A Review of Early Archean Geology in the Godtha˚Bsfjord Region, and Reappraisal of ﬁeld Evidence for \3850 Ma Life on Akilia

$A Review of Early Archean Geology in the Godtha˚Bsfjord Region, and Reappraisal of ﬁeld Evidence for \3850 Ma Life on Akilia$

Precambrian Research 103 (2000) 101–124 www.elsevier.com/locate/precamres

Vestiges of life in the oldest Greenland rocks? A review of early Archean geology in the Godtha˚bsfjord region, and reappraisal of ﬁeld evidence for \3850 Ma life on Akilia

John S. Myers *, James L. Crowley

Department of Earth Sciences, Memorial Uni6ersity of Newfoundland, St John’s, Newfoundland, Canada A1B 3X5 Received 30 November 1999; accepted 18 May 2000

Abstract

The Godtha˚bsfjord region of West Greenland contains the most extensive, best exposed and most intensely studied early Archean rocks on Earth. A geological record has been described of numerous magmatic events between 3.9 and 3.6 Ga, and evidence of life at \3.85 Ga and 3.8–3.7 Ga has been proposed from two widely-separated localities. Some of these claims have recently been questioned, and the nature of the best preserved remnants of the oldest known terrestrial volcanic and sedimentary rocks in the Isua greenstone belt are being reinvestigated and substantially reinterpreted. The ﬁrst part of this article reviews the evolution of geological research and interpretations, outlining the techniques by which the geological history has been determined and the ensuing controversies. The second part re-examines crucial ﬁeld evidence upon which the antiquity of the oldest terrestrial life is claimed from the island of Akilia. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Early Archean; Gneiss complex; Geochronology; Oldest life; Greenland

1. Introduction sedimentary rocks during the late Archean. These rocks were repeatedly deformed and metamor- The Godtha˚bsfjord region lies in the centre of the phosed, developing gneissosity that was folded, refolded and transposed into new tectonic layering, Archean gneiss complex on the west coast of and recrystallizing during and after deformation at Greenland in the vicinity of Nuuk (Fig. 1, inset amphibolite or granulite facies (Bridgwater et al., map). Most of the Archean gneiss complex is 1976). derived from sheets of tonalite and granodiorite, In a belt 25–75 km wide, extending for 200 km and a small amount of granite and diorite, that were through Godtha˚bsfjord (Fig. 1), similar late intruded into basaltic volcanic rocks and minor Archean plutonic rocks were intruded into, and tectonically interleaved with, early Archean * Corresponding author. Tel.: +1-709-7378417; fax: +1- tonalitic and granodioritic gneisses, and both early 709-7372589. and middle Archean metavolcanic and metasedi- E-mail address: [email protected] (J.S. Myers). mentary schists and gneisses.

The ﬁrst part of this article (Sections 2–4) vious observations and interpretations, and out- reviews knowledge of the early Archean rocks lining current controversies. Most of the early that formed before 3600 Ma, synthesizing pre- Archean rocks were intensely, and repeatedly, de-

