Deformation features and critical field relationships of early Archaean rocks, Akilia, southwest...

Precambrian Research 126 (2003) 259–271

Deformation features and critical field relationships of earlyArchaean rocks, Akilia, southwest Greenland

Martin J. Whitehousea,∗, Christopher M. Fedoba Swedish Museum of Natural History, P.O. Box 50007, SE-10405 Stockholm, Sweden

b Department of Earth and Environmental Sciences, George Washington University, Washington, DC 20052, USA

Accepted 5 February 2003

Abstract

Lithologically varied early Archaean rocks on the island of Akilia, Godthåbsfjord, southwest Greenland include a quartz–pyroxene unit that has been proposed as a BIF. Carbon isotope data from graphite particles in apatites within this unit havefurther been proposed to represent evidence for Earth’s earliest life. We present field observations which demonstrate that alllithologies on SW Akilia have been intensely deformed during both early and late Archaean high-grade metamorphism. In ourview, this deformation precludes unambiguous demonstration of purported sedimentary features in the quartz–pyroxene rock,a unit which we interpret as the result of metasomatic, metamorphic, and tectonic reworking of a mafic–ultramafic protolith.Such an origin is completely incompatible with the quartz–pyroxene rock hosting any signs of early biogenic activity. We alsoreappraise contact relationships between Amı̂tsoq gneisses and mafic–ultramafic rocks and can find no convincing evidence forthe existence of intrusive relationships that have been central to geochronological interpretations on Akilia.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Archaean; Greenland; Godthåbsfjord; Protoliths; Relative age; Deformation

1. Introduction

The southwest tip of the island of Akilia, Godthåbs-fjord, West Greenland (Fig. 1) contains exposuresof early Archaean rocks whose origin and preciseage have been at the centre of a vigorous debatefor the past few years. The most significant catalystto this debate was the identification of isotopicallylight carbon (Mojzsis et al., 1996), considered to bebiogenic in origin, as graphite inclusions in apatitesall hosted in a quartz–pyroxene rock interpreted atthe time as a banded iron formation (BIF). Taken to-gether with a claimed age of >3.85 Ga (Nutman et al.,

∗ Corresponding author. Tel.:+46-8-519-551-69;fax: +46-8-519-540-31.

E-mail address: [email protected] (M.J. Whitehouse).

1997), this study suggested that a robust hydrosphereand life was established on Earth during a period ofheavy asteroid bombardment that has generally beenassumed to preclude or frustrate life’s development(e.g. Chyba, 1993). The various controversies canbe divided into: (1) direct challenges to the originof the quartz–pyroxene rock as a sediment (Fedoand Whitehouse, 2002a,b,c; cf. Friend et al., 2002;Mojzsis and Harrison, 2002a); (2) questioning of thefield relationships that have been used to constrainthe age of the quartz–pyroxene rock (Myers andCrowley, 2000); (3) differing opinions on the inter-pretation of feldspar Pb-isotope and ion-microprobeU–Pb geochronological data from complex polyphasezircons in tonalite–trondhjemite–granodiorite (TTG)gneisses that are claimed to cross-cut the purportedlife-bearing rocks on Akilia (Kamber and Moorbath,

0301-9268/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0301-9268(03)00098-6

260 M.J. Whitehouse, C.M. Fedo / Precambrian Research 126 (2003) 259–271

Fig. 1. Location maps of (a) Archaean craton in southern Greenland and (b) Akilia in the outer Godthåbsfjord. Main map (c) shows thegeology of the southwest part of Akilia (modified afterMyers and Crowley, 2000). Localities A and B show where vigorously debatedfield relationships between mafic–ultramafic rocks and TTG gneisses, the latter subject to zircon U–Pb geochronological studies, have beenused to propose age constraints on the mafic–ultramafic enclave (see text for in depth discussion). Locality C refers to the location of thedetailed lithological log of the quartz–pyroxene unit (Fedo and Whitehouse, 2002a) and the inferred location of light C-isotope bearingsample G91-26 (Mojzsis et al., 1996).

1998; Whitehouse et al., 1999, 2001; cf. Nutman et al.,1997, 2001, 2002; Mojzsis and Harrison, 2002b).Rather than repeating these debates in detail, we referthe reader to the primary literature.

The purpose of the present paper is two-fold, and isbased on the importance of fully documenting criticalfield relationships that help to identify lithologies andprocesses that have affected them. Firstly, we reap-praise the field relations and deformation state of theenclosing, and possibly younger, Amı̂tsoq gneissesat and near the localities where all workers whofavour a ca. >3.85 Ga age for the quartz–pyroxenerock obtain their ages. Secondly, we present previ-ously unpublished field observations in support ofour interpretation that the quartz–pyroxene rock on

Akilia, once claimed to host early terrestrial life is, infact a highly deformed, metasomatised zone in a lay-ered mafic–ultramafic enclave (Fedo and Whitehouse,2002a). Our objective is to present clear documen-tation of evidence showing the intense metasomaticand tectonic overprinting that has profoundly affectedthese rocks, which cannot be ignored when trying tointerpret the 3.65 Ga or older history contained within.

