolites are easily dis- categories: (1) rocks with preserved primary features that allow tinguished from later undeformed dikes, such as the ubiquitous set identification of their protoliths, (2) rocks with ambiguous affinities, of basaltic, -phyric Ameralik dikes, both petrographically and (3) rocks that are demonstrably metasomatic in origin. The first and in the field (Furnes et al., 2007b). This pre-Ameralik tectono- category is dominated by deformed basaltic extrusive and shallow metamorphic event was contemporaneous with a major phase of intrusive rocks intercalated with abundant chemical sediments Amitsoq gneiss magmatism ca. 3750 Ma (Crowley et al., 2002; Frei including – banded formation (BIF) and rare et al., 2002; Nutman et al., 2002, 2007), which likely provided the heat clastic sediments. The lithologies with ambiguous affinities are for metamorphism and the production of metasomatic fluids. The dominated by ultramafic and felsic rocks. The ultramafic rocks come deformation, , and metamorphism associated with this in a great variety of types depending on metamorphic mineralogy, but event have obliterated most primary contacts between the supracrus- they can be divided into metadunites and metaperidotites based on tal belt and the enveloping , and between different lithostrati- their geochemistry and field appearance. They may represent mantle- graphic units within the belt. However, the precursors of the Amitsoq derived tectonic slivers, ultramafic sills, and cumulates from mafic gneiss were probably dominantly intrusive into the supracrustal intrusions, although it has not been possible to assign any specific belt, as evidenced by several well-preserved intrusive contacts with affinity to most individual bodies in the field. The felsic rocks gneisses ranging in age from 3810 Ma to 3710 Ma (Nutman et al., 1993, commonly share petrographic and geochemical similarities with the 1997, 2007; Crowley, 2003). Amitsoq gneiss that encloses the supracrustal belt. They include Whether the ISB constitutes one continuous -sedimentary detrital sediments and volcaniclastic flows but are dominated by succession or whether it comprises tectonic units with different ages granitoid intrusions into the supracrustal pile and slivers of the and origins has not been well established in the literature. Isotopic Amitsoq gneiss that were tectonically intercalated with the supra- ages of the supracrustal lithologies are strongly influenced by crustal rocks during late-stage orogenic events. The third category metamorphic and metasomatic overprinting, and while often precise represents metasomatic rocks, which are derived from all lithologies these data are unlikely to reflect accurate ages of different units. of the former categories by reaction with hydrothermal solutions Consequently, the age of the ISB rocks is mainly constrained by U–Pb during one or more tectonometamorphic events (Rose et al., 1996; ages of crosscutting felsic intrusives. This principle has in itself Rosing et al., 1996). The metasomatic transformations have been been the focus of major controversy because of the possibility of locally extensive, and some lithologies that have been characterized as inheritance in the zircon populations of felsic rocks (Kamber and metasediments in the literature can be shown to be metasomatic in Moorbath, 1998; Nutman et al., 2000; Whitehouse et al., 2001). origin (Rosing et al., 1996; Myers, 2001). Some of these metasomatites However, most felsic units studied have simple igneous zircon include metacarbonate rocks, – , and muscovite populations with tightly clustered age distributions. From the and fuchsite schists. available data, there are a large number of felsic dikes with dates of A prominent unit in the ISB is the Garbenschiefer amphibolites ca. 3800 Ma in the ISB near the outward convex border of the belt, (GA) (Fig. 1). This unit is composed of schistose metabasic rocks that whereas the studied felsic dikes near the inward concave margin of display distinctive sheave textures (defined by bundles of the belt have dates close to 3700 Ma. This has given rise to the idea amphibole crystals set in a matrix of plagioclase and quartz) in most that the belt is composed of at least two tectonostratigraphic units outcrops. Chlorite is present in many Garbenschiefer rocks. This has separated by 100 Ma in time of formation (Nutman et al., 1997, 2007). led to the common suggestion that the Garbenschiefer Unit represents The finding of 3690–3700 Ma in BIF and felsic rocks in the a greenschist facies domain (Hayashi et al., 2000; Komiya et al., 2002; inferred inner unit has been taken as evidence for the ca 3700 Ma age Appel et al., 2003) within the otherwise dominantly of this part of the ISB. However, the significance of these zircon dates facies of the ISB (Boak and Dymek, 1982; Rollinson et al., 2002). builds on the interpretation of the felsic rocks as extrusive and the However, the chlorites in the Garbenschiefer Unit are Mg-rich, zircons in the BIF as detrital. Both claims are contentious and have no reflecting the high Mg/Mg+Fe and high Al content of the amphibo- independent observational support. In the absence of more robust lites. Magnesian chlorite is stable well into amphibolite facies evidence for a composite origin of the ISB, we treat it as one (Winkler, 1967), and there is no basis for separating the Garbenschie- tectonostratigraphic unit constrained to be older than the enveloping fer Unit out as a separate metamorphic zone. The distinctive Amitsoq/Itsaq gneiss complex (Fig. 1) by intrusive relationships along metamorphic paragenesis discriminates the Garbenschiefer metaba- both the inner and outer margins. sic rocks from “normal” hornblende–plagioclase–quartz amphibolites that are common throughout the belt. These metabasites are derived 3. Field relationships of the mafic–ultramafic rocks of the ISB from tholeiitic extrusive and shallow intrusive protoliths (Gill and Bridgwater, 1979; Nutman et al., 1984; Gill et al., 1988; Komiya et al., For the field and geochemical description of magmatic rocks in the 2004). The tholeiitic metabasic rocks are everywhere strongly ISB we basically maintain the subdivision of the complex into central, deformed, but in places relic pillow structures (often with ocelli) outer and inner arc domains, as defined by Polat et al. (2002) and Polat and dike-in-dike relationships are preserved. In the SW part of the belt and Hofmann (2003). However, this description is not entirely pillow and dike lithologies are followed along-strike by a large representative when it comes to the eastern arm of ISB, and we area of homogeneous meta-gabbroic amphibolite. These rock therefore propose the following terms for the units: 1. 118 H. Furnes et al. / Lithos 113 (2009) 115–132

