Vestiges of life in the oldest Greenland rocks? A

Transcription

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 Godthåbsfjord 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 Godthåbsfjord 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
The Godthåbsfjord region lies in the centre of the
Archean gneiss complex on the west coast of
Greenland in the vicinity of Nuuk (Fig. 1, inset
map). Most of the Archean gneiss complex is
derived from sheets of tonalite and granodiorite,
and a small amount of granite and diorite, that were
intruded into basaltic volcanic rocks and minor
* Corresponding author. Tel.: +1-709-7378417; fax: +1709-7372589.
E-mail address: jmyers@sparky2.esd.mun.ca (J.S. Myers).
sedimentary rocks during the late Archean. These
rocks were repeatedly deformed and metamorphosed, developing gneissosity that was folded,
refolded and transposed into new tectonic layering,
and recrystallizing during and after deformation at
amphibolite or granulite facies (Bridgwater et al.,
1976).
In a belt 25–75 km wide, extending for 200 km
through Godthåbsfjord (Fig. 1), similar late
Archean plutonic rocks were intruded into, and
tectonically interleaved with, early Archean
tonalitic and granodioritic gneisses, and both early
and middle Archean metavolcanic and metasedimentary schists and gneisses.
0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 9 2 6 8 ( 0 0 ) 0 0 0 8 9 - 9
102
J.S. Myers, J.L. Crowley / Precambrian Research 103 (2000) 101–124
The first part of this article (Sections 2 – 4)
reviews knowledge of the early Archean rocks
that formed before 3600 Ma, synthesizing pre-
vious observations and interpretations, and outlining current controversies. Most of the early
Archean rocks were intensely, and repeatedly, de-
Fig. 1. Geologic map of the Godthåbsfjord 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.
formed and metamorphosed at high grade during
late Archean events and, except in the vicinity of
Isua, the present structure and appearance largely
reflect these late Archean events (cover of Precambrian Research, this volume). Therefore a brief
outline of the late Archean tectonic evolution is
included in this review (Section 5), indicating current interpretations of how the early Archean
rocks were intruded by, and tectonically interleaved with, younger Archean rocks.
The second part of this article (Section 6) re-examines the field evidence from the island of Akilia
upon which the antiquity of life is claimed to be
\ 3800 Ma (Mojzsis et al., 1996) and ] 3850 Ma
(Nutman et al., 1997a; Mojzsis and Harrison,
2000), in the vicinity of the gneiss shown on the
cover of Precambrian Research (this volume).
2. Pioneering studies in outer Godthåbsfjord
Current understanding of the geology stems
from the pioneering work of Berthelsen (1955)
and McGregor (1966, 1968, 1973), who demonstrated that basic dykes could be used as field
criteria to subdivide the Archean gneisses into
older and younger components. The basic dykes
cut across older rocks, structures and gneissosity,
and are cut by younger granitoid rocks that were
subsequently deformed, metamorphosed and converted to gneisses.
This technique had previously been developed
in gneiss complexes elsewhere, especially in northwest Scotland by Peach et al. (1907) and Sutton
and Watson (1951), and in southern Finland by
Sederholm (1923, 1926) and Wegmann (1931).
Wegmann (1938) introduced this concept to
southern Greenland, distinguishing major tectonic
and metamorphic episodes (subsequently recognized as Archean and Proterozoic) on the basis of
intervening dyke swarms. During field studies between 1946 and 1955, Ramberg (1948) and NoeNygaard and Ramberg (1961), assisted by
Berthelsen, applied this concept further north and
presented the first fundamental subdivision of the
Precambrian gneiss complex from Godthåbsfjord
northwards for 600 km. They used early Proterozoic dykes to distinguish unmodified Archean
103
rocks cut by undeformed dykes from Archean
rocks and dykes that were deformed and metamorphosed during the early Proterozoic.
Berthelsen and McGregor were the first to use
basic dykes to distinguish between Archean tectonic, magmatic and metamorphic events in
Greenland. Berthelsen (1955) recognized that basic dykes could be used to distinguish between
older and younger geological events in the southeastern part of the island of Qilángârsuit (Fig. 1).