2. Geological setting

Like all early Archaean rocks of the outer Godthåbs-fjord region, there are two main groups on Akilia: (1)ca. 3.65–3.85 Ga composite, banded, tonalitic gneisses

M.J. Whitehouse, C.M. Fedo / Precambrian Research 126 (2003) 259–271 261

(Nutman et al., 1996, 1997; Whitehouse et al., 1999),commonly referred to as Am̂ıtsoq gneisses, and (2)coarse-grained, mafic/ultramafic rocks, that are gen-erally assumed to be older. These latter rocks, termedthe Akilia Association (McGregor and Mason, 1977),contain a less common, banded, quartz–pyroxenelithology, which has been interpreted as BIF (Mojzsiset al., 1996; Nutman et al., 1997, 2002; Mojzsis andHarrison, 2000, 2002a; Anbar et al., 2001; Friendet al., 2002). Early Archaean gneisses and Akilia As-sociation rocks are recognized and distinguished fromsimilar late Archaean rocks in the field on the basis oftheir intrusion by a suite of dolerites known as Amera-lik dykes that are probably similar in age to ca. 3.5 Gaintrusions dated in the Isua region (White et al., 2000).When considering the pre-Ameralik-dyke history ofthese rocks and the nature of their protoliths, it is es-sential to appreciate that they have been subjected tomultiple episodes of regional metamorphism, includ-ing two granulite-facies events (ca. 3.6 and 2.8 Ga),where temperatures and pressures ranged from∼600to 900◦C and∼8–12 kbar (Griffin et al., 1980). Theseevents have imposed a penetrative tectonic fabric (e.g.Myers and Crowley, 2000), such that all lithologies,including the Ameralik dykes which only experi-enced the ca. 2.8 Ga event, are strongly schistose (orbanded) and have been transposed into parallelism.

3. Deformation state of the Am̂ıtsoqgneisses on Akilia

The quartz–pyroxene rock (Fedo and Whitehouse,2002a,b,c) that has been interpreted as BIF is hostedin a mafic/ultramafic enclave on the southwestern tipof Akilia, and is itself enclosed in polyphase TTGsuite Am̂ıtsoq gneisses. As a prelude to our discus-sion of the quartz–pyroxene rock itself, we examinethe deformational state of these polyphase gneisseswithin a few tens of metres of their contact with themafic–ultramafic enclave. Because these lithologicallydiverse rocks preserve evidence of a very complex tec-tonic history and are generally thought to be youngerthan the mafic/ultramafic enclave, deformation that af-fected the gneisses also affected the enclave, but strainfeatures may be difficult to observe in the monotonouslithologies of the enclave itself (with the notable ex-ception of the quartz–pyroxene rock).

The gneisses exposed on SW Akilia (Fig. 2A) aretypical polyphase gneisses consisting of TTG rocksthat range in likely protolith age from ca. 3.85 to3.65 Ga (Nutman et al., 1996, 1997, 2002; Whitehouseet al., 1999), with the younger phases representing ei-ther injection of melts, in situ partial melting, or a com-bination of both. These early phases, including earlymafic–ultramafic rocks that may be disrupted parts ofthe Akilia Association, were strongly deformed in theearly Archaean (Griffin et al., 1980) prior to intrusionby ca. 3.5 Ga mafic Ameralik dykes.Fig. 2B shows athick mafic dyke that originally would have been dis-cordant to foliation and/or layering in the host tonal-ities or gneisses but is now transposed parallel to theintense mm–cm scale gneissic banding. This transpo-sition indicates a very high degree of post-mafic dyke(Ameralik?) strain, probably occurring during late Ar-chaean (Myers and Crowley, 2000). The high degreeof strain is further emphasized by a thin quartzosevein within the dyke itself which is now preserved asa rootless, isoclinal, and intrafolial fold. Consideredentirely in isolation, it could be argued that the veinhas been cut by ayounger mafic dyke, but the oppo-site is true. Such relationships occur at many differentscales within complexly deformed gneisses such asthese, so any “discordant” contact must be considereda priori as structural in the present state and onlyconsidered as representing a primary igneous relation-ship if it can confidently be retrodeformed.Fig. 2Cfurther illustrates the high degree of strain recordedby mafic dykes (Ameralik?) which are younger thanthe host Am̂ıtsoq gneisses and the mafic/ultramaficenclave that hosts the quartz–pyroxene lithology. Inthis example, siliceous veins have been intenselystretched and define a very high strain foliationthat is commensurate with the strain visible in theAmı̂tsoq gneisses. Another apparent discordance thatis clearly the result of tectonic processes is illustratedin Fig. 2D. In this case banded gneisses have beenfolded and one limb has been sheared off, leaving thefold hinge truncated against other strongly foliatedrocks. The foldappears to be cross-cut by youngergneisses. If the “cross-cutting” banded gneisses wereinstead a more homogeneous layer of TTG, it wouldbe possible, but erroneous, to assign magmatic agesignificance to the contact. Relationships such asthese must be borne in mind when interpreting anylocalized field relationships with the aim of assigning