Undifferentiated amphibolites (UA) for the formerly named “outer perpendicular to the cleavage and 200–250% extension along a well and inner domains”, and 2. Garbenschiefer amphibolites (GA) (Fig. 1) defined lineation plunging at 72 S. for the “central domain”. In the following we consequently use these two terms, UA and GA. 3.1.2. Dikes At locality 2 (see Fig. 1) the sequence consists of tabular, sub- 3.1. Undifferentiated amphibolites (UA) parallel dikes with intervening cm- to dm-thick zones of lenticular to irregular screens of volcanic material (Fig. 3). Approximately 500 m All our field data pertaining to our interpretation of the ISB as an further south, at locality 3 (see Fig. 1), the mixed dike/volcanic ophiolite come from the undifferentiated amphibolites of the western sequence changes structurally downward into a sheeted complex, arm of the ISB. The rocks in this region are exposed along the eastern which consists of 100% tabular dikes (Fig. 4A–C). This dike complex is side of the belt referred to as the “inner arc tectonic domain” by Polat in tectonic contact with metagabbro and ultramafic sheets to the west. and Frei (2005). Below are brief field and petrographic descriptions of Individual dikes range in width from 2 to 50 cm. Dikes have both one- the lithological components (of the eastern part of the western arm) and (mostly) two-sided, fine grained, chilled planar margins (Fig. 4C). that together suggest the existence of an ophiolite—the Isua ophiolite Crosscutting dikes are also observed (Fig. 4D). This sequence was (Furnes et al., 2007a). interpreted as part of a sheeted dike complex by Furnes et al. (2007a). Petrographically the central parts of sheeted dikes consist of fine- 3.1.1. grained (grain size ∼300 μm) plagioclase, amphibole (predominant), Pillow structures can be found at many outcrops around locality 1 and biotite with remnant subophitic textures. Dark green, commonly (Fig. 1). In general they are highly deformed, and a penetrative schistose, marginal zones are interpreted to represent chilled margins cleavage occurs parallel to the lithological layering of different rock (Fig. 4C) and are composed of dense (∼100 μm) monomineralic zones units. However, in places (at locality 1, see Fig. 1) the pillows are of amphibole. These chilled margins of dikes are mineralogically and relatively well-preserved, and between pillows, small pockets of inter- texturally similar to the chilled margins of pillows in this area. pillow hyaloclastite are common (Fig. 2A, B). Ocelli-bearing pillows (Fig. 2C), characteristic of Archean pillow lavas (e.g. de Wit and 3.1.3. Plagiogranite Ashwal, 1997), are useful for estimating the degree of deformation Plagiogranite occurs as small pockets associated with the sheeted these rocks experienced. Deformed ocelli (originally spherical dike complex (Fig. 5A). Petrographically it contains of objects) (Fig. 2D) indicate deformation with 80–90% shortening plagoioclase and amphibole set in a fine-grained quartz–plagioclase

Fig. 2. A–C. Pillow lavas, interpillow hyaloclastite (IPH) and ocelli. A pillow with dense ocelli-development in the central part is shown in C. D. Deformed ocelli within a pillow. Location: Western arm of Isua Supracrustal Belt, location 1. Scale information: A. Pocket knife in upper left corner is 9 cm long. B. Number label in lower right corner is 10 cm long. C. GPS bag in lower left is ca. 12 cm long. D. Hammer head is 13 cm long. H. Furnes et al. / Lithos 113 (2009) 115–132 119

Fig. 3. Dikes and screens of volcanic rocks at location 2 in Fig. 1. Length of hammer shaft is ca. 60 cm. matrix (Fig. 5B, C). The geochemical composition of this rock type 3.1.4. (sampled from the plagiogranite occurrence shown in Fig. 5A) is given Metagabbro is rather rare in the ISB. However, in the southernmost in Furnes et al. (2007a). part of the western arm of the ISB, a relatively small area of metagabbro

Fig. 4. Dike complex of the Isua Supracrustal belt. A. Ca. 30 m long continuous outcrop section (in the foreground of the picture) across part of the sheeted dike complex. B. A series of dike units. C. Detail of individual dikes with pronounced chilled margins. D. Cross-cutting dikes. From location 3 on Fig. 1. Hammer head in B, C and D is 13 cm long. 120 H. Furnes et al. / Lithos 113 (2009) 115–132

Fig. 5. A. Plagiogranite (pale grey, irregular pods) associated with dikes and screens of volcanic rocks. Hammer for scale (32 cm long shaft). From location 2 on Fig. 1. B and C. Photomicrographs showing the quartz–albite–amphibole groundmass with plagioclase phenocrysts (Pl) in B, and an amphibole (Amph) in C.

is exposed (Fig. 6). Locally this gabbro displays distinctive layering 4. Geochemistry (Fig. 6B) and has been transformed to a flasergabbro due to solid-state deformation as evidenced by the elongated streaks of plagioclase In this section, we summarise the published, available geochemical (Fig. 6C). data from the meta-ultramafic to metabasaltic rocks of the GA and UA of the ISB, together with new data from the GA (Table 1). The samples were