Fig. 1. Geologic map of the Godtha˚bsfjord region, compiled mainly from maps by Allaart (1982), Chadwick and Coe (1983, 1988), Garde (1987, 1989) and McGregor (1984), with terrane boundaries and Ikkattoq gneiss from McGregor et al. (1991) and geochronology from Nutman et al. (1996) and Nutman (1997). The inset map of Greenland locates Nuuk and the main regions of Archean gneiss that escaped pervasive ductile Proterozoic deformation. J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 103 formed and metamorphosed at high grade during rocks cut by undeformed dykes from Archean late Archean events and, except in the vicinity of rocks and dykes that were deformed and meta- Isua, the present structure and appearance largely morphosed during the early Proterozoic. reflect these late Archean events (cover of Precam- Berthelsen and McGregor were the first to use brian Research, this volume). Therefore a brief basic dykes to distinguish between Archean tec- outline of the late Archean tectonic evolution is tonic, magmatic and metamorphic events in included in this review (Section 5), indicating cur- Greenland. Berthelsen (1955) recognized that ba- rent interpretations of how the early Archean sic dykes could be used to distinguish between rocks were intruded by, and tectonically inter- older and younger geological events in the south- leaved with, younger Archean rocks. eastern part of the island of Qilańgaˆrsuit (Fig. 1). The second part of this article (Section 6) re-ex- McGregor (1966, 1968, 1973), during a detailed amines the field evidence from the island of Akilia investigation of the gneisses in the vicinity of upon which the antiquity of life is claimed to be Nuuk (formerly Godtha˚b and Nuˆk) between \3800 Ma (Mojzsis et al., 1996) and ]3850 Ma 1965–71, discovered that these basic dykes could (Nutman et al., 1997a; Mojzsis and Harrison, be used as a regional mapping criterion separating 2000), in the vicinity of the gneiss shown on the older from younger events. McGregor (1968) cover of Precambrian Research (this volume). called the dykes Ameralik dykes and recognized that intense deformation had severely modified the original structure of the dykes and their host 2. Pioneering studies in outer Godtha˚bsfjord rocks such that most dykes and host rocks were rotated into a new common gneissosity (e.g. cover Current understanding of the geology stems of Precambrian Research, this volume). Cross- from the pioneering work of Berthelsen (1955) cutting relationships were preserved only in small and McGregor (1966, 1968, 1973), who demon- areas of relatively low strain. Likewise, McGregor strated that basic dykes could be used as field (1973) described how the cross-cutting relation- criteria to subdivide the Archean gneisses into ships of granitoid rocks that were intruded after older and younger components. The basic dykes the dykes were also widely obliterated by intense cut across older rocks, structures and gneissosity, deformation that rotated most components of the and are cut by younger granitoid rocks that were gneiss complex into subparallel layers. subsequently deformed, metamorphosed and con- Many were sceptical of McGregor’s interpreta- verted to gneisses. tion, and some considered metamorphic grade to This technique had previously been developed be a key indicator of the relative age of rocks and in gneiss complexes elsewhere, especially in north- events. Based on a wider regional survey of the west Scotland by Peach et al. (1907) and Sutton Archean gneisses of West Greenland, Windley and Watson (1951), and in southern Finland by (1968, 1969) regarded the linear NNE-trending Sederholm (1923, 1926) and Wegmann (1931). belt of amphibolite facies gneisses in the Wegmann (1938) introduced this concept to Godtha˚bsfjord region, including the gneisses de- southern Greenland, distinguishing major tectonic scribed by McGregor, as derived by tectonic re- and metamorphic episodes (subsequently recog- working and retrogression of older granulite nized as Archean and Proterozoic) on the basis of facies gneisses to the north and south. He consid- intervening dyke swarms. During field studies be- ered that the Ameralik dykes of McGregor (1968) tween 1946 and 1955, Ramberg (1948) and Noe- cut older granulite facies gneisses to the south of Nygaard and Ramberg (1961), assisted by Ameralik (Fig. 1) and ‘can be followed… into the Berthelsen, applied this concept further north and linear belts where they become progressively de- presented the first fundamental subdivision of the formed and migmatised’ (Windley, 1969, p. 159). Precambrian gneiss complex from Godtha˚bsfjord Thus, in contradiction of McGregor’s hypothesis, northwards for 600 km. They used early Protero- the dykes and relatively straight NNE-trending zoic dykes to distinguish unmodified Archean belt of highly deformed amphibolite facies 104 J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 gneisses through Godtha˚bsfjord were interpreted gneisses were highlighted by Coe et al. (1976) and as younger than regional granulite facies meta- Chadwick and Coe (1983) who discovered intra- morphism. However, McGregor’s hypothesis was Nuˆk dykes (Neriuneq and Qa´qatsiaq dykes) south subsequently supported by geochronology. Black of Ameralik (Fig. 1), broadly similar in appear- et al. (1971) discovered that the whole rock RbSr ance to many Ameralik dykes. Chadwick (1981) and PbPb isochron ages of gneisses that were cut described another, smaller and more controversial by Ameralik dykes in the Godtha˚bsfjord belt were group of dykes he called Tinissaq dykes, thought one billion years older than the widespread gran- to be intruded into an early phase of Nuˆk ulite facies metamorphism, and were far older gneisses. On the island of Qilańgaˆrsuit to the west than any other terrestrial rocks known at that (Fig. 1), Chadwick (1981) recognized eight types time. of Ameralik dykes intruded into Amıˆtsoq gneisses Black et al. (1971) called the rocks that were and two types of amphibolite dykes, ‘similar to older than the Ameralik dykes ‘Amıˆtsoq gneisses’ two main categories of Ameralik dykes’ (ib. cit. P. and discovered that they were characterized by 221) cutting the Malene supracrustals. Chadwick extremely unradiogenic whole-rock lead isotopes. (1981), pp. 223–224) concluded that ‘limited evi- They described the sequence of events following dence of dyke intersections suggests some Amera- the intrusion of the Ameralik dykes as: eruption lik dykes may be older than the Malene of basic volcanic rocks (now mostly amphibolite) supracrustal rocks, and others, including Malene and deposition of sedimentary rocks (mainly dykes, may have been injected after the tectonic quartz-biotite-garnet schist), collectively called interleaving’ of Amıˆtsoq gneisses and Malene Malene supracrustals by McGregor (1969); tec- supracrustals. tonic interleaving with the Amıˆtsoq gneisses; em- The Malene supracrustals were thought to have placement of stratiform anorthosites; and been deposited on Amıˆtsoq gneisses (after 3600 syntectonic intrusion of a large volume of calc-al- Ma) before the intrusion of Nuˆk gneisses at 3070 kaline rocks that were deformed and metamor- Ma (Chadwick and Nutman, 1979; Nutman and phosed to become gneisses that they called Nuˆk Bridgwater, 1983). However, UPb studies of gneisses. The protoliths of the Nuˆk gneisses were metasedimentary rocks within the Malene largely intruded as sills and were further inter- supracrustals by Schiøtte et al. (1988) delineated a leaved with all the older rocks by tectonic pro- range in the age of detrital zircons from 2900 cesses, producing a complex tectono-stratigraphy to 2650 Ma. They found some rocks with B2800 (Bridgwater et al., 1974). Ma detrital zircons, younger than Nuˆk gneisses Isotopic ages of the gneisses were extensively and high grade regional metamorphism at 2800 studied during the 1970s. Protolith ages of the Ma. Therefore ‘the concept of the Malene Amıˆtsoq gneisses were found to range between supracrustals as a single lithostratigraphic unit’ 3.8–3.65 Ga and those of the Nuˆk gneisses was found to be ‘no longer valid’ (Schiøtte et al., between 3.0–2.8 Ga. Results included RbSr 1988, p. 56). This work also indicated a more whole rock isochrons of 3.75 (Moorbath et al., prolonged and complex sequence of late Archean 1972), PbPb whole rock isochrons of 3.62 Ga tectonic, magmatic and metamorphic events than (Black et al., 1971) and 3.8 Ga (Moorbath et previously recognized. al., 1975), and UPb discordia upper intercepts of The Amıˆtsoq gneisses contain small bodies of 3.65 Ga from bulk zircons (Baadsgaard, 1973) highly metamorphosed basic, ultrabasic and sedi- and single zircons (Michard-Vitrac et al., 1977). mentary rocks that were first described, and called Ages of Nuˆk gneisses determined during the 1970s the Akilia association, by McGregor and Mason included RbSr whole rock isochrons of 3.04 (1977). They stated that ‘one of the largest and Ga (Pankhurst et al., 1973) and 2.93–2.78 Ga best exposed occurrences… is on the southwestern (Moorbath and Pankhurst, 1976). tip of the island of Akilia. This is designated the Complications in the use of amphibolite dykes type locality for the Akilia association, an infor- as time markers between Amıˆtsoq and Nuˆk mal term introduced here for all the enclaves of J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 105 older rocks in the Amıˆtsoq gneisses in the and early 1980s. This research included stratigra- Godtha˚b region with the exception of those in phy and sedimentology (Dimroth, 1982; Nutman the Isua supracrustal belt’ (p. 889). The geology et al., 1984), structure (James, 1976), petrology, and current controversies of southwestern Akilia mineralogy and geochemistry (Schidlowski et al., (Fig. 1) are described later in this paper (Section 1979; Gill and Bridgwater, 1979; Gill et al., 1981; 6). Boak et al., 1983; Nutman and Bridgwater, In this region south of Nuuk, the Akilia associ- 1986), metamorphism (Boak and Dymek, 1982), ation and the main, older, components of the geochronology (Moorbath et al., 1975; Baads- Amıˆtsoq gneisses were metamorphosed to gran- gaard, 1976; Michard-Vitrac et al., 1977; Hamil- ulite facies at 3600 Ma (Griffin et al., 1980). ton et al., 1978), oxygen and sulphur isotope The original extent, and tectonic and magmatic studies (Oehler and Smith, 1977; Oskvarek and context, of this metamorphism are unknown. Perry, 1976; Perry and Ahmad, 1977) and organic The Isua supracrustal belt (Isua greenstone belt chemistry (Nagy et al., 1975). The supracrustal of Appel et al., 1998) (Fig. 1) was considered by rocks and adjacent tonalitic gneisses were McGregor and Mason (1977) to be an exception- mapped and described by Allaart (1976), Nutman ally large, temporal equivalent of the Akilia asso- et al. (1984) and Nutman (1986). ciation, at lower metamorphic grade and with a Many interpretations from this era of research well preserved stratigraphy. have since been questioned and many fundamental aspects of the geology remain controversial. Claimed discoveries of early life by Walters, 3. Isua 1979, cited in Nagy et al., 1981) based on hydrocarbons, and ‘yeast-like microfossils’ called Isuas- The metavolcanic and metasedimentary rocks phaera by Pflug (1978) and Pflug and at Isua were initially thought to be Proterozoic Jaeschke-Boyer (1979), were refuted by Nagy et (Windley, 1969) because of their relatively low al. (1981) and Bridgwater et al. (1981) respec- metamorphic grade and a K/Ar whole rock age tively, on the grounds that these claims were of 1940950 Ma on a ‘low-grade metabasic’ rock incompatible with the highly deformed and meta- ‘forming…sills in the weakly metamorphosed morphosed state of the rocks. quartzitic component’ (Lambert and Simons, More extensive controversy has ranged over 1969, p. 69). However, Moorbath et al. (1972) the interpretation of rocks claimed to be of sedi- obtained a RbSr whole rock isochron of 37009 mentary origin, especially conglomerates and sed- 140 Ma on the adjacent tonalitic gneiss (subse- imentary carbonates that were interpreted as quently recalculated to 3640950 Ma, see indicating shallow-water deposition (Dimroth, Moorbath and Whitehouse, 1996), and a whole 1982; Nutman et al., 1984; Nutman, 1986). Rose rock PbPb age of 3760970 Ma on a banded et al. (1996) and Rosing et al. (1996) demon- iron formation (Moorbath et al., 1973) (subse- strated that the carbonates are largely of metaso- quently recalculated to 3710970 Ma, see Moor- matic origin, and Fedo (2000) has reinvestigated bath and Whitehouse, 1996), indicating that this the conglomerates and associated clastic rocks greenstone belt comprised the oldest known and reviewed the controversies about their origin. metavolcanic and metasedimentary rocks on Recently, there have been fundamental reinter- Earth. Bridgwater and McGregor (1974) outlined pretations of the original nature of the rocks that the main features of the geology and postulated comprise most of the Isua greenstone belt. Previ- that the well-preserved metadolerite dykes cutting ous interpretations were based on detailed map- the Isua greenstone belt and adjacent tonalitic ping by Allaart and Nutman between 1974–82 gneiss were equivalent to the Ameralik dykes. (Nutman et al., 1984; Nutman, 1986). They inter- Recognition of the extreme antiquity of the preted the greenstone belt as an upward-facing metavolcanic and metasedimentary rocks at Isua syncline on the basis of stratigraphy and small- led to a spate of diverse research during the 1970s scale way-up criteria. The mapped stratigraphy 106 J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 was defined by a mixture of features including quence of originally low-K tholeiitic basalt, metamorphic rock types, metamorphic textures chert-BIF and turbidite. They interpreted the and interpreted protoliths. Studies during the greenstone belt as an accretionary complex that 1990s, especially of the metacarbonate rocks by developed in an intraoceanic environment, Rose et al. (1996) and Rosing et al. (1996), rec- analogous to the late Phanerozoic tectonic evolu- ognized that ‘most of the supracrustal rocks have tion of Japan, and attributed most of the struc- undergone pervasive metasomatism, which has tural evolution of the belt to this episode of obscured their origin’ (ib. cit., p. 43), and ques- intraoceanic accretion. However, this interpreta- tioned the significance of the stratigraphy de- tion does not appear to recognize the multiple scribed by Nutman et al. (1984) and Nutman episodes of intense regional ductile deformation, (1986). metamorphism and metasomatism, described by Remapping of parts of the greenstone belt in most other investigators such as Bridgwater and 1997–99 by Myers (2000) supports the general McGregor (1974), James (1976), Nutman et al. conclusions of Rose et al. (1996) and Rosing et (1984), Rosing et al. (1996) and Myers (2000), al. (1996). Myers (2000) found that, although that converted most of the Isua belt to schists, most of the rocks were intensely deformed and and formed and deformed the schistosity in the many were substantially altered by metasoma- adjacent tonalitic gneiss. tism, the deformation and metasomatism were The new mapping by Komiya et al. (1999) and heterogeneous, and transitional stages could be Myers (2000) indicates that a major component traced from rocks with recognizable primary vol- of the Isua greenstone belt was derived from canic and sedimentary structures into schists in basaltic pillow lava, in contrast to previous de- which all primary features were obliterated. This tailed accounts by Nutman et al. (1984) and mapping demonstrated that most of the Isua Nutman (1986), who described many of these greenstone belt was derived from basaltic and rocks in terms of metamorphic textures (such as high-Mg basaltic pillow lava and pillow lava ‘garbenschiefer unit’, ib. cit., p. 15) and inter- breccia, with a smaller amount derived from preted the protoliths as mafic intrusions (such as chert-BIF, layered mafic-ultramafic intrusive metagabbro, Nutman, 1997). sheets, ultramafic rocks (of autochthonous extru- Another controversial major component of the sive and/or intrusive origin, and/or al- Isua greenstone belt is quartzo-feldspathic schist lochthonous origin), and a minor component of and mylonite described as felsic formations A6 clastic sedimentary rocks (see Fedo, 2000). The and B1 by Nutman et al. (1984) and Nutman mapping also revealed a complex tectonic history (1986), and dated as \3790 Ma and 3710 Ma of polyphase deformation and high grade meta- respectively by Nutman et al. (1997b). Parts of morphism in which compositional layering and this schist contain ellipsoidal fragments of grani- foliations were isoclinally folded and refolded be- toid rocks that have been interpreted as of vol- fore the greenstone belt was assembled as a num- canic pyroclastic origin (Allaart, 1976; ber of fault-bounded packages of strongly to Bridgwater et al., 1976; Dimroth, 1982). How- intensely deformed rocks. These structures and ever, Nutman et al. (1984) and Nutman (1986) schistosities were cut by swarms of basic and attributed the fragmentation to tectonic processes ultrabasic dykes (equivalent to Ameralik dykes) in a felsic volcanic rock. They described how the and then deformed again, probably in the late ellipsoidal fragments were located in fold hinges, Archean. and that away from fold hinges the fragments Komiya et al. (1999) also interpreted the passed into tabular bodies and then continuous northeastern part of the belt as a stack of fault- layers. Rosing et al. (1996) reinterpreted the sup- bounded rock packages on the basis of detailed posed felsic volcanic rocks as products of ‘meta- mapping. They identified a number of duplex somatic alteration of intrusive Amıˆtsoq structures and reconstructed the lithostratigraphy tonalite-granite gneiss’ (p. 44). Myers (2000) de- as a tectonically repeated, simple upward se- scribed how some layers with supposed pyroclas- J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 107 tic fragments were derived from metasomatically gether of rocks of different ages’ (p. 415) and altered basaltic pillow lavas and tectonically dis- proposed a plate tectonic scenario. rupted intrusive sheets of tonalite. In other parts This interpretation has been challenged in a of the felsic formation A6 of Nutman et al. number of papers by Kamber, Moorbath and (1984), Myers (2000) found large scale, along- Whitehouse. Kamber and Moorbath (1998) dis- strike gradations between quartzo-feldspathic cussed new and published Pb isotope data for schist and mylonite, and tonalitic gneiss. Amıˆtsoq gneiss feldspars and whole rocks and Frei et al. (1999) discovered heterogeneity in concluded ‘that the magmatic precursors of the late Archean metamorphic recrystallization of the Amıˆtsoq orthogneisses separated from a depleted Isua greenstone belt by Pb stepwise leaching of mantle-like source at ca. 3.65 Ga’ (p. 19). They magnetite from banded iron formations. They proposed that zircons described by Nutman et al. recognized that the amphibolite facies metamor- (1993, 1996) with ages significantly older than phic mineral assemblages of banded iron forma- 3.65 Ga were inherited from older rocks. This tions in the northeastern part of the belt conclusion was supported by cathodolumines- developed at 3.69 Ga. These rocks were cut by cence imaging and ion-microprobe UThPb dat- magnetite and sulphide-bearing veins associated ing of zircons from Amıˆtsoq gneisses by with metasomatic alteration at 3.63 Ga. In Whitehouse et al. (1999), who found 3.8 Ga contrast, in the western part of the Isua green- cores mantled by overgrowths at 3.65 Ga. stone belt, step leaching of magnetite from simi- They interpreted the cores as inherited grains and lar banded iron formations yielded a PbPb the overgrowths as indicating the magmatic em- isochron age of 2.84 Ma, interpreted as the age placement age, in contrast to Nutman et al. of peak amphibolite facies metamorphism in this (1993), who interpreted similar overgrowths as region. metamorphic, resulting from granulite facies Carbon isotopic ratios, considered to be of metamorphism first described from the same re- biogenic origin, were determined by Rosing gion south of Nuuk by Griffin et al. (1980). (1999) from graphite in schistose rocks inter- Subsequently, from a region unaffected by early preted as metagraywackes in the Isua greenstone Archean granulite facies metamorphism, Nutman belt. These metasedimentary rocks were recog- et al. (1999) provided a detailed description of the nized in a lenticular zone of relatively low strain 3800 Ma age of little deformed metatonalites within a thick sequence of deformed amphibolite and metaquartz-diorites immediately south of the schist, mainly derived from basaltic volcanic Isua greenstone belt. rocks. This interpretation, and the \3700 Ma Controversy also surrounds the interpretation age of the graphite, remains unchallenged. of SmNd isotopic data from the Amıˆtsoq gneisses. Bennett et al. (1993) interpreted the wide m range in initial Nd values of Amıˆtsoq gneisses as 4. Numerous magmatic events between 3870 indicating transient, highly depleted mantle reser- and 3625 Ma? voirs, and proposed a major change in processes between early Archean and later styles of de- Nutman et al. (1993) proposed on the basis of pleted-mantle evolution. These hypotheses were SHRIMP UPb zircon data that ‘the Amıˆtsoq challenged by Vervoort et al. (1996), Gruau et al. gneisses comprise 3870, 3820–3810, 3760, 3730, (1996), Moorbath et al. (1997) and Frei et al. 3700, and 3625 Ma TTG and 3660–3650 and (1999), who argued that the SmNd systems, as- 3625 Ma granites, and their inclusions belong to sumed to have been closed systems by Bennett et several supracrustal sequences with a similar al. (1993), were modified by subsequent tec- spread of ages’ (ib. cit., p. 415). They concluded tonothermal events. This prompted substantial that ‘these results show that the complex grew by debate by Bennett and Nutman (1998) and Kam- episodic addition of new TTG and welding to- ber et al. (1998). 108 J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124