Fig. 2. Features of the Am̂ıtsoq gneisses adjacent to the mafic–ultramafic enclave on southwest Akilia. (A) Extreme polyphase deformationand lithological heterogeneity on a millimeter to 10 cm scale. Note younger mafic dykes (probably ca. 3.5 Ga Ameralik dykes) that havebeen rotated into parallelism with the gneissic foliation and layering. The quartz–pyroxene rock located within a few tens of metres ofthis locality would undoubtedly have experienced the same degree of deformation. (B) Ameralik dyke, now parallel with foliation in theAmı̂tsoq gneisses; note the small quartzose vein that has been isoclinally folded and left rootless by deformation. (C) Mafic dyke exhibitingstrong foliation and highly strained quartz stringers. Scale units are in centimeters in (B) and (C). (D) Part of a late fold hinge in bandedAmı̂tsoq gneisses that has been tectonically truncated by other banded Amı̂tsoq gneisses generating a >30◦ “discordance” (indicated bywhite arrow) that has no protolith age significance. This boundary clearly formed after imposition of an early foliation in the gneisses andmay be late Archaean in age.

age significance to them (seeSection 4). Because thequartz–pyroxene rock is generally assumed to haveformed in the early Archaean (as evidenced by thepresence of ca. 3.65 Ga metamorphic zircons),beforeintrusion of the Am̂ıtsoq gneisses, it requires that thequartz–pyroxene rocks experienced at least the samedeformation as the gneisses. Consequently, interpret-ing the fine layering as “depositional” (Mojzsis et al.,1996) or as evidence of sedimentary banding as inclassical BIF (Mojzsis et al., 1996; Nutman et al.,1997) is totally inappropriate, a point which will bediscussed in more detail inSection 5.

4. Contact relations between gneisses and themafic–ultramafic enclave

4.1. Current debate

Prior to the study ofFedo and Whitehouse (2002a,b,c), the single most contentious issue surroundingclaims for evidence of >3.85 Ga biogenic activity inthe rocks of Akilia was the assignment of this age it-self. Here we summarize the main points of the debateand provide new field observations and interpretationsthat bear on this controversy.


It is generally agreed that direct dating of the pro-tolith of the Akilia quartz–pyroxene rock is unattain-able given its lithology. Indeed, if the metasomaticorigin proposed byFedo and Whitehouse (2002a,b)is correct, then it seems likely that all chronome-ters will yield ages that are either profoundly dis-turbed or record event(s) younger than the protolithage. This is exactly the case with the U–Pb dating ofapatites from the key isotopically light-C bearing sam-ple G91-26 which yield an age of ca. 1.5 Ga (Sanoet al., 1999). Older ages have been reported from zir-cons found in this same sample but it is likely thatboth the exclusively late Archaean ages ofNutmanet al. (1997)and the rare early Archaean (ca. 3.65 Ga)ages ofMojzsis and Harrison (2000)represent meta-morphic zircon growth and/or recrystallisation duringwell-documented granulite-facies metamorphic events(Griffin et al., 1980).

In the absence of direct dating for the quartz–pyroxene rock, the next best possibility rests withdating of the adjacent mafic and ultramafic rocks(commonly and collectively referred to as “amphibo-lites”). However, this approach requires a significantand, to date, untested assumption that the quartz–pyro-xene rock and the mafic–ultramafic rocks representsome kind of temporally connected volcano-sedimen-tary association. A problem with this assumption isthat recognition of textural evidence that must be usedto support an extrusive origin for the mafic–ultramaficrocks, such as pillow basalts like those at Isua (Myers,2001; Fedo et al., 2001), is impossible because of therepeatedly deformed, metamorphosed, and recrys-tallised nature of the Akilia rocks. Similarly, undocu-mented claims for “flow top breccias” (Friend et al.,2002) are pure speculation. Nonetheless, assignmentof a protolith age to the Akilia mafic–ultramaficenclave remains a worthwhile geochronologicalendeavor, whatever its bearing on the age of thequartz–pyroxene rock. Two largely overlooked studieshave applied Sm–Nd isochron (Moorbath et al., 1997)and feldspar Pb-isotopic (Kamber and Moorbath,1998) methods to propose a protolith age of ca. 3.7 Ga.It is of course possible that the Sm–Nd isochronmight represent granulite-facies resetting in an olderprotolith (see for exampleWhitehouse, 1988, 1989),although the feldspar Pb-isotope data require a morecomplex, quantitative model in order to be explainedmerely as resetting, and the concordance of these two

ages from different methods remains an important ob-servation that demands a plausible explanation fromproponents of a metamorphic origin for the 3.7 Gaages.