3.1.5. Ultramafic rocks fused with LiBO2 and dissolved in HNO3. Major and minor elements Ultramafic rocks constitute a significant part of both the western were analysed by ICP-Emission and all the trace elements by ICP-MS at and eastern arms of the ISB (Fig. 1). They are composed of medium to SARM (Service d'Analyses Roches et des Mineraux) at Centre de coarse grained, red to dark grey/black rocks with alternating reddish Researches Petrographic et Geochemique, Vendoeuvre, France. We and dark grey bands of various thicknesses (Fig. 7). In general the also compare these ISB data with representative geochemical data from unspecified ultramafic rocks are enveloped by calc-silicate rocks. The other ophiolite types. calc-silicates are interpreted to have been formed by carbonation and desilication of country rock by fluids flowing out of the ultra- 4.1. Isua geochemistry maficrocks(Rose et al., 1996; Rosing et al., 1996). Possible protoliths t of the highly altered ultramafic rocks (mantle or cumulates) are Fig. 9 shows Bowen diagrams for some major (SiO2,TiO2,Al2O3,FeO, hence difficult to decipher, although locally they may represent CaO) and trace (Cr, Ni, Zr) elements. Both with respect to the major and ultramafic sills within the volcanosedimentary units (Dilek and trace elements the magmatic rocks define two distinct trends. This is well t Polat, 2008). demonstrated with respect to TiO2,Al2O3,FeO and to some extent CaO, and Zr. The rocks of the GA, together with the rocks referred to as “MORB”

3.2. Garbenschiefer amphibolites (GA) by Komiya et al. (2004),havedistinctlylowerTiO2 and Zr, and generally t lower FeO and CaO, and higher Al2O3 contents than the UA of the ISB The Garbenschiefer amphibolites (GA) define a significant part of (outer and inner arc domains of Polat and Frei (2005)), and the “OIB” the ISB (Fig. 1). Mostly this unit consists entirely of recrystallized rocks of Komiya et al. (2004). Both types show continuous ranges in the rocks, mostly highly schistose, composed of a hornblende–garnet– MgO contents between ca. 17.5–5.5 wt.% for the GA rocks, and ca. 20.5– biotite–chlorite mineral assemblage (Fig. 8A, B) (Rosing, 1999). 1 wt.% for the UA (Fig. 9). All the elements presented show relatively

Although the protoliths of these rocks are hard to recognize, it is well-defined magmatic trends with steadily increasing SiO2,TiO2,Al2O3, still possible to distinguish locally between metavolcanic/intrusive Zr, and decreasing Cr and Ni contents with decreasing MgO. With and metasedimentary (volcaniclastic) protoliths. In the lowest-strain decreasing MgO, CaO increases up to ca. 7 wt.%, and decreases thereafter. domains well-preserved sedimentary structures such as graded- Fig. 10 shows MORB-normalised multi-element diagrams of all bedding can be found (Fig. 8C, D) (Rosing, 1999), and pillow structures available samples from the UA and GA of the ISB. The two former show have been reported (Komiya and Maruyama, 1995). rather flat, but slightly enriched patterns towards the most-incompatible H. Furnes et al. / Lithos 113 (2009) 115–132 121

Fig. 6. Deformed metagabbro from the southernmost part of the western arm of ISB. A. General view of gabbro exposure. B. Rhythmically phase-layered gabbro. C. Close-up of flasergabbro, showing the lenticular appearance of plagioclase. Location of C is shown by the boxed area in A. elements (to the left in the diagram), and also display minor negative to briefly review the most-recent classification of ophiolites based on Nb-anomalies and positive Pb-anomalies. The samples of the GA show Phanerozoic examples (Dilek, 2003). Subsequent to the definition of highly depleted and convex-downward MORB-normalized patterns, the Penrose-type ophiolite (Anonymous, 1972), a wealth of informa- with slight negative Ta and Nb anomalies, and considerable positive Pb tion on structural architecture, geochemical fingerprints, and evolu- anomalies. The most incompatible elements, i.e. Th and Ba, mostly show tionary paths, that suggest different tectonic environments of ophiolite pronounced enrichment relative to the most depleted elements (e.g. Ti formation. Dilek (2003) compiled this information and proposed through La of Fig. 10). the following classification into seven ophiolite types: (1) Ligurian-, Fig. 11 shows the mafic and ultramafic rocks in the tectonic domains (2)Chilean-,(3)Macquarie-,(4)Caribbean-,(5)Franciscan-,(6)Sierran-, plotted inTh/Yb vs. Nb/Yb diagram. In this diagram the samples from the and (7) Mediterranean (or suprasubduction zone)-type ophiolites. GA show a pronounced spread, but define a different field than the As indicated above, Bowen diagrams of the ISB data (Fig. 9) are samples from the UA. Both groups, however, plot above the mantle array compared with the data from two of the above-mentioned ophiolite defined by the fields for N-MORB, E-MORB and OIB (Fig. 11). types, i.e. the LIP-type Caribbean- and suprasubduction zone (SSZ) type Mediterranean ophiolites (Fig. 12). The UA of the ISB, particularly 4.2. Isua geochemistry compared to Phanerozoic ophiolites the high-MgO samples, can be closely matched with volcanic rocks from the Caribbean ophiolites with In order to compare the ISB lithologies and the geochemical (LIP) affinities. In the same diagram (Fig. 12) the ISB data are com- character of the pillow lavas and dikes with ophiolites, it is pertinent pared with the volcanic rocks from the Tethyan SSZ-type ophiolites. 122 H. Furnes et al. / Lithos 113 (2009) 115–132

Fig. 7. Ultramafic rocks from the eastern part of the western arm of the ISB (in the area between locations 1 and 3 in Fig. 1). A. Large-scale, alternating reddish-brown (meta-dunitic to -harzburgitic) and black (metapyroxenite-rich) layers. B. Close-up photograph of the reddish-brown metaperidotite. C. Small-scale, alternating red and dark grey layers of metaperidotite. Scale information: A. Hammer shaft is ca. 60 cm long. B. Shoes in upper left are ca. 35 cm long. C. Black GPS bag is 12 cm long.