5. Terranes and late Archean tectonic evolution The Tasiusarsuaq terrane to the south (Fig. 1) is dominated by 2920 and 2860–2820 Ma tonalitic Detailed mapping of the geology between Tre gneisses derived from sheets of tonalite intruded Brødre and Ameralik (southwestern part of Fig. into metavolcanic rocks (now amphibolite), lay- 1) led to the recognition of three tectonostrati- ered anorthosite complexes, and minor metasedi- graphic terranes, distinguished by different com- mentary rocks, and was metamorphosed to ponents and geological histories, that were called granulite facies at 2820 Ma (McGregor et al., the Færingehavn, Tre Brødre and Tasiusarsuaq 1991; Nutman, 1997). The Tasiusarsuaq terrane terranes (Friend et al., 1987). Regional reconnais- was thrust northwards over the southern margin sance studies by Friend et al. (1988) and Nutman of the Akulleq terrane, causing the development et al. (1989) extended this concept throughout the of new LS fabrics at amphibolite facies conditions Godtha˚bsfjord region and defined a fourth ter- before the intrusion of 2710 Ma granitic sheets rane (called Akia) to the north (Fig. 1). McGregor that cut all three terranes (Nutman et al., 1996; et al. (1991) subsequently combined the Nutman, 1997). To the north of the Akulleq Færingehavn and Tre Brødre terranes into a sin- terrane, the Akia terrane (Fig. 1) consists of gle terrane termed Akulleq. The Akia, Akulleq 3230 and 3000 Ma dioritic, tonalitic, trond- and Tasiusarsuaq terranes (Fig. 1) were inter- hjemitic and granitic gneisses, amphibolite largely preted as originally independent rafts of continen- derived from volcanic rocks, and intrusions of leucogabbro and norite, all of which experienced tal crust, distinguished by different rock assemblages with different SHRIMP UPb zircon granulite facies metamorphism at 3000 Ma (Riciputi et al., 1990; McGregor et al., 1991; ages and metamorphic histories, that were assem- Nutman, 1997). bled at 2720 Ma (Friend et al., 1996). This subdivision of the Godtha˚bsfjord region The early Archean Amıˆtsoq gneisses and into terranes led to revision of the previous younger Ameralik dykes are confined to the nomenclature because rocks formerly called Akulleq terrane, where they are tectonically inter- Malene supracrustals (McGregor, 1969) and Nuˆk calated with younger metavolcanic and metasedi- gneisses (Black et al., 1971) were found to be of mentary rocks and layered anorthosite complexes. different ages in the different terranes. McGregor During this tectonic intercalation and associated et al. (1991) recommended that the name Malene amphibolite facies metamorphism, the Akulleq supracrustals be discontinued and the name Nuˆk terrane was intruded by granodiorite (Ikkattoq gneiss be restricted to rocks in the Akia terrane. gneisses) at 2820 Ma (McGregor et al., 1991; Nutman et al. (1996) introduced a term ‘Itsaq Nutman, 1997). Nutman et al. (1996) attributed Gneiss Complex’ to include all the early Archean this event to the development of the Akulleq rocks in the Godtha˚bsfjord region: Amıˆtsoq terrane as an accretionary complex in which di- gneisses, Isua supracrustal belt, and Akilia associ- verse crustal fragments accreted to the margin of ation. These rocks are restricted to, and partly a southern continent marked by the Tasiusarsuaq define, the Akulleq terrane (McGregor et al., terrane. They envisaged that the protoliths of the 1991). Ikkattoq gneisses formed during an early stage of a 2830 and 2720 Ma episode of accretion and subduction of the Akulleq terrane beneath the 6. Life on Akilia ]3850 Ma? Tasiusarsuaq terrane. ‘Final juxtaposition of the Akia, Akulleq and Tasiusarsuaq terranes occurred Here we re-examine fundamental evidence re- after the intrusion of the 2820 Ma Ikkattoq lating to the discovery claimed by Mojzsis et al. gneisses (which are truncated by the terrane (1996) of the oldest trace of terrestrial life in boundaries…), but before the intrusion of 2710 carbonaceous inclusions in apatite crystals from a Ma granitic sheets that cut all three terranes’ (ib. thin layer of quartz-rich rock on the small island cit., p. 8). of Akilia south of Nuuk (Figs. 1 and 2). We focus J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 109