An alternative to direct dating of the mafic–ultra-mafic enclave is to use “cross-cutting” granitoid sheetscontaining zircons that are amenable to precise U–Pbdating, an approach that will, however, yield only aminimum age. A critical prerequisite to this approachis unambiguous demonstration of aprimary intrusiverelationship of TTG gneiss protoliths into the enclave.In Fig. 3 we show a portion of the contact exposedclose to the highest elevation in SW Akilia,<100 mnorth of the main quartz–pyroxene rock outcrop. Theintense folding and transposition exhibited at this con-tact zone precludes unambiguous determination of rel-ative age and thus, any primary igneous relationships(if present to begin with) have been totally obliterated.Despite this observation, igneous intrusion of TTGinto mafic–ultramafic enclave has been claimed at twoplaces along the main gneiss-amphibolite contact, bothwithin a few tens of metres of the locality we illus-trate inFig. 3. Field relationships and geochronologyfrom these localities are the subject of the followingdiscussion.

Nutman et al. (1996, 1997)presented ion-micro-probe U–Pb zircon data from TTG samples at twolocalities (marked A and B inFig. 1c) within oradjacent to the mafic–ultramafic enclave on the south-west tip of Akilia (sample G93-05 from Locality Aand sample G88-66 from Locality B). Both samplesyielded “mostly optically homogeneous” (Nutmanet al., 1996) zircon populations with a wide spreadin ages whose maximum of ca. 3.85 Ga was inter-preted as the age of the granitoid protoliths with allyounger ages explained as Pb-loss during later meta-morphic events. A subsequent ion-microprobe studyby Whitehouse et al. (1999)utilized cathodolumines-cence (CL) imaging of zircons prior to analysis andrevealed previously unrecognized internal complexityin zircons from a TTG sample (SM/GR/97/2) fromLocality B (Fig. 1c) that was considered equivalentto sample G88-66 ofNutman et al. (1996). Takentogether with new U–Pb data and integration of thecombined multi-system isotopic results ofKamber andMoorbath (1998), Whitehouse et al. (1999)proposedthat the older ages could represent inherited cores ina granitoid whose emplacement age is ca. 3.65 Ga, an


Fig. 3. Intense deformation at the contact between TTG gneisses and the mafic–ultramafic enclave, close to the highest elevation in SWAkilia and <100 m north of the main quartz–pyroxene rock locality. Fifty millimeter lens cap for scale is located close to the centre ofthe photograph.

age that is consistent with the bulk of the Amı̂tsoqgneisses in the Godthåbsfjord region. A rare-earthelement study of zircons from sample SM/GR/97/2(Whitehouse and Kamber, 2003) also supports anigneous origin for the 3.65 Ga overgrowths, either inan introduced magma or a large degree in situ partialmelt. The original proposal of discordance (Nutmanet al., 1996) between the mafic–ultramafic enclaveand TTG gneisses represented by sample G88-66(and SM/GR/97/2) at Locality B (Fig. 1c) was later,and in our opinion correctly, considered equivocal byNutman et al. (1997). As an alternative,Nutman et al.(1997) presented a field sketch of the outcrop fromwhich their sample G93-05 was collected (Locality A,Fig. 1c) purporting to show a ca. 10◦ discordance be-tween the mafic–ultramafic enclave and a “∼1 m wide. . . homogeneous” and specifically “not compositebanded” quartz diorite sheet that yielded G93-05.Since then, all workers have concentrated on the fieldrelations and/or geochronology at this locality. Of par-ticular importance is the study ofMyers and Crowley(2000) whose detailed mapping showed that thegneiss sheet that yielded sample G93-05 is in fact ca.70 cm wide composite sheet comprising “heteroge-neous gneiss” (thinly banded mafic and felsic layers),

“homogeneous tonalitic gneiss” (nomenclature weadopt throughout the present paper), and a number ofminor mafic and pegmatitic components. Followingthis study,Nutman et al. (2001)revealed that G93-05was collected from the homogeneous tonalitic gneiss,subsequently providing a field photograph of its exactlocation (Fig. 10b ofNutman et al., 2002).