The large variations in the magmatic rocks from these ophiolite types none of the ophiolitic rocks match the high-Al2O3 rocks of the ISB in define broad fields with respect to most elements. The ISB rocks do not the ca.14–16 wt.% MgO range. occupy the entire fields defined by these ophiolites in the Bowen Fig. 13 shows the various ophiolite types plotted in the Th/Yb vs. diagrams, particularly the highly fractionated (SiO2-rich) rocks, Nb/Yb diagram. Whereas the Ligurian- and Caribbean-types, and although some important similarities appear. For example, the Californian ophiolites plot within the mantle array from N-MORB to E- complete range in MgO and the low concentrations of TiO2 and Zr, MORB to OIB, the SSZ-type plot above the mantle array (Fig. 13). and partly the high Al2O3 content of the rocks of the GA are highly With respect to the multi-element diagrams we have compared similar to the trends defined by the typical boninitic to IAT rocks of the UA and GA with the characteristic multi-element diagrams for these Tethyan SSZ-type ophiolites. It should be noted, however, that Caribbean- and SSZ-type ophiolites (Fig. 14). The high-MgO basaltic H. Furnes et al. / Lithos 113 (2009) 115–132 123

Fig. 8. Garbenschiefer amphibolites from the western arm of ISB. A. General view of the Garbenschiefer amphibolite; B. Close-up from picture A, showing cm-long amphibole sheaves; C. Volcaniclastic sediments showing graded bedding; D. Close-up from picture C. Finger points to the grey graphite-bearing pelitic top of a graded bed (younging to the left). All photos from the western arm of ISB. rocks from the Caribbean-type ophiolites compare well with those of are little affected by metamorphism. Previous geochemical studies of the UA (Fig. 14). One difference between the Caribbean and the UA is the Isua ultramaficandmafic magmatic rocks suggest that Zr, REE, Th, Ti, that the latter shows a negative Nb anomaly, a feature that is not Nb and Y are relatively immobile (Polat et al., 2002; Polat and Hofmann, represented by the Caribbean-type. The samples from the high-MgO 2003; Polat and Frei, 2005). This is consistent with other studies of rocks of the GA, on the other hand, show a pattern in the multi- Archean volcanic rocks that also find that the relative mobilities of Al, Cr element diagram that is nearly indistinguishable from a typical SSZ- and Ni are notably less than the commonly large scale mobility of Rb, K, type boninite (Fig. 14). Na, Sr, Ba, Si, Ca, Mg, Fe, P and Pb (see Polat et al., 2002 and references therein). The Bowen diagrams shown in Fig. 9 should, therefore, be 5. Geochemical and petrogenetic implications interpreted with care, since variations in Si, Mg, Fe and Ca may partly have resulted from secondary element mobility. However, the large 5.1. Element mobility range in the concentrations of both major and trace elements as shown in Fig. 9 (Si, Ti, Al, Mg, Ca, Zr, Cr, Ni), and the relatively high stability of Al, During alteration and metamorphism of basaltic rocks elements are Ti, Zr, Cr and Ni can hardly be accounted for by alteration processes variably mobilized (e.g. Cann, 1970; Coish, 1977; Humphris and alone. Moreover, all of the Bowen diagrams shown in Fig. 9,withthe Thompson,1978). Several studies investigating element mobility during exception of the FeOt vs. MgO diagram, show systematic, positive or alteration and metamorphism have identified, however, some elements inverse correlations with MgO content that are reminiscent of the trends that retain approximately their original concentration (Nicollet and caused by fractional crystallization. Furthermore, the patterns defined Andribololona, 1980; Weaver and Tarney, 1981), or element ratios that by the multi-element diagrams (Fig. 10)andtheTh/Ybvs.Nb/Yb 124

Table 1 Major and trace element analyses of Garbenschiefer amphibolites from the Isua supracrustal complex.

Sample # 810381a 242790A 242670 242744 242743 242742 242733 242737 242729 242727H 242725 242719c 242718 242717A 242689 242673b 242671b