and created difﬁcult conditions for the development and survival of life (Sleep et al. 1989). Therefore the claimed traces of terrestrial life ]3800 Ma are especially signiﬁcant and have wider consequences for interpreting the cause and extent of the lunar bombardment event.

6.1. Geology of southwestern Akilia

The small island of Akilia south of Nuuk (Figs. 1 and 2) consists of early Archean Amıˆtsoq gneisses containing numerous layers of amphibolite (Ameralik dykes) that were intensely deformed during the late Archean (Chadwick and Nutman, 1979; Chadwick and Coe, 1983) (cover of Precambrian Research, this volume). The Amıˆtsoq gneisses range in composition from diorite to granite; their plutonic origin, and the intrusive origin of the amphibolite layers (Ameralik dykes), were recognized nearby on Qilańgaˆrsuit by Berthelsen (1955) and around Nuuk by Mc- Gregor (1973). In the region south and southeast of Nuuk, the Amıˆtsoq gneisses comprise two main components, ‘banded grey gneisses’ and ‘augen and ferrodioritic gneisses’ (Chadwick and Coe, 1983), both of which exist on Akilia (Fig. 2). Augen gneisses range from quartz-diorite to gran- Fig. 2. Geologic map showing the location of Akilia within the regional tectonic framework, after Chadwick and Nutman ite and are characterized by megacrysts of micro- (1979) and Chadwick and Coe (1983). cline and by a high iron content (Chadwick and Coe, 1983). Amıˆtsoq gneisses contain fragments on new field evidence of the nature of the rocks, of diverse high grade metamorphic rocks, called their geological history and relative ages. We do Akilia association by McGregor and Mason not discuss whether the isotopic composition of (1977), that were thought to be mainly of the carbon described by Mojzsis et al. (1996) is of supracrustal origin and were interpreted as the unequivocal organic origin or ‘why the apatite host rocks into which the igneous precursors of the Amıˆtsoq gneisses were intruded. should act as a safe-deposit vault’ (Hayes, 1996, On the southwestern peninsula of Akilia (Fig. p. 22). Such questions were raised by Hayes 3), the type locality of the Akilia association (1996) in his Nature review that accompanied the (McGregor and Mason, 1977) comprises dis- publication of Mojzsis et al. (1996). ] rupted layers of mafic and ultramafic gneiss The age of 3850 Ma ‘for the Akilia island (mainly hornblende-diopside amphibolite, horn- sediments is… significant because it would place blende-pyroxenite and minor serpentinite) and a their deposition simultaneous with the Late thin layer of quartz-rich rock interpreted by Nut- Heavy Bombardment of the Moon at 3800–3900 man et al. (1997a) as banded iron formation and Ma’ (Mojzsis and Harrison, 2000, p. 3). During the oldest terrestrial record of water-lain sedimen- this event, the Earth is thought to have suffered tary deposits. more severely from frequent and large meteorite The adjacent quartzo-feldspathic gneiss is het- impacts that repeatedly vapourized the oceans erogeneous and comprises numerous phases of 110 J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 mainly tonalitic or quartz-dioritic composition that range from melanocratic to leucocratic and medium to coarse grained (Figs. 4 and 5). Further heterogeneity reflects variations in the intensity of the deformation, and the irregular distribution of numerous small fragments of tectonically dis-