U–Pb geochronological studies of zircons fromsamples of the composite gneiss sheet at Locality A(Fig. 1c) have been presented by several independentgroups using both ion-microprobe (Nutman et al.,1997; Whitehouse et al., 1999; Mojzsis and Harrison,2002b) and conventional (Krogh et al., 2002) meth-ods. The studies ofNutman et al. (1997)andMojzsisand Harrison (2002a,b)yield a wide spread ages fromca. 3.85 to<3.65 Ga, and both groups consistently in-terpret the oldest ages as dating the igneous protolithwhile younger ages, commonly but not exclusivelyaccompanied by low Th/U ratios, are interpreted asmetamorphic in origin and/or the result of Pb-loss.Whitehouse et al. (1999)presented ion-microprobeU–Pb data on zircons from a sample (SM/GR/97/7)that was collected from the composite gneiss sheet byS. Moorbath in 1997, before the mapping ofMyersand Crowley (2000)demonstrated its true complexity


and beforeNutman et al. (2001)revealed the exactposition of G93-05. Single-phase oscillatory-zonedzircons from SM/GR/97/7 have high Th/U and yieldca. 3.65 Ga ages (Whitehouse et al., 1999). AlthoughSM/GR/97/7 was initially assumed byWhitehouseet al. (1999)to be equivalent to the sample G93-05of Nutman et al. (1997), it is quite obvious from thecontrasting U–Pb zircon data and internal structuresthat it cannot be the precise equivalent of G93-05, or asample studied byMojzsis and Harrison (2002b)thathas yielded similar U–Pb data to G93-05. The conven-tional U–Pb zircon study reported recently byKroghet al. (2002)yields ca. 3.76 Ga, an anomalous age thatcannot be used to support the older ages, as incorrectlyclaimed byFriend et al. (2002), or directly comparedwith the younger ages ofWhitehouse et al. (1999).

Petrographic analysis of sample SM/GR/97/7 (pro-vided by S. Moorbath) reveals that this sample is infact banded on a<5 mm scale, suggesting that it wascollected from banded heterogeneous gneisses thatborder to the east the homogeneous tonalitic gneissrepresented by G93-05, an interpretation supported byV.R. McGregor (personal communication, 1999) whowas present at the time sample SM/GR/97/7 was col-lected. Despite its clear non-equivalence to samplesthat have yielded older zircons, we will demonstratebelow that SM/GR/97/7 may indeed be a crucialsample with regard to interpretation of field relationsand geochronology of the composite gneiss sheet andsurrounding rocks of the mafic–ultramafic enclave.

Myers and Crowley (2000)claimed on the basis oftheir mapping and observations that the ca. 10◦ dis-cordance between the composite gneiss sheet and themafic–ultramafic enclave shown in the (back-to-front)field sketch ofNutman et al. (1997)does not exist.Fedo and Whitehouse (2002a)also did not identifydiscordant igneous contacts with the mafic–ultramaficenclave at this locality based upon their investigationof the outcrop in 2001. Despite these observations,Nutman et al. (2002)persist with an interpretationthat the homogeneous tonalitic gneiss intruded themafic–ultramafic enclave, an interpretation that isaccepted byMojzsis and Harrison (2002b).

4.2. Field constraints and model

The specific part of the composite gneiss sheet atLocality A (Fig. 1c) where discordance is explicitly

noted by Nutman et al. (1997, 2002)on the basisof “locally truncated” compositional layering in themafic–ultramafic enclave has surprisingly never beendocumented by a close-up field photograph, whichis now shown inFig. 4B. In accord with the obser-vations of Myers and Crowley (2000), our photo-graph shows that the claimed 10◦ discordance simplydoes not exist, but represents an optical illusion ofa three-dimensional outcrop. Within measurement,the gneissic banding in the two lithologies and thecontact between them are concordant. Furthermore,the border between the homogeneous tonalitic gneissand the mafic–ultramafic enclave is complicated bya thin, tightly folded pegmatite vein (Fig. 4C) whichis consistent with a tectonic nature of the westernmargin of the composite gneiss sheet. Elsewhere thestructural nature of this margin is even more unam-biguous. Approximately 3 m north of the claimeddiscordance discussed above and still in the compos-ite gneiss sheet, (Fig. 4D and clearly visible in themiddle of Fig. 10b ofNutman et al., 2002), a flatlying, wave-washed bench exposes heterogeneousgneisses that preserve a (locally) concordant contactwith the homogeneous tonalitic gneiss sheet (a–a inFig. 4D). Although the margin between the compos-ite gneiss sheet and the mafic–ultramafic enclave isintruded by ca. 10 cm late Archaean (or younger) peg-matite sheet (enclosed by dashed lines in Fig. 10b ofNutman et al., 2002), it is clear fromFig. 4D that thehomogeneous tonalitic gneiss–heterogeneous gneisscontact intersects the pre-pegmatite margin with themafic–ultramafic enclave at a relatively high angle(ca. 30◦), a relationship that is clearly consistent witha tectonic margin that was established sometime priorto intrusion of the late pegmatite. Furthermore, thetruncation of a contact that is internal to the compos-ite gneiss sheet by the contact between the sheet andthe mafic–ultramafic enclave clearly establishes thatthe latter contact post-dates the former. Indeed, thefield observations at this outcrop, together with theabsence of evidence for an original extrusive originfor the mafic–ultramafic enclave equally permits thepossibilities that the enclave might in fact represent ayounger intrusion intoolder TTG gneisses rather thanthe other way around, or that the two units were jux-taposed by faulting and then later deformed together.