McGregor (1966, 1968, 1973), during a detailed
investigation of the gneisses in the vicinity of
Nuuk (formerly Godthåb and Nûk) between
1965–71, discovered that these basic dykes could
be used as a regional mapping criterion separating
older from younger events. McGregor (1968)
called the dykes Ameralik dykes and recognized
that intense deformation had severely modified
the original structure of the dykes and their host
rocks such that most dykes and host rocks were
rotated into a new common gneissosity (e.g. cover
of Precambrian Research, this volume). Crosscutting relationships were preserved only in small
areas of relatively low strain. Likewise, McGregor
(1973) described how the cross-cutting relationships of granitoid rocks that were intruded after
the dykes were also widely obliterated by intense
deformation that rotated most components of the
gneiss complex into subparallel layers.
Many were sceptical of McGregor’s interpretation, and some considered metamorphic grade to
be a key indicator of the relative age of rocks and
events. Based on a wider regional survey of the
Archean gneisses of West Greenland, Windley
(1968, 1969) regarded the linear NNE-trending
belt of amphibolite facies gneisses in the
Godthåbsfjord region, including the gneisses described by McGregor, as derived by tectonic reworking and retrogression of older granulite
facies gneisses to the north and south. He considered that the Ameralik dykes of McGregor (1968)
cut older granulite facies gneisses to the south of
Ameralik (Fig. 1) and ‘can be followed… into the
linear belts where they become progressively deformed and migmatised’ (Windley, 1969, p. 159).
Thus, in contradiction of McGregor’s hypothesis,
the dykes and relatively straight NNE-trending
belt of highly deformed amphibolite facies
104
gneisses through Godthåbsfjord were interpreted
as younger than regional granulite facies metamorphism. However, McGregor’s hypothesis was
subsequently supported by geochronology. Black
et al. (1971) discovered that the whole rock RbSr
and PbPb isochron ages of gneisses that were cut
by Ameralik dykes in the Godthåbsfjord belt were
one billion years older than the widespread granulite facies metamorphism, and were far older
than any other terrestrial rocks known at that
time.
Black et al. (1971) called the rocks that were
older than the Ameralik dykes ‘Amı̂tsoq gneisses’
and discovered that they were characterized by
extremely unradiogenic whole-rock lead isotopes.
They described the sequence of events following
the intrusion of the Ameralik dykes as: eruption
of basic volcanic rocks (now mostly amphibolite)
and deposition of sedimentary rocks (mainly
quartz-biotite-garnet schist), collectively called
Malene supracrustals by McGregor (1969); tectonic interleaving with the Amı̂tsoq gneisses; emplacement of stratiform anorthosites; and
syntectonic intrusion of a large volume of calc-alkaline rocks that were deformed and metamorphosed to become gneisses that they called Nûk
gneisses. The protoliths of the Nûk gneisses were
largely intruded as sills and were further interleaved with all the older rocks by tectonic processes, producing a complex tectono-stratigraphy
(Bridgwater et al., 1974).
Isotopic ages of the gneisses were extensively
studied during the 1970s. Protolith ages of the
Amı̂tsoq gneisses were found to range between
3.8–3.65 Ga and those of the Nûk gneisses
between 3.0–2.8 Ga. Results included RbSr
whole rock isochrons of 3.75 (Moorbath et al.,
1972), PbPb whole rock isochrons of 3.62 Ga
(Black et al., 1971) and 3.8 Ga (Moorbath et
al., 1975), and UPb discordia upper intercepts of
3.65 Ga from bulk zircons (Baadsgaard, 1973)
and single zircons (Michard-Vitrac et al., 1977).
Ages of Nûk gneisses determined during the 1970s
included RbSr whole rock isochrons of 3.04
Ga (Pankhurst et al., 1973) and 2.93 – 2.78 Ga
(Moorbath and Pankhurst, 1976).
Complications in the use of amphibolite dykes
as time markers between Amı̂tsoq and Nûk
gneisses were highlighted by Coe et al. (1976) and
Chadwick and Coe (1983) who discovered intraNûk dykes (Neriuneq and Qáqatsiaq dykes) south
of Ameralik (Fig. 1), broadly similar in appearance to many Ameralik dykes. Chadwick (1981)
described another, smaller and more controversial
group of dykes he called Tinissaq dykes, thought
to be intruded into an early phase of Nûk
gneisses. On the island of Qilángârsuit to the west
(Fig. 1), Chadwick (1981) recognized eight types
of Ameralik dykes intruded into Amı̂tsoq gneisses
and two types of amphibolite dykes, ‘similar to
two main categories of Ameralik dykes’ (ib. cit. P.