Si02 48.61 49.60 49.50 50.03 46.72 47.10 48.62 45.14 46.62 43.68 44.88 46.50 47.22 46.29 48.74 49.31 49.47 Al203 17.73 17.60 18.52 17.82 16.34 19.56 17.37 15.99 13.42 9.91 14.52 16.83 16.08 15.11 17.83 16.19 18.20 t Fe203 10.41 11.29 10.72 11.54 9.69 8.71 10.92 9.80 11.02 9.09 11.38 9.12 9.97 9.89 11.03 11.08 11.13 MnO 0.13 0.17 0.23 0.17 0.14 0.13 0.15 0.15 0.23 0.15 0.23 0.20 0.16 0.19 0.22 0.19 0.19 MgO 12.76 9.72 8.80 8.05 14.83 10.88 9.31 16.37 14.28 23.44 14.80 11.31 13.25 12.85 9.37 17.15 9.58 CaO 7.57 8.10 9.32 7.37 6.28 8.07 8.60 5.84 10.17 7.22 8.71 11.68 7.13 9.74 9.33 2.78 9.02 Na2O 0.96 2.00 1.23 1.80 0.66 1.68 1.65 1.13 0.62 0.07 0.95 1.18 1.02 0.78 1.88 0.50 1.43 K20 0.15 0.02 0.08 0.15 0.02 0.10 0.10 0.04 0.07 0.15 0.04 0.06 0.05 0.10 0.11 0.02 TiO2 0.27 0.32 0.31 0.39 0.21 0.21 0.36 0.20 0.19 0.14 0.21 0.23 0.22 0.16 0.30 0.22 0.29 P205 0.18 0.21 0.21 0.20 0.19 0.20 0.22 0.18 0.20 0.17 0.22 0.23 0.21 0.18 0.19 0.15 0.18 LOI 1.15 0.91 1.01 2.42 4.85 3.29 2.64 5.10 3.11 6.07 3.69 2.32 4.62 3.75 0.95 2.24 0.40 Total 99.92 99.94 99.93 99.94 99.93 99.93 99.94 99.94 99.93 99.94 99.74 99.64 99.94 98.99 99.94 99.92 99.91 Be 0.53 0.12 0.19 0.1 0.13 0.19 0.1 0.24 0.36 V 207 217 209 233 185 198 230 160 157 136 184 183 177 163 238 184 211 Cr 255 217 254 55.6 701 359 106 1207 1705 2203 1343 908 878 728 363 1165 271 Co 68.0 67.9 62.7 45.4 63.4 44.8 53.7 65.8 73.8 81.8 77.7 61.3 70.3 56.0 65.9 73.7 62.3 Ni 210 112 177 59.1 342 149 122 472 625 811 586 366 403 330 174 469 160 Cu 52.1 1.1 2.6 4.3 2.5 2.7 47.2 11.9 23.8 20.1 0.8 18.5 18.0 14.9 24.6 83.9 1.8 Zn 64.8 81.8 77.7 78.0 62.2 52.1 49.5 59.3 101 75.1 84.1 60.6 66.4 56.7 84.3 74.9 65.7 Ga 12.9 14.6 15.1 14.1 11.0 12.3 14.4 10.0 9.84 7.61 10.9 11.1 10.8 9.43 14.6 12.1 14.0 115 (2009) 113 Lithos / al. et Furnes H. Ge 1.66 1.65 1.54 1.43 1.65 1.97 1.91 1.35 1.76 2.22 1.63 1.26 1.32 1.33 1.69 1.55 1.43 As 0.58 0.47 0.63 0.36 0.27 0.06 0.87 0.31 0.63 5.0 0.48 0.9 0.51 0.96 0.63 0.7 0.48 Rb 10.01 0.71 4.87 3.28 1.08 1.24 0.97 1.51 1.49 0.49 4.36 1.15 2.38 1.37 6.78 16.29 1.50 Sr 48.4 65.9 74.3 84.1 52.2 72.8 56.8 54.7 25.3 1.32 36.5 115 58.8 60.0 52.5 8.45 53.8 Y 11.1 12.5 13.3 15.6 10.2 9.54 13.3 7.42 7.38 6.28 7.93 10.1 10.0 9.1 12.9 11.0 13.6 Zr 14.5 20.3 25.7 23.4 12.5 15.3 19.3 10.4 13.1 6.25 9.82 12.9 12.8 10.4 21.2 12.5 24.0 Nb 0.43 0.45 0.79 0.55 0.23 0.39 0.39 0.21 0.28 0.13 0.14 0.25 0.26 0.16 0.40 0.22 0.64 Mo 0.13 0.14 0.22 0.21 0.11 0.09 0.13 0.21 0.25 0.22 0.15 0.10 0.20 0.17 0.27 0.30 0.24 Cd 0.11 0.01 0.05 0.04 0.08 0.13 0.04 0.15 0.11 0.07 0.10 0.06 In 0.06 0.08 0.06 0.11 0.07 0.08 0.09 0.09 0.07 0.06 0.09 0.07 0.06 0.05 0.07 0.05 0.05 Sn 0.20 0.36 0.32 0.44 0.26 0.35 0.18 0.51 0.25 0.18 0.32 0.09 0.09 0.13 0.57 0.17 0.44 Sb 0.17 0.29 0.26 0.13 0.20 0.05 0.21 0.22 0.31 0.40 0.30 0.44 0.27 0.22 0.31 0.09 0.23 Cs 0.923 0.090 0.827 0.163 0.096 0.108 0.094 0.116 0.140 0.078 0.166 0.108 0.157 0.749 0.299 –

Ba 8.4 12 11 22 5.0 10 4.8 3.4 6.8 0.6 29 9.8 15 7.5 1.5 4.1 3.7 132 Hf 0.43 0.51 0.67 0.75 0.35 0.46 0.54 0.33 0.43 0.22 0.3 0.37 0.4 0.32 0.65 0.4 0.68 Ta 0.0718 0.0468 0.0647 0.0480 0.0179 0.0318 0.0315 0.0236 0.0247 0.0101 0.0147 0.0241 0.0314 0.0175 0.0487 0.0208 0.0602 W 66.3 107 75.5 0.22 29.8 0.05 0.06 30.4 0.11 16.2 38.6 90 88.0 36.5 69.0 42.0 82.8 Pb 85.6 6.29 3.36 1.22 0.53 1.05 1.0 0.63 4.95 0.98 1.73 3.92 2.57 2.82 3.86 1.79 2.67 Bi 0.05 0.07 0.01 0.0 0.0 0.0 0.02 0.01 0.02 0.03 0.03 0.02 0.52 0.13 0.02 Th 0.140 0.140 0.430 0.180 0.022 0.170 0.043 0.067 0.010 0.022 0.052 0.063 0.053 0.230 0.016 0.260 U 0.046 0.030 0.110 0.014 0.011 0.008 0.003 0.043 0.052 0.002 0.022 0.120 0.058 La 0.85 0.96 2.08 1.25 0.59 1.02 0.57 0.13 0.40 0.35 0.32 0.37 0.61 0.51 1.13 0.33 1.90 Ce 1.82 2.51 4.53 3.21 1.22 2.47 1.88 0.38 0.89 0.27 0.96 1.03 1.64 1.09 3.32 1.09 4.24 Pr 0.23 0.38 0.59 0.47 0.20 0.31 0.33 0.09 0.14 0.11 0.18 0.20 0.23 0.21 0.43 0.19 0.56 Nd 1.15 1.73 2.62 2.31 0.95 1.35 1.91 0.45 0.55 0.62 0.83 1.07 1.24 0.82 1.96 1.17 2.75 Sm 0.55 0.69 0.82 0.86 0.37 0.54 0.64 0.26 0.19 0.21 0.38 0.41 0.43 0.36 0.69 0.51 0.86 Eu 0.16 0.21 0.29 0.21 0.16 0.17 0.22 0.10 0.08 0.04 0.11 0.15 0.15 0.12 0.27 0.16 0.29 Gd 0.73 0.98 1.07 1.15 0.59 0.64 1.05 0.48 0.40 0.38 0.59 0.74 0.70 0.61 1.13 0.83 1.16 Tb 0.16 0.20 0.22 0.22 0.15 0.17 0.22 0.09 0.08 0.09 0.11 0.14 0.15 0.14 0.21 0.15 0.23 Dy 1.35 1.56 1.54 2.06 1.16 1.27 1.56 0.81 0.75 0.64 0.98 1.20 1.25 1.10 1.66 1.37 1.68 Ho 0.37 0.40 0.43 0.49 0.32 0.27 0.41 0.23 0.22 0.18 0.28 0.31 0.33 0.26 0.40 0.32 0.45 Er 1.19 1.24 1.29 1.53 1.00 0.99 1.28 0.83 0.78 0.61 0.83 0.97 1.08 0.83 1.21 1.13 1.36 Tm 0.20 0.22 0.25 0.27 0.16 0.18 0.23 0.14 0.13 0.11 0.17 0.15 0.16 0.18 0.19 0.17 0.23 Yb 1.37 1.32 1.65 1.91 1.29 1.14 1.36 0.92 1.08 0.84 0.98 1.20 1.28 1.11 1.58 1.33 1.54 Lu 0.23 0.23 0.27 0.27 0.23 0.19 0.22 0.16 0.18 0.16 0.14 0.21 0.18 0.18 0.26 0.22 0.26 t Fe2O3 = total iron as Fe2O3. LOI = loss on ignition. a Sample 810381 is from the eastern arm of the ISB; all the others are from the western arm of ISB. H. Furnes et al. / Lithos 113 (2009) 115–132 125