Fig. 4. Heterogeneous gneiss comprising layers of pegmatite- banded tonalitic and dioritic Amıˆtsoq gneiss (grey), parallel layers and boudins of amphibolite (black) derived from Amer- alik dykes, and pegmatite veins (white). The superﬁcial sim- plicity reﬂects repeated episodes of intense ductile deformation by which layers of rock initially at high angles to each other were rotated, attenuated and transposed into parallelism. View to the south on southwestern Akilia, in the steep attenuated limb of a gently south-plunging fold that refolded isoclinal folds, boudins and previously attenuated parallel layering. Located on Fig. 3. Part of the same rock surface appears on the cover of Precambrian Research (this volume). The hammer is 33 cm long.

rupted layers of hornblende-pyroxene rocks, similar to parts of the larger bodies of mafic and ultramafic gneiss shown on Fig. 3. After deformation and the development of gneissosity, these rocks were intruded by sheets of more homogeneous melanocratic tonalitic rock before the intrusion of the Ameralik dykes. All the rocks were repeatedly, intensely deformed, and layers were isoclinally folded, refolded and segmented into boudins, resulting in a superficially simple banded gneiss composed of thin, greatly attenuated, parallel layers and boudins (Fig. 4 and cover of Fig. 3. Simplified geologic map of southwestern Akilia. Precambrian Research, this volume). J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 111

The amphibolite layers interpreted as deformed rotated into parallelism with the Ameralik dykes. remnants of dykes (Ameralik dykes) are generally Thus, although the gneissosity of both quartzo- more homogeneous than the Amıˆtsoq gneisses feldspathic Amıˆtsoq gneisses and mafic and ultra- and mafic and ultramafic gneiss, do not contain mafic gneiss of the Akilia association is parallel, it abundant thin layers of pegmatite that are an did not all form at the same time and reflects a integral part of the gneiss, and can locally be seen long history of development, reworking and trans- cutting across heterogeneity in the gneiss and position. Recognition of the complete history of folded gneissosity. The Ameralik dykes were in- development of the gneissosity on southwestern truded into the Amıˆtsoq gneisses and mafic and Akilia is rendered difficult, and probably impossi- ultramafic gneiss after the protoliths of these ble, because the late Archean tectonic modifica- rocks had already been converted into gneisses by tions were so intense that the present structure is deformation and metamorphism. This early gneis- superficially simple. Except for a few relatively sosity was substantially modified during late little-deformed pegmatites that mostly strike east- Archean episodes of deformation when it was west or northwest-southeast, the rock layers are generally parallel, subvertical and strike north- south, and most fold axes and lineations plunge 20–45° to the south (Fig. 3). However, the map (Fig. 3) indicates that the main layer of mafic and ultramafic gneiss (the type Akilia association) is a complex fold interference structure. The two trains of lenticular bodies of mafic and ultramafic gneiss define the limbs of a major isoclinal fold that, in the southwest, is refolded about a steep, northwest-southeast trending axial surface. The development of the main isoclinal fold was accompanied and followed by further attenuation of the Amıˆtsoq gneisses and Ameralik dykes. Iso- clinally folded remnants of Ameralik dykes were Fig. 5. Heterogeneous tonalitic and dioritic gneiss (Amıˆtsoq disrupted into trains of boudins (Figs. 4 and 6, gneiss) with parallel layers of pegmatite-veined mafic and and cover of Precambrian Research, this volume) ultramafic gneiss (Akilia association) and thin layers of amphi- and then deformed by tight upright southerly bolite. View to the south on southwestern Akilia. Located on plunging folds (Fig. 7). These younger folds are Fig. 3. probably minor structures associated with the fold with a northwest-southeast trending axial trace that deformed the major isoclinal fold in the southwest of Fig. 3. The lenticular outcrop pat- tern of the mafic and ultramafic gneiss reflects older small-scale fold interference structures, such as those on the western limb of the major isoclinal fold. Similar structures indicating refolding with axial surfaces at high angles to each other are marked in the lenticular outcrops on the eastern limb of this major isoclinal fold by thin layers of serpentinite that are too thin to show at the scale Fig. 6. Amphibolite boudins derived from an Ameralik dyke in of Fig. 3. tonalitic gneiss (Amıˆtsoq gneiss). The sampled pegmatite (270493MaUPb age) formed during the development of The context of Akilia in the regional tectonic the boudins. Located on Fig. 3. The hammer to the right of framework (Fig. 2) indicates that all the folds seen the arrow tip is 40 cm long. on Fig. 3 were refolded on a larger scale. The 112 J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124

pegmatite intruded between boudins of Ameralik dyke amphibolite (Figs. 3 and 6) during development of the boudins. The pegmatite has an undeformed igneous texture. Therefore, the age of the pegmatite may date a late stage of the last major episode of intense ductile deformation, when the dykes and their host rocks underwent further attenuation following repeated isoclinal folding, dismemberment and transposition into the main gneissosity (cover of Precambrian Research, this volume). Large (200–400 mm long), pink to colourless, elongate (aspect ratio of 3:1:1) to stubby (aspect ratio of 4:3:1), prismatic zircon grains were obtained from the pegmatite. The clearest and least magnetic grains with the fewest inclusions and cracks were abraded (Krogh, 1982). UPb dating by isotope dilution thermal ionization mass spectrometry was performed on Fig. 7. Upright, gently plunging folds of composite gneissosity five single grains. Routine sample preparation and of heterogeneous Amıˆtsoq gneiss on southwestern Akilia. analytical procedures were similar to those described in Dube´ et al. (1996). Two analyses are concordant with 207Pb/206Pb ages of 2706.2 and 2701.6 Ma (Table 1, Fig. 8). Three other grains yielded discordant ages. The two concordant analyses and two of the discordant analyses are collinear (MSWD=0.03) with a poorly defined, early Archean upper intercept. The igneous crystallization age of the pegmatite is interpreted as 270493 Ma, which is an age that spans the uncertainties of both the concordant analyses. The discordant analyses are interpreted as indicating the presence of a minor amount of zircon that was inherited from the host gneiss. This interpretation is supported by the discordia with an upper Fig. 8. UPb concordia plot. Ellipses for analyses represent the intercept that agrees with zircon ages in nearby s 2 uncertainty. See Table 1 for analytical data. tonalitic gneiss (Nutman et al., 1996, 1997a; Whitehouse et al., 1999). youngest fold axial trace in the southwestern part In summary, the rocks on Akilia indicate a of Fig. 3 may equate with D3 structures on Fig. 2, complex history of repeated intense ductile defor- where it can be seen that D3 structures are folded mation in which most geological features were by D4. The older two episodes of folding in attenuated, rotated and transposed into a new southwestern Akilia post-date the Ameralik dykes tectonic fabric (Figs. 4 and 5 and cover of Pre- and appear to be additional to the D2 thrusts of cambrian Research, this volume). Early Archean Chadwick and Nutman (1979) (Fig. 2) and their granulite facies metamorphism at 3600 Ma D1 structures that predate the Ameralik dykes. (Griffin et al., 1980), that predated the Ameralik To obtain precise control on the age of some dykes, was followed by probably multiple late Archean deformation and metamorphism on episodes of late Archean amphibolite facies meta- Akilia, we determined the UPb age of zircons in morphism at 2830–2710 Ma (Nutman et al., J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 113 in s 1.1 1.0 1.2 1.1 0.9 9 9 9 9 9 Pb Pb e 207 206 2706.2 Pb U Age (Ma) 206 238 g. m 10 \ 0.030.03 27120.04 27260.04 2719.7 27190.03 2744.3 2704 2749.2 2705 2701.6 9 9 9 9 9 Pb Pb 207 206 0.100.11 0.18742 0.10 0.19025 0.12 0.19082 0.10 0.18538 0.18589 9 9 9 9 9 10% for grains weighing Pb U B 207 235 g, m 10 0.090.10 13.516 0.09 13.804 0.11 13.805 0.10 13.322 13.366 B 9 9 9 9 9 Pb ratio corrected for spike and fractionation only. Errors are 1 204 / Pb U Pb 206 238 W %% 206 d 47 % Pb Pb 208 206 1 pg. Pb Pb B 10% for grains weighing Atomic ratios 206 204 N, long. 51°40 \ %% 06 % (pg) c (ppm) Pb f U (ppm) Pb c , total common Pb in analysis corrected for spike and fractionation. c g) m ( b in Ma. Pb blanks were 10–15 pg and U blanks were 9 223 122.5 34 97 53.9 1523 192 123 107.3 67.5 40 31 2386 2932 0.035 0.5215 s Wt. Concentration a Weight of grain estimated visually using aRatios microscope. corrected for spike, fractionation, blank, and initial common Pb, except Fraction code, followed by grain colour (pk, pink; co, colourless) and length to width ratio. Concentration uncertainty varies with sample weight: estimated Errors are 2 Pb, radiogenic Pb; Pb Pb analytical data a b c d e f percent. Analysis JC-001-99. Pegmatite between amphibolite boudins at lat. 63°56 Z1 pk, 5:2Z2 pk, 3:1Z3 pk, 3:2Z5 co, 4:3 8 244 135.4 48 35 1342 0.040 3150 0.5247 0.043 0.5230 0.043 0.5262 Z4 pk, 4:3 34 1830 0.041 0.5212 Table 1 U 114 J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124