221) cutting the Malene supracrustals. Chadwick
(1981), pp. 223–224) concluded that ‘limited evidence of dyke intersections suggests some Ameralik dykes may be older than the Malene
supracrustal rocks, and others, including Malene
dykes, may have been injected after the tectonic
interleaving’ of Amı̂tsoq gneisses and Malene
supracrustals.
The Malene supracrustals were thought to have
been deposited on Amı̂tsoq gneisses (after 3600
Ma) before the intrusion of Nûk gneisses at 3070
Ma (Chadwick and Nutman, 1979; Nutman and
Bridgwater, 1983). However, UPb studies of
metasedimentary rocks within the Malene
supracrustals by Schiøtte et al. (1988) delineated a
range in the age of detrital zircons from 2900
to 2650 Ma. They found some rocks with B 2800
Ma detrital zircons, younger than Nûk gneisses
and high grade regional metamorphism at 2800
Ma. Therefore ‘the concept of the Malene
supracrustals as a single lithostratigraphic unit’
was found to be ‘no longer valid’ (Schiøtte et al.,
1988, p. 56). This work also indicated a more
prolonged and complex sequence of late Archean
tectonic, magmatic and metamorphic events than
previously recognized.
The Amı̂tsoq gneisses contain small bodies of
highly metamorphosed basic, ultrabasic and sedimentary rocks that were first described, and called
the Akilia association, by McGregor and Mason
(1977). They stated that ‘one of the largest and
best exposed occurrences… is on the southwestern
tip of the island of Akilia. This is designated the
type locality for the Akilia association, an informal term introduced here for all the enclaves of
older rocks in the Amı̂tsoq gneisses in the
Godthåb region with the exception of those in
the Isua supracrustal belt’ (p. 889). The geology
and current controversies of southwestern Akilia
(Fig. 1) are described later in this paper (Section
6).
In this region south of Nuuk, the Akilia association and the main, older, components of the
Amı̂tsoq gneisses were metamorphosed to granulite facies at 3600 Ma (Griffin et al., 1980).
The original extent, and tectonic and magmatic
context, of this metamorphism are unknown.
The Isua supracrustal belt (Isua greenstone belt
of Appel et al., 1998) (Fig. 1) was considered by
McGregor and Mason (1977) to be an exceptionally large, temporal equivalent of the Akilia association, at lower metamorphic grade and with a
well preserved stratigraphy.
3. Isua
The metavolcanic and metasedimentary rocks
at Isua were initially thought to be Proterozoic
(Windley, 1969) because of their relatively low
metamorphic grade and a K/Ar whole rock age
of 19409 50 Ma on a ‘low-grade metabasic’ rock
‘forming…sills in the weakly metamorphosed
quartzitic component’ (Lambert and Simons,
1969, p. 69). However, Moorbath et al. (1972)
obtained a RbSr whole rock isochron of 3700 9
140 Ma on the adjacent tonalitic gneiss (subsequently recalculated to 3640950 Ma, see
Moorbath and Whitehouse, 1996), and a whole
rock PbPb age of 3760 970 Ma on a banded
iron formation (Moorbath et al., 1973) (subsequently recalculated to 3710970 Ma, see Moorbath and Whitehouse, 1996), indicating that this
greenstone belt comprised the oldest known
metavolcanic and metasedimentary rocks on
Earth. Bridgwater and McGregor (1974) outlined
the main features of the geology and postulated
that the well-preserved metadolerite dykes cutting
the Isua greenstone belt and adjacent tonalitic
gneiss were equivalent to the Ameralik dykes.