Fig. 9. Bowen diagrams for the mafic and ultramafic magmatic rocks of the Isua supracrustal belt (ISB). Data are from: Polat et al. (2002); Polat and Hofmann (2003); Komiya et al. (2004); Furnes et al. (2007a); this work (Table 1). The undifferentiated amphibolites (1) and (2) are for the western and eastern part, respectively, of the western arm of the ISB (Polat and Hofmann, 2003). diagram (Fig. 11) are based predominantly on elements that are some of the samples analyzed in this study have experienced Th- considered to be stable during metamorphism (except Ba and Pb). enrichment above their primary values. However, Rosing and Frei (2004) showed extensive metasomatic Th enrichment in some Isua rocks. Based on 208/204–206/204 Pb 5.2. The ophiolite rock association systematics, Frei et al. (2002) demonstrated that Th was mobilized and introduced into these rocks during 2800 Ma tectonometamorphic The lithological components of the UA, namely the pillow lavas, events that affected most Isua rocks. It may, therefore, be expected that sheeted dikes, transitional zone between volcanic rocks and dikes,

Fig. 10. Multi-element diagrams of representative samples from the Undifferentiated amphibolites (UA) (A and B), and Garbenschiefer amphibolites (GA) (C).Datafrom:AandB:Polat and Hoffman (2003);C:Polat et al. (2002) and this work (Table 1). Normalizing MORB values (in ppm) (after Pearce and Parkinson, 1993) are: Ba (6,3); Th (0.12), Nb (2.33), La (2.5), Ce (7.5), Pb (0.3), Pr (1,32); Sr (90), Nd (7.3), Zr (74), Sm (2.63), Eu (1.02), Ti (7620), Gd (3.68), Tb (0.67), Dy (4.55), Y (28), Ho (1.01), Er (2.97), Tm (0.456), Yb (3.05), V (300), Cr (275), Ni (100). 126 H. Furnes et al. / Lithos 113 (2009) 115–132

Fig. 11. The Isua data plotted in Th/Yb vs. Nb/Yb diagram. Modern MORB (N-MORB and E-MORB) and OIB define a diagonal array with N-MORB, E-MORB and OIB at its centre. that have interacted with on ascent, or have a subduction component, are, at a given Nb/Yb ratio, displaced to higher Th/Yb values. After Pearce (2008). Isua data from: Polat et al. (2002); Polat and Hofmann (2003); this work (Table 1). The undifferentiated amphibolites (1) and (2) are for the western and eastern part, respectively, of the western arm of the ISB (Polat and Hofmann, 2003).The field defined by the Mariana arc is taken from Pearce (2008). plagiogranite, gabbro and ultramafic rocks (Figs. 2–7), together tional range from high- to low-Mg basalts represented by dikes and comprise all the components of a complete Penrose-type ophiolite pillow lavas, and thus strengthen the cogenetic origin of these (Dilek, 2003). Furnes et al. (2007a) suggested that this rock lithological units. On the basis of the present knowledge of the UA, we association, together with geochemical affinities of pillow lavas and thus maintain our previous conclusion that this unit represents a dikes, represents an ophiolite. Given that this rock sequence has been dismembered Archean ophiolite (Furnes et al., 2007a, b). strongly deformed it is admittedly difficult on the basis of field relationships alone to prove that these rock components once were a 5.3. Geochemical characteristics coherent slab of oceanic crust. Current geochronological constraints on the UA do not however, exclude the cogenetic origin of the various The geochemical characteristics of the metabasaltic rocks of the UA lithological components. Furthermore, the geochemistry of the are, in most respects, compatible with the magmatic evolution of the metabasaltic rocks of the UA (Fig. 9), shows a continuous composi- MORB-type Ligurian and LIP-type Caribbean ophiolites (Dilek, 2003;