1996). These episodes of metamorphism accompa- the southern shore sampled by Mojzsis et al. nied the main deformation features now seen on (1996) (Fig. 3), and defines the eastern limb of a Akilia and extended to 2704 Ma (new data third-generation fold of the thickest layer of mafic above) and the younger D3 and D4 events de- and ultramafic gneiss. Thinner layers of this scribed by Chadwick and Nutman (1979). The quartz-rich rock were also seen to the north- rocks on Akilia were also subjected to a major northwest in the mafic and ultramafic gneiss on thermal and magmatic episode at 2550 Ma the west coast in the vicinity of the word ‘sea’ on during the emplacement of the nearby Qoˆrqut Fig. 3, but are too thin to be shown on this map. granite (Figs. 1 and 2) (Baadsgaard, 1976; Brown The rock mainly consists of quartz with a few et al., 1981; Moorbath et al., 1981), and some of thin layers and lenses rich in diopside and mag- the cross-cutting pegmatites on the island could netite, and thin cross-cutting layers of diopside be of this age. Another major thermal episode of and quartz of similar thickness. Smaller amounts regional extent is recorded by biotites that suf- of hornblende, orthopyroxene and garnet occur fered extensive loss of radiogenic 87Sr at 1700– with the magnetite and diopside. These thin 1600 Ma (Pankhurst et al., 1973). melanocratic layers are unevenly distributed The possibility of any trace of early Archean within the layer of quartz-rich rock. The rock has life that may have existed in southwestern Akilia a post-tectonic, very coarse grained metamorphic surviving these multiple episodes of intense ductile texture. The large grain size suggests substantial deformation and high grade metamorphism, and diffusion during metamorphism. being recognizable as early Archean, appears The age of the recrystallization is unknown, but miraculous. Yet such a miracle was claimed by in view of the tectonic history, the rock must have Mojzsis et al. (1996) and received world-wide undergone substantial ductile deformation and re- publicity. crystallization during the late Archean. Sano et al. (1999) determined UPb and PbPb isotopic ages 6.2. Host rock of ‘oldest life’ of apatites from this locality to be 1500 Ma and concluded that ‘either the apatites in the BIFs Mojzsis et al. (1996) measured ‘the carbon-iso- grew about 1500 Myr ago, or that they grew tope composition of carbonaceous inclusions earlier than that but were substantially affected by within grains of apatite’ (p. 55) and found compo- recrystallization, and/or diffusive exchange with sitions that they interpreted to be the result of the environment, which reset the UPb system of ‘biological activity’ ‘at least 3800 Myr before the samples’. As noted by Sano et al. (1999), this present’ (p. 56). metamorphic event, which they estimated reached The apatite grains were extracted from a thin 600°C, could have been part of the regional ther- layer of highly deformed and metamorphosed mal event recognized by Pankhurst et al. (1973). quartz-rich rock within mafic and ultramafic In reply to the discussion of Sano et al. (1999), gneiss on the southwestern tip of Akilia (Fig. 3). Mojzsis et al. (1999), p. 128) asserted that ‘this This quartz-rich rock was interpreted as a banded rock, which is essentially a quartzite, has no in- iron formation and is described in detail by Nut- trinsic capacity to form apatite by metamorphic man et al. (1997a). The rock is claimed to be ‘the reactions following diagenesis. Thus, the apatite oldest known sediment … and the oldest known present in this rock must be original (formed rock with evidence of biological processes active before 3850 Myr) unless phosphate-bearing fluids during the time of deposition’ (Mojzsis and Har- were subsequently introduced metasomatically.’ rison, 2000). The latter possibility cannot be lightly dismissed We found that this layer of rock is more than in view of the very coarse metamorphic grain size twice as extensive as previously indicated by Nut- of the rock and the presence of syn- and/or pre- man et al. (1997a, their Fig. 1) and Mojzsis and metamorphic veins of quartz and quartz-diopside, Harrison (2000, their Fig. 4). The layer extends with the same coarse grain size and texture as the north and north-northeast from the locality near host rock. Nor can the rock with certainty be J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 115 classified as ‘essentially quartzite’ (or metamor- The tonalitic gneiss adjacent to the eastern mar- phosed banded iron formation) with apatites that gin of the mafic and ultramafic gneiss, near the have suffered little more than diagenesis. A sedi- southern shore of Akilia (Fig. 3), was dated by mentary origin is not impossible, but alternative Nutman et al. (1996) at 3872910 Ma (SHRIMP origins or multiple origins, such as all or part of UPb zircon date, sample G88-66). Mojzsis et al. the rock being derived from vein quartz, need to (1996), referring to this locality, stated that the be considered and discounted. The apparent ab- mafic and ultramafic gneiss was ‘cut by a de- sence of ‘apatites with graphite inclusions in the formed quartz-dioritic sheet’ (p. 57), the same immediately adjacent, encompassing gneisses’ gneiss dated by Nutman et al. (1996). The age of noted by Mojzsis et al. (1999), p. 128) as an the ‘sheet’ was considered to be the minimum age indication that apatite was not derived from the of the quartz-rich rock hosting the apatites with adjacent rocks does not preclude metasomatic carbonaceous inclusions (Mojzsis et al., 1996). transfer along the quartz-rich layer. If all or part Nutman et al. (1997a) contended that the of the quartz-rich layer was derived from vein ‘sheet’ should not be used to constrain the age of quartz then mechanical transport of mineral the mafic and ultramafic gneiss and associated grains along a ductile fault zone is also a possibil- quartz-rich layer because ‘given the strong defor- ity to be considered. mation at the contact, it cannot be totally ruled Thus the age of crystallization, and primary out that the gneisses and supracrustal rocks have source, of the carbonaceous inclusions in the ap- been tectonically juxtaposed’ (p. 2477). Therefore, atite crystals are uncertain. The rock may be the age of another layer of quartz-dioritic gneiss derived from a banded iron formation, but it in contact with the mafic and ultramafic gneiss appears to have been so substantially modified by was investigated by Nutman et al. (1997a) (sample deformational and metamorphic processes that G93-05, located by the arrow marked (Figs. 9–11 this interpretation is a hypothesis. This hypothesis in Fig. 3). They described this layer as a ‘quartz- needs to be substantiated, and alternative hy- dioritic sheet within amphibolites of the potheses, such as that some or all of this layer supracrustal body… Where sampled, this sheet is could have been derived from vein quartz that 1 m wide and its contacts are up to 10° was repeatedly deformed and metamorphosed, discordant to the compositional layering in host need to be dismissed before a sedimentary origin layered amphibolites’ (p. 2477). They stated that can unequivocally be established. ‘apart from the mafic inclusions and the sparse pegmatite veins, the quartz diorite sheet is homogeneous—it is not a composite banded gneiss. On 6.3. Age of host rock the basis of the field relations, the quartz-diorite sheet definitely cuts the supracrustal rocks’ (p. Nutman et al. (1997a) extracted zircons from 2477). The age for this sheet, and thus the mini- the quartz-rich rock (Fig. 3) ‘in the hope that mum age for the mafic and ultramafic gneiss, was either a few volcanogenic or extremely old detrital interpreted from SHRIMP UPb zircon dating as grains might be found’ (p. 2481). However, all being ‘definitely 3840, and possibly 3865911 zircons yielded ages of 2700–2590 Ma that Ma’ (pp. 2481–2482). were interpreted as growing ‘during one or more Whitehouse et al. (1999) reinvestigated the age episodes of late Archaean high grade amphibolite of this ‘sheet’ by performing UPb zircon ion-mi- facies metamorphism…and provide no informa- croprobe dating in conjunction with cathodolu- tion on the deposition of the sediment’ (p. 2481). minescence imaging on a sample (SM/GR/97/7) Consequently, Nutman et al. (1997a) concluded collected from the same locality as sample G93-05. that only a minimum age of the quartz-rich rock The imaging revealed cores and overgrowths of could be determined by dating granitoid rocks zircon, and in contrast to Nutman et al. (1997a) that intruded the quartz-rich rock or the spatially little evidence for zircon older than 3650 associated mafic and ultramafic gneiss. Ma was found (only one core with an age of 116 J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124