Recognition of the extreme antiquity of the
metavolcanic and metasedimentary rocks at Isua
led to a spate of diverse research during the 1970s
105
and early 1980s. This research included stratigraphy and sedimentology (Dimroth, 1982; Nutman
et al., 1984), structure (James, 1976), petrology,
mineralogy and geochemistry (Schidlowski et al.,
1979; Gill and Bridgwater, 1979; Gill et al., 1981;
Boak et al., 1983; Nutman and Bridgwater,
1986), metamorphism (Boak and Dymek, 1982),
geochronology (Moorbath et al., 1975; Baadsgaard, 1976; Michard-Vitrac et al., 1977; Hamilton et al., 1978), oxygen and sulphur isotope
studies (Oehler and Smith, 1977; Oskvarek and
Perry, 1976; Perry and Ahmad, 1977) and organic
chemistry (Nagy et al., 1975). The supracrustal
rocks and adjacent tonalitic gneisses were
mapped and described by Allaart (1976), Nutman
et al. (1984) and Nutman (1986).
Many interpretations from this era of research
have since been questioned and many fundamental aspects of the geology remain controversial.
Claimed discoveries of early life by Walters,
1979, cited in Nagy et al., 1981) based on hydrocarbons, and ‘yeast-like microfossils’ called Isuasphaera by Pflug (1978) and Pflug and
Jaeschke-Boyer (1979), were refuted by Nagy et
al. (1981) and Bridgwater et al. (1981) respectively, on the grounds that these claims were
incompatible with the highly deformed and metamorphosed state of the rocks.
More extensive controversy has ranged over
the interpretation of rocks claimed to be of sedimentary origin, especially conglomerates and sedimentary carbonates that were interpreted as
indicating shallow-water deposition (Dimroth,
1982; Nutman et al., 1984; Nutman, 1986). Rose
et al. (1996) and Rosing et al. (1996) demonstrated that the carbonates are largely of metasomatic origin, and Fedo (2000) has reinvestigated
the conglomerates and associated clastic rocks
and reviewed the controversies about their origin.
Recently, there have been fundamental reinterpretations of the original nature of the rocks that
comprise most of the Isua greenstone belt. Previous interpretations were based on detailed mapping by Allaart and Nutman between 1974–82
(Nutman et al., 1984; Nutman, 1986). They interpreted the greenstone belt as an upward-facing
syncline on the basis of stratigraphy and smallscale way-up criteria. The mapped stratigraphy
106
was defined by a mixture of features including
metamorphic rock types, metamorphic textures
and interpreted protoliths. Studies during the
1990s, especially of the metacarbonate rocks by
Rose et al. (1996) and Rosing et al. (1996), recognized that ‘most of the supracrustal rocks have
undergone pervasive metasomatism, which has
obscured their origin’ (ib. cit., p. 43), and questioned the significance of the stratigraphy described by Nutman et al. (1984) and Nutman
(1986).
Remapping of parts of the greenstone belt in
1997 –99 by Myers (2000) supports the general
conclusions of Rose et al. (1996) and Rosing et
al. (1996). Myers (2000) found that, although
most of the rocks were intensely deformed and
many were substantially altered by metasomatism, the deformation and metasomatism were
heterogeneous, and transitional stages could be
traced from rocks with recognizable primary volcanic and sedimentary structures into schists in
which all primary features were obliterated. This
mapping demonstrated that most of the Isua
greenstone belt was derived from basaltic and
high-Mg basaltic pillow lava and pillow lava
breccia, with a smaller amount derived from
chert-BIF, layered mafic-ultramafic intrusive
sheets, ultramafic rocks (of autochthonous extrusive and/or intrusive origin, and/or allochthonous origin), and a minor component of
clastic sedimentary rocks (see Fedo, 2000). The
mapping also revealed a complex tectonic history
of polyphase deformation and high grade metamorphism in which compositional layering and
foliations were isoclinally folded and refolded before the greenstone belt was assembled as a number of fault-bounded packages of strongly to
intensely deformed rocks. These structures and
schistosities were cut by swarms of basic and
ultrabasic dykes (equivalent to Ameralik dykes)
and then deformed again, probably in the late
Archean.
Komiya et al. (1999) also interpreted the
northeastern part of the belt as a stack of faultbounded rock packages on the basis of detailed
mapping. They identified a number of duplex
structures and reconstructed the lithostratigraphy
as a tectonically repeated, simple upward se-
quence of originally low-K tholeiitic basalt,
chert-BIF and turbidite. They interpreted the
greenstone belt as an accretionary complex that
developed in an intraoceanic environment,
analogous to the