Fig. 12. Bowen diagrams for different types of Phanerozoic ophiolites (suprasubduction zone and Caribbean types), shown together with the Isua undifferentiated- (UA) and Garbenschiefer amphibolites (GA). Data sources for the Isua amphibolites are provided in Fig. 9. The suprasubduction zone type ophiolites are represented Mirdita: Dilek et al. (2008), Pindos: Pe-Piper et al. (2004); Saccani and Photiades (2004), : Lippard et al. (1986); Einaudi et al. (2003); Godard et al. (2003); Troodos: Rautenschlein et al. (1985); Auclair and Ludden (1987); Taylor (1990), Kizildag: Dilek and Thy (1998); Y. Dilek (unpublished data). Caribbean-type data: Klaver (1987); Kerr et al. (1996). H. Furnes et al. / Lithos 113 (2009) 115–132 127

Fig. 13. The Phanerozoic ophiolite data plotted in Th/Yb vs. Nb/Yb diagram, onto which the fields for the two Isua populations are shown. See Fig. 11 for further information on data sources for the Isua amphibolites. Data sources for ophiolites are: Suprasubduction zone and Caribbean types: see Fig. 12 Ligurian ophiolites: Ferrara et al. (1976); Beccaluva et al. (1977); Ottonello et al. (1984); Vannucci et al. (1993); Rampone et al. (1998); Californian and Philippine ophiolites: Harper (1984, 2003), Harper et al. (1988); Dilek et al. (1991); Evans et al. (1991); Yumul et al. (2000); Metzger et al. (2002); Shervais (1990); Giaramita et al. (1998); Shervais et al. (2005).

Figs. 12 and 13). The MORB-type Ligurian ophiolites represent the (Dilek, 2003). None of these ophiolite types are early stages of opening of an , whereas the LIP-type associated with subduction-related magmatism. However, in the Th/ Caribbean ophiolites represent the oceanic crustal assemblages of an Yb vs. Nb/Yb diagram (Fig.11) all of the analyses plot above the mantle

Fig. 14. Trace element comparison between amphibolites of the Isua supracrustal belt and representative basaltic and boninitic rocks from Phanerozoic ophiolites. A. MORB- normalized multi-element diagram of the undifferentiated amphibolites (UA) compared with basaltic samples from the Caribbean type ophiolite. B. Multi-element diagram of the Garbenschiefer amphibolites (GA) compared with a typical boninite sample from a suprasubduction zone type ophiolite (Mirdita ophiolite, Albania, see Dilek et al., 2008). Data sources for the Undifferentiated- and the Garbenschiefer amphibolites, see Fig. 10. 128 H. Furnes et al. / Lithos 113 (2009) 115–132 array, a feature that may result from crustal contamination or globally unique 0.3 e-unit 142Nd/144Nd anomaly, and together they subduction-related recycling influence. The same conclusion may be define a statistically significant 3.78±0.04 Ga Sm–Nd isochron (Boyet inferred from the small negative Nb-anomalies (Figs. 10 and 14). et al., 2003). We find these geochemical and isotopic similarities in The geochemical characteristics of the rocks of the GA show strong support of a similar age and origin for UA and consider this similarity with the boninitic to IAT magmatic rocks of SSZ-type evidence stronger than those arguments for their dissimilarity based on ophiolites in the Mediterranean region (Figs. 12–14).Asinthecaseof the apparent lack of observed ∼3800 Ma felsic dikes crosscutting these the rocks of the UA, whether this geochemical trend was influenced amphibolites. Consequently, we regard the UA as part of a single basaltic by crustal contamination or subduction processes, is a fundamental volcanic complex formed at ca. 3800 Ma. question. Although the commonly-accepted evidence for the oldest The 143Nd/144Nd systematics of the Isua metabasic rocks has been a continental crust is around 4 Ga (Bowring and Williams, 1999), more matter of continued attention during more than 30 years of geochemical recent Hf-isotope studies of ca. 4.4 Ga zircons from Jack Hills, research. The main conclusion we derive from these studies is that there Western , indicate that continental crust may be traced back has been one or more phases of severe metasomatic disturbance of the for an additional 400 My (Harrison et al., 2005). Therefore, it is Sm/Nd isotopic system, and that the metabasaltic rocks most likely had probable that at the time of formation of the Isua rocks (UA and GA), an initial positive 143Nd/144Nd anomaly of ca. + 1.5 e-units (Hamilton continental crust might have been recycled into the mantle. et al., 1978; Gruau et al., 1996). This has been taken as evidence for the However, the amount of recycled continental crust was probably existence of a MORB-like depleted mantle reservoir at the time of minor in the early stages of the Isuan time span (ca. 3800–3500 Ma), formation of the Isua rocks (e.g. Hamilton et al., 1983). and the Nd-isotope studies of the metabasalts (see below) do not favour significant continental involvement in their genesis. It is, 6. Geodynamic model: discussion and conclusions therefore, considered more likely that the negative Nb-anomalies, the Th/Yb vs. Nb/Yb relationships, and typical boninite-like patterns The term suprasubduction zone (SSZ) was first introduced by Pearce of the GA originated as a result of geochemical processes in a et al. (1984) for ophiolites that have the crustal components and subduction zone. architecture of oceanic crust with geochemical signatures of subduction There are no systematic geochemical differences between the effects. These ophiolites have been interpreted to have formed in intra- amphibolites from the UA, but there are marked differences in major oceanic immature arc systems. Hawkins and Evans (1983) and Hawkins and trace element compositions between the UA, and the rocks of the et al. (1984) expanded the meaning of the term suprasubduction zone to GA. Interestingly, the pillow lava amphibolites from the UA are not only include all of the components formed in a subduction-related system indistinguishable in major and trace element patterns but also share a from arc, to backarc. A good example of this SSZ environment is