Nutman et al. (2001, 2002)suggested that the homo-geneous tonalitic gneiss, while mostly concordant to


Fig. 4. Field relationships of a composite gneiss sheet within the mafic–ultramafic enclave at Locality A (Fig. 1c; GPS location: 63◦55.853′N,51◦41.018′W). (A) Reference map base, reproduced fromMyers and Crowley (2000)and annotated with the position of sample G91-05as revealed byNutman et al. (2002); note that we retain the distinction between ultramafic rocks (hornblendites) within the 70 cm widecomposite gneiss sheet and mafic–ultramafic rocks external to it (cf.Nutman et al., 2002). Location of photographs shown in (B) and(D) are indicated by boxes; all photographs have a consistent orientation with top to the west and scale units are in centimeters, in (B)and (D). (B) Photograph showing the nature of the contact at which discordance has repeatedly been claimed to support an intrusiverelationship of the homogeneous tonalitic gneiss (ton) and mafic–ultramafic rock (m/um); a late pegmatite (peg) intrudes at lower left. Ourphotograph shows no obvious discordance and does not support these claims. (C) Close up of region outlined by white box in (B). Noteisoclinally folded late pegmatite vein, further supporting a tectonised, not original igneous intrusive, nature of this margin. (D) Photographof relationships ca. 3 m north of (B) showing a similar area to that of Fig. 10c ofNutman et al. (2002)but expanded to show the contactwith the external mafic–ultramafic enclave to the west (top in photograph). Note that banding in the heterogeneous gneiss (gn) as well asan apparently concordant margin between homogeneous tonalitic gneiss and heterogeneous gneiss (along a–a) is truncated at the marginwith the mafic–ultramafic enclave in the bottom right corner of the photograph which is consistent with a tectonised margin (the contactis locally intruded by a late pegmatite (peg) and the dashed line boundary of this is taken as a proxy for the original contact). Below thelabel “ton”, the homogeneous tonalitic gneiss has been interpreted byNutman et al. (2002)to cut banding in the heterogeneous gneiss aswell as an apparently concordant contact between heterogeneous gneiss and an ultramafic boudin (um). Given the degree of strain in thisoutcrop, it is in our view possible that the contact of the homogeneous tonalitic gneiss with these two units is also tectonic although weacknowledge the preference ofNutman et al. (2002)that it is intrusive.


banding in heterogeneous gneisses as described above,may locally cross-cut them (Fig. 4D; see also Fig. 10cof Nutman et al., 2002). This relationship, if correctlyinterpreted byNutman et al. (2001, 2002), suggeststhat dating the homogenous tonalitic gneiss providesa minimum age for lithological banding in the hetero-geneous gneisses. On the basis of lithological similar-ity alone,Nutman et al. (2002)also consider small,isolated ultramafic lenses within the composite sheet,including the one at the centre ofFig. 4D, to be dis-rupted parts of the mafic–ultramafic enclave implyingthat the apparent cross-cutting relationship describedabove may also be used to provide a minimum age forthe entire SW Akilia enclave, although this relation-ship has not been rigourously demonstrated.

We propose here that geochronological data forsample SM/GR/97/7 (Whitehouse et al., 1999) of-fers a new insight into the relative ages of differentunits within the composite gneiss sheet. The zirconsin this sample preserve clear evidence for igneouscrystallization at ca. 3.65 Ga, i.e. typically igneousoscillatory banding (see Fig. 7 ofWhitehouse et al.,1999), moderate Th/U ratios (0.06–0.7), rare, partlyresorbed >3.8 Ga cores. Accepting that this sampleis part of the heterogeneous gneiss (see discussionabove) and, for the purpose of the current discus-sion, Nutman et al.’s (2001, 2002)interpretation ofthe cross-cutting relationship shown inFig. 4D asan original igneous intrusive contact, we considerthat foliation in the heterogeneous gneiss, definedby injected felsic bands and layers that presumablyhost the ca. 3.65 Ga zircon, must be older than theclaimed ca. 3.85 Ga homogeneous tonalitic gneiss.Clearly therefore, the field relationships are in severeconflict with the geochronology, with an apparentlyolder rock yielding younger zircons (3.65 Ga) than anapparentlyyounger rock that yields older zircons (ca.3.85 Ga). However, this conflict has a straightforwardsolution and one which has been mentioned on severalprevious occasions (Kamber and Moorbath, 1998;Whitehouse et al., 1999, 2001). The older zircon agesfrom the homogeneous tonalitic gneiss are simplyinherited from a precursor rock which probably par-tially melted during one (or both) of the high-grademetamorphic events that have affected the region. Inour opinion, the homogeneous tonalitic gneiss sheet,as we see it today is therefore best interpreted as alate-syn- or post- (ca. 3.65 Ga) kinematic intrusion

into complex banded, polyphase gneisses, some ofwhich might still have much older protolith ages.