3800 Ma was discovered). Whitehouse et al. (1999) concluded that the rock ‘shows unambigu- ous evidence for a ca. 3.65 Ga magmatic event’ (p.

Fig. 10. Composite layer of granitoid gneiss shown in map form on Fig. 9. View to the north showing the whole outcrop of this layer. Pebbles mark a 30 cm grid used for mapping.

217) and that pre-3.65 Ga zircon dates obtained by Nutman et al. (1997a) were from inherited grains.

6.4. Field relations of ‘quartz-diorite sheet’

Given the importance of the contacts between the ‘quartz-diorite sheet’ and the mafic and ultramafic gneiss, we mapped the outcrop at a scale of 1:12. The outcrop lies within mafic and ultramafic gneiss, on the eastern limb of the major isoclinal Fig. 9. Simplified map of a composite layer of granitoid gneiss, fold (Fig. 3; outcrop located by the arrow marked the ‘quartz-diorite sheet’ interpreted by Nutman et al. (1997a) as an intrusion into the mafic and ultramafic gneiss of the Figs. 9–11 on Figs. 3 and 5). Our map is shown Akilia association. Simplified after mapping at 1:12 scale. in Fig. 9 and a photograph of the outcrop is Located by arrows on Figs. 3 and 5, on the shore of south- shown in Fig. 10. Part of this outcrop (shown in western Akilia. The layer is too thin to be represented on Fig. detail in our Fig. 11) was previously portrayed by 3. The outcrop slopes gently northwards except for a small Nutman et al. (1997a) as evidence of discordance vertical portion marked by the thick line. Therefore the geology of this vertical rock face cannot be seen on this map. A between the ‘quartz-diorite sheet’ and the mafic photograph and sketch of this vertical face are shown in Fig. and ultramafic gneiss. Their figure (ib. cit., Fig. 2, 11. p. 2478), a sketch from a photograph, was unfor- J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 117 tunately drawn in reverse image and the scale claimed by Nutman et al. (1997a), p. 2477) does indicated was incorrect (the scale bar should be not exist; apparent discordance seen in Fig. 11 is 40% longer). due to the perspective of the photograph and the Figs. 9–11 show that geologic relationships in three-dimensional outcrop surface. The gneissos- this outcrop are considerably more complex than ity in the mafic and ultramafic gneiss is also depicted by Nutman et al. (1997a). The ‘sheet’ is a parallel to the contact with, and foliation in, the strongly deformed composite layer, 0.6–0.8 m tonalitic gneiss across strike to the east as can be wide, consisting of discontinuous layers of: het- seen in Fig. 5 (outcrop located on Fig. 3 by the erogeneous gneiss (thinly banded mafic and felsic arrow marked Fig. 5). layers), relatively homogeneous tonalitic gneiss, Nutman et al. (1997a) and Whitehouse et al. leucocratic pegmatite, hornblendite, and vein (1999) did not indicate where their UPb samples quartz (Figs. 9–11). These rocks are cut by an were collected, but V.R. McGregor and M.J. undeformed, gently north-dipping pegmatite dyke Whitehouse (personal communications, 1999) in- (probably late Archean). Compositional layering dicated the samples were taken from the tonalitic in the heterogeneous gneiss parallels the foliation gneiss at location 3 in Fig. 11b. The tonalitic in the tonalitic gneiss, the margins of the com- gneiss appears to have a less complex history than posite layer, and the gneissosity in the adjacent the heterogeneous gneiss (Figs. 9–11), but there is mafic and ultramafic gneiss (Figs. 9–11). The ‘up no field evidence suggesting that it intruded into to 10°’ discordance between the composite the heterogeneous gneiss or the adjacent mafic layer and gneissosity in the adjacent rocks that is and ultramafic gneiss. Likewise, the fragmentary