Fig. 15. Schematic model showing proposed tectonic environment for the generation of the two units (UA–the Undifferentiated amphibolites, and GA–the Garbenschiefer amphibolites) of magmatic rocks of the Isua supracrustal belt, and their geochemical and geological characteristics. Formation and subsequent aggregation, differentiation and modification of melts (in A and B) are similar to a model suggested by Grove et al. (1992). H. Furnes et al. / Lithos 113 (2009) 115–132 129 the West Philippine Basin– system, where subduction SSZ-type ophiolites and modern analogues (e.g. Hawkins, 2003; rollback over the last 48 My has generated a series of backarc basins, Pearce, 2003; Dilek et al., 2008). In this way the Isua ophiolite can be remnant arcs and a frontal arc system (e.g. Crawford et al., 1981; Fryer, defined as a composite segment of Archean oceanic crust derived from 1992). In this convergent margin tectonic setting, the arc–forearc– a first-stage generation of MORB-like magmas with little to moderate backarc systems have been undergoing lithospheric extension. When subduction influence (UA), followed by the generation of IAT-boninite slab rollback exceeds the plate convergence rates, extensional condi- magmas (GA) from a hydrated and depleted mantle. tions prevail, producing seafloor spreading generated oceanic crust Polat and Frei (2005) also proposed a suprasubduction geody- (Leitch, 1984; Dilek et al., 2008). namic setting for the boninite-like and non-boninitic rocks of Isua. In We propose a new geodynamic model for the magmatic develop- their model, oceanic ridge subduction is proposed as a necessity to ment of the UA and GA in the Isua supracrustal belt (Fig. 15). In this generate sufficient heat for the production of the high-MgO magmatic model the UA represents the oldest unit of the ISB, and the field rocks. However, if the high-MgO magmatic rocks of the GA represent geology shows that this unit contains all the components of an true boninites generated above a hydrated mantle wedge, in a similar ophiolite. The amphibolites (pillow lavas and sheeted dikes) display a manner to the production of modern boninites, the necessity for MORB-like geochemistry with a wide compositional range from generating extra heat by oceanic ridge subduction for their generation primary to highly fractionated magmas. We suggest that the UA unit of may not be required (e.g. Stone et al., 1997; Parman et al., 2001, 2003; the ISB originally represented a coherent slab of oceanic crust formed Parman and Grove, 2004; Grove et al., 2006). by seafloor spreading (Fig. 15A). Compared with Phanerozoic The inferred SSZ origin of the Isua supracrustal units has significant ophiolites, the geochemistry of the UA unit is most comparable with implications for the ongoing debate about the timing of the onset of the Ligurian- and Caribbean-type ophiolites that represent oceanic the modern plate-tectonic processes (seafloor spreading and subduc- crust without subduction imprint. It is possible, however, that some of tion) in the Archean. These rocks represent some of the oldest intact the UA magmas were influenced by subduction as indicated by minor rocks on Earth, and their lithological components and mutual negative Nb-anomalies (Fig. 10) and the relatively high Th/Yb ratios at relationships, combined with the geochemical signatures of the a given Nb/Yb value (Fig. 11). In various discriminant diagrams (Ti–V, lavas and dikes, can be found in Phanerozoic ophiolite terrains. This Zr–Zr/Y, Ti–Zr, Y–Cr), the UA pillow lavas and dikes straddle the observation implies in turn that the magmatic and tectonic processes boundary between mid-ocean ridge basalts (MORB), island arc and geodynamic setting during the formation of the 3.8 Ga Isua belt tholeiites (IAT) and boninites (Furnes et al., 2007a). To what extent may have been similar to those of the Phanerozoic SSZ ophiolites. the UA crust was influenced by subduction processes depends on the Therefore, we maintain that Phanerozoic-like plate tectonic was location of the mantle source producing the melts relative to the operative at 3.8 Ga (Furnes et al., 2007a,b; Dilek and Polat, 2008). subduction zone. In our model we suggest that the oceanic crust of the UA–the Isua ophiolite–formed at an early stage and in a setting away Acknowledgements from major influence of a subduction zone (Fig. 15A, and see Dilek and Flower, 2003). Hence, it was little affected by subduction processes. This study was financed by a series of research grants from the The GA consists of volcaniclastic rocks, pillow lavas and gabbroic Norwegian Research Council, the Geological Museum of Copenhagen, intrusions (collectively referred to as Garbenschiefer). The geochem- the GFZ-Potsdam, and the Agouron foundation. We thank Nicola ical composition of the amphibolites defines a typical boninite-like McLoughlin for comments on an early version of the paper, and Jane pattern, with strong depletion in incompatible elements. It thus Ellingsen for helping with the illustrations. We further thank the two differs from the UA unit both with respect to the lithological referees, Simon A. Wilde and Gouchun Zhao, and the guest editor of components, structural architecture, and the geochemistry. Boninites this issue, Paul T. Robinson, for constructive comments and sugges- are invariably associated with the early stages of intra-oceanic island tions that improved the manuscript. This is AEON contribution arc generation, and are generally attributed to extension of forearc number 63. lithosphere in response to slab rollback (e.g. Crawford et al., 1989; Bedard et al., 1998). We suggest that the GA unit represents an extended incipient island-arc fed by the magmas that were generated References from a subduction-influenced, repeatedly depleted and hydrated, – refractory mantle (Fig. 15B). This model thus suggests that the GA Anonymous, 1972. Penrose Field Conference on ophiolites. Geotimes 17, 24 25. Appel, P.W.U., Moorbath, S., Touret, J.L.R., 2003. Early Archaean processes and the Isua formed synchronously, or after the formation of the UA, reminiscent of Greenstone Belt, West Greenland—preface. Precambrian Research 126 (3–4), most SSZ-type Tethyan ophiolites in the Mediterranean region (Dilek 173–179. et al., 2008). The geochemical composition and trends of the boninite- Arndt, N.T., 2003. Komatiites, , and boninites. 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