This interpretation renders meaningless any argu-ments that old inherited zircons would have beendissolved in the homogeneous tonalitic gneiss (e.g.Nutman et al., 2001; Mojzsis and Harrison, 2002a,b).Additionally, by proposing a<3.65 Ga age for thehomogeneous tonalitic gneiss, it addresses a largelyignored paradox in the composite gneiss sheet thatwas noted byMyers and Crowley (2000), namelythat the homogeneous tonalitic gneiss, consistentlyclaimed to be ca. 3.85 Ga, appears to have escapedthe effects of extreme deformation during high-grademetamorphism at both ca. 3.65 and 2.7 Ga (Griffinet al., 1980), the effects of which are pervasive else-where on Akilia, not least within the heterogeneousgneisses of the composite sheet itself.

We reiterate that discussion of the detailed fieldrelationships and relative chronology of the Amı̂tsoqgneisses and mafic–ultramafic enclaves is only rele-vant to the age of the quartz–pyroxene rocks on SWAkilia if the latter can be unambiguously shown to bepart of a temporally continuous volcano-sedimentarysequence and not the result of metasomatic and meta-morphic processes (Fedo and Whitehouse, 2002a;Myers and Crowley, 2000).

5. Deformation features in thequartz–pyroxene rock

In their original study of Akilia Association rocks,McGregor and Mason (1977)stated: “At many local-ities rocks. . . with unusually high contents of quartzhave been noted. Superficially they resemble bandedironstones, but. . . they are rocks of varied parentagewhich have suffered intense silica metasomatism, of-ten with quartz introduced along the layering of theparent rocks.” The caution expressed in this commenthas largely been ignored by subsequent workers (e.g.Mojzsis et al., 1996; Nutman et al., 1997; Anbar et al.,2001) who have used tenuous criteria to claim a BIForigin for the entire 5 m wide quartz–pyroxene unit onsouthwest Akilia. Specifically, two lines of evidencehave been presented to support a BIF origin (Nutmanet al., 1997): (1) the supposed presence of magnetitelayering akin to that seen at Isua in quartz–magnetiteBIF, and (2) a non-specific comparison with classical


Fig. 5. Features of the quartz–pyroxene rock. Scale units are in centimeters in all panels. (A) Boudinage, pinch, and swell and formationof fine banding; note quartz accumulation in boudin necks (arrowed); (B) coarsely crystalline quartz and pyroxene banding with pyroxeneboudins in the quartz–pyroxene rock resulting from intense ductile strain; (C) garnet-rich ductile shear zone that cross-cuts foliation in thequartz–pyroxene rock and is clearly a late feature. The quartz–pyroxene rock at this location has itself been strongly sheared leading thepresently observed fabric.


BIF based on the work of James (1954). Our collec-tive field observations between 1997 and 2001 wereundertaken to investigate the veracity of these claimsconsidering the comment ofMcGregor and Mason(1977)as our null hypothesis—i.e. is any positive ev-idence for a BIF origin preserved on Akilia or are therocks simply the product of extreme silica (and other)metasomatism?

Much of the primary literature pertaining to Ak-ilia is difficult to interpret because of false statementsabout the quartz–pyroxene unit, for example: “. . . onemetachert-BIF unit is broken up neither by boudinage,nor minor shearing, nor grossly disrupted by silica mo-bility. It forms a parallel-sided,∼5 m thick unit . . . ”(Nutman et al., 1997) and “A 40 m long section of BIF. . . has escaped boudinage, shearing, or disruption ofthe unit by silica mobility and forms a parallel-sided,∼5 m thick outcrop. . . ” (Anbar et al., 2001). Suchstatements convey the impression that the unit is anamazingly well-preserved, relatively low-strain, sedi-mentary rock, an impression that is reinforced by thestatement that “finely laminated >3850 million yearBIF” escaped “perturbation” by meteorite bombard-ment (Mojzsis et al., 1996). Fedo and Whitehouse

Fig. 6. Metamorphosed and folded mafic igneous rocks with secondary veins of quartz (qtz), mafic–ultramafic rocks (m/um) and carbonate(carb) from the Isua greenstone belt. This is a likely lower metamorphic grade and lower strain analog for rocks on Akilia. Scale unitsare in centimeters.

(2002a)presented a detailed lithological section acrossthe quartz–pyroxene unit on southwest Akilia whichwas used to show features within the quartz–pyroxenerock that refute the statements quoted above.