Fig. 11. (a) View to the south showing part of the mapped outcrop presented in Fig. 9, including the vertical face (behind the hammer) located by the thick line on Fig. 9. (b) Sketch of the same photograph showing rock units and trend lines of gneissosity, with ornament as on Fig. 9. A sketch of this outcrop, viewed from the same perspective, was presented by Nutman et al. (1997a, Fig. 2) as evidence that the ‘quartz-diorite sheet’ was discordant and intrusive into the mafic and ultramafic gneiss of the Akilia association. Notes for locations indicated by numbered circles: (1) The contact and gneissosity are concordant. This contrasts with the sketch by Nutman et al. (1997a, Fig. 2) indicating discordance at this location. (2) The apparent discordance between the contact and gneissosity is due to the three-dimensional outcrop surface and the perspective of the photograph. (3) Collection site of samples G93-05 (Nutman et al., 1997a) and SM/GR/97/7 (Whitehouse et al., 1999), (V.R. McGregor and M.J. Whitehouse, personal communications, 1999). 118 J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 nature of the hornblendite lenses in the composite the Akilia association on southwestern Akilia layer probably reflects deformational rather than were intruded by any of the protoliths of the intrusive processes. adjacent tonalitic gneiss and may therefore be This outcrop on southwestern Akilia, which older than 3.85 Ga (Nutman et al., 1997a) or 3.65 forms the basis for the age attributed to the oldest Ga (Whitehouse et al., 1999). The layer of gneiss life claimed by Mojzsis et al. (1996), comprises (Figs. 9–11) is heterogeneous and clearly contains highly strained rocks. These rocks underwent components of different composition and age. multiple deformation and high grade metamor- The current controversy between Nutman et al. phic events that obliterated primary relationships (2000) and Whitehouse et al. (2000) over the between the tonalitic gneiss and the mafic and interpretation of disparate ages of zircon popula- ultramafic gneiss (Figs. 9–11). Although our ob- tions in this rock could reflect differences in the servations do not rule out intrusion of part of the components, or different mixtures of components, protolith of the tonalitic gneiss into the protolith that were studied from within this composite of the mafic and ultramafic gneiss, they do suggest layer. that other primary relationships are equally plau- The southwestern tip of Akilia is the type local- sible. These relationships include (i) deposition of ity of the Akilia association (McGregor and Ma- the protoliths of the mafic and ultramafic gneiss son, 1977) and therefore it would be important to as lava flows and/or clastic or pyroclastic frag- obtain unequivocal protolith ages directly from ments onto the tonalitic gneiss; (ii) intrusion of the mafic and ultramafic gneiss at this location. the protoliths of the mafic and ultramafic gneiss However, in order to constrain the age of any into the tonalitic gneiss; (iii) intrusion of the pro- early life on Akilia, it would also be necessary to tolith of a younger component of the tonalitic determine the primary age relationships between gneiss (carrying xenoliths of older heterogeneous the mafic and ultramafic gneiss, the quartz-rich gneiss) into the mafic and ultramafic gneiss or its rock, and the apatite with carbonaceous inclu- protoliths, and (iv) tectonic juxtaposition, such as sions (Mojzsis et al., 1996). At present these cru- the D2 thrusting of Chadwick and Nutman cial relationships are unknown and appear to be (1979). extremely difficult to resolve in the intensely de- Nutman et al. (1997a) stated that ‘extreme cau- formed and metamorphosed rocks on Akilia. tion has to be exercised in establishing the age of any unit of supracrustal rocks…given evidence for supracrustal rocks of more than one age in the 7. Conclusions Itsaq Gneiss Complex (Nutman et al., 1996), (p. 2477). Need for caution was further stressed by What do we know about the Earth’s oldest Nutman et al. (2000): ‘bearing in mind that the continental crust in the Godtha˚bsfjord region? older components of the geology on southern The early Archean gneisses were mainly derived Akilia underwent granulite-facies metamorphism from plutonic igneous rocks, tonalite and gran- and some partial melting at ca. 3650 Ma and that odiorite and a smaller amount of quartz-diorite, most of them were highly deformed in both the trondhjemite, diorite, ferrodiorite and granite. early and late Archaean, a degree of caution may The precise ages of intrusion as magmas is contro- be warranted regarding whether the dates from versial. Largely on the basis of SHRIMP UPb samples G88-66 and G93-05 give a minimum age data on zircons, Nutman et al. (1996) considered for the unit of banded iron formation… that is that the magmas that formed these early Archean interpreted as the oldest evidence of a hydro- rocks were intruded over a protracted period be- sphere on Earth’. tween 3870 and 3625 Ma. In contrast, Kamber We agree that the evidence needs to be rigor- and Moorbath (1998) used PbPb data from ously evaluated and that even greater care than feldspars and whole rocks to conclude that ‘the that exhibited by Nutman et al. (1997a) must be magmatic precursors of the Amıˆtsoq or- taken in order to prove that any components of thogneisses separated from a depleted mantle-like J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124 119 source at ca. 3.65 Ga’ (p. 19). A broadly similar The Isua greenstone belt is generally considered conclusion was reached by Whitehouse et al. to be a better preserved equivalent of the Akilia (1999), based on UPb zircon data and cathodo- association at lower metamorphic grade. Never- luminescence imaging. They found that the Amıˆt- theless, these rocks are also intensely deformed soq gneisses they studied were derived from rocks and suffered numerous episodes of deformation intruded at 3.65 Ga, and older zircons are and metamorphism to amphibolite facies. The xenocrysts. ages of these rocks are poorly constrained but The Amıˆtsoq gneisses are diverse, extensive, thought to be 3.8–3.7 Ga (Nutman et al., 1996, and heterogeneous, and although the amount of 1997b; Moorbath and Whitehouse, 1996). geochronology carried out on these rocks is prob- Is there any direct trace of life in the early ably greater than on any other region of early Archean rocks of the Godtha˚bsfjord region? The Archean rocks, it is still far from adequate to answer appears to be very little. The age and define the whole temporal evolution of the gneiss significance of the graphite described from apatite complex. However, it is probably significant that crystals on Akilia by Mojzsis et al. (1996) are the age determinations all fall in the range of controversial. The age and origin of the host rock 3850–3600 Ma and that no significantly older are unknown. Previous claims that the rock is zircon xenocrysts have been found. This contrasts \3850 Ma are unsupported by reinvestigation of with the Acasta gneiss complex in the Slave the field evidence. The discovery by Rosing (1999) Province of Canada and the Narryer gneiss com- of carbon of possible biogenic origin in \3700 plex in the Yilgarn Craton of Australia that have Ma schists interpreted as metasedimentary rocks both yielded much older zircons. Tonalitic and of turbiditic and pelagic origin in the Isua green- granodioritic gneisses from the Acasta region con- stone belt is more plausible. These rocks are from tain zircons with SHRIMP UPb zircon ages a zone of relatively low strain where primary between 4.0–4.03 Ga (Bowring et al., 1989; features can be recognized, but the tectonostratig- Stern and Bleeker, 1998; Bowring and Williams, raphy of the greenstone belt is complicated and 1999), and in the Narryer region, late Archean there is insufficient precise geochronology to granites contain \4.0 Ga zircon xenocrysts (Nel- define the ages of all the tectonostratigraphic son et al., 1997), and detrital zircons in late units. However, existing data suggest that much Archean metasedimentary rocks range back, be- of the greenstone belt is \3700 Ma, and life, if it yond the oldest known 3.73 Ga rocks of the then existed, is more likely to be preserved in region, from 3.9–4.3 Ga (Froude et al., 1983; these rocks than in any other early Archean rocks Compston and Pidgeon, 1986; Maas et al., 1992). in the region. The wide range of extremely ancient zircons in the Banded iron formations are widely considered Narryer gneiss complex indicates the presence, or to reflect the precipitation of insoluble ferric iron former presence, of substantially older and more as a result of oxidation of soluble ferrous iron in complex continental crust in this region than ap- seawater (e.g. Awramik, 1981, p. 357). The re- pears likely in the Godtha˚bsfjord region. quired oxygen is generally thought to have been Small fragments of diverse composition (Akilia generated by organic photosynthetic processes. association) are widespread in the Amıˆtsoq Thus the presence of banded iron formation gneisses. They are generally considered to be within the \3700 Ma Isua greenstone belt, and derived from highly metamorphosed mafic and possibly within the Akilia association, could be ultramafic intrusions and sedimentary rocks, and macroscopic indications of photosynthetic to be older than the Amıˆtsoq gneisses. Nutman et processes. al. (1996, 1997a) considered that the age of the Although these ancient rocks of the Akilia association ranges back to over 3850 Godtha˚bsfjord region have been dead and cold Ma, whereas Kamber and Moorbath (1998) ar- for a long time, there is still plenty of heat and life gued that these rocks formed at 3700–3650 left in the current research and controversies sur- Ma. rounding their interpretation. The geology around 120 J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124

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