The quartz–pyroxene rock possesses a strong tec-tonic foliation defined by alternations of quartz-richbands and pyroxene-rich bands. Pyroxene-rich bandsshow pervasive evidence of pinch-and-swell struc-ture and boudinage at millimeter to centimeter scales(Fig. 5A) throughout the∼5 m thickness of the unit.Much of the millimeter-scale banding represents tailsof boudins suggesting that boudinage is not onlypresent, contrary toMojzsis et al. (1996)andNutmanet al. (1997), but also is prevalent in this highly de-formed unit. Where thicker pyroxenite bands beginto segment, coarse clear quartz was deposited inthe neck areas (Fig. 5A), suggesting the presence ofmobilized quartz. A very coarse post-tectonic gra-noblastic texture is responsible for annealing internalmineral strain, but well-defined boudins with longtails demonstrate the ductile deformation (Fig. 5B).Our observations are consistent with the boudinage ofcentimeter-wide mafic dykes in the adjacent Amı̂tsoqgneisses and indicate the overall high strain in the


area. The high-strain foliation in the quartz–pyroxenerock is coaxial with the foliation in the adjacentmafic/ultramafic rocks, and is cross-cut by minorbrittle fractures filled by later mobilized pyroxene.

Suggestions that the quartz–pyroxene lithologyhas escaped shearing, minor, or otherwise, are com-pletely inaccurate. The quartz–pyroxene rock itselfis a product of intense shearing coupled with meta-somatic additions during multiple high-grade defor-mational events. In the absence of usable markers, aquantitative strain estimate for the quartz–pyroxenerock is not attainable. However, the intensity of thestrain recorded in the adjacent gneisses indicates itis very high. Furthermore, minor ductile shear zonescross-cut the quartz–pyroxene rock and enclosingmafic–ultramafic rocks every few metres along strike.These steeply dipping shear zones are commonly sev-eral centimeters wide, show an apparent right lateraloffset, and contain abundant garnet where they passthrough the quartz–pyroxene rock (Fig. 5C).

Consequently, we argue that these rocks, reputedlyclaimed to host life (e.g.Mojzsis et al., 1996; Nutmanet al., 1997), are actually the products of highly meta-somatized and deformed mafic and ultramafic igneousrocks. A similar interpretation has emerged in theIsua greenstone belt, where rocks claimed to be BIFand hosting graphite interpreted as biogenic in origin(Mojzsis et al., 1996) are now recognized as meta-somatized mafic rocks with graphite as a product ofsiderite decomposition or contamination (van Zuilenet al., 2002). We envision a similar scenario for therocks on Akilia, and show a photograph from Isua(Fig. 6) as a potential lower metamorphic grade anal-ogy for comparison. Mafic rocks show abundant whiteveins of quartz that have entered along foliation and athicker band of replacement carbonate, also followingfoliation. These secondary veins are clearly early, asthey are folded and metamorphosed with the maficrocks that host them. Once this assemblage is raised togranulite-facies with accompanying ductile strain, thecarbonate would decompose (e.g.Perry and Ahmad,1977) and a prominent parallel foliation would result.

6. Summary

All lithologies on the southwestern corner of Akiliaexhibit extremely intense deformation that is the result

of both early and late Archaean tectonometamorphicevents in the region. All internal features of the crit-ical, supposedly life-bearing (Mojzsis et al., 1996)quartz–pyroxene rock can be explained by a combi-nation of metasomatism, high-grade metamorphismand tectonic transposition forming the heterogeneous,sheared, boudinaged and, in places, finely bandedlithology seen today (Fedo and Whitehouse, 2002a).Primary evidence for a BIF origin is completely lack-ing and, without such support, the carbon isotopearguments for early biogenic activity become irrele-vant. Furthermore, intense deformation has affectedall lithological contacts so that claimed geochronolog-ical constraints from “intrusive” gneisses cannot besubstantiated. In our view, the search for life greaterthan ca. 3.7 Ga ago should concentrate on localitieswithin the Isua greenstone belt where there is at leastunambiguous evidence for early Archaean sedimentsat 3.7–3.8 Ga and the effects of deformation, metaso-matism, and metamorphism are better understood.

Acknowledgements

We acknowledge financial and logistic support bythe Swedish Research Council, National GeographicSociety, and George Washington University. TheDanish Natural Science Research Council, the Com-mission for Scientific Research in Greenland and theBureau of Minerals and Petroleum, Nuuk, Greenland,generously support the Isua Multidisciplinary Re-search Project, from which this is a contribution. TheGreenland Home Rule Government generously gavepermission to conduct studies on Akilia. We thankJim Crowley and Hugh Rollinson for thorough andconstructive reviews. We also thank L.L. Sørensenand V.L. Pease for field support in 2001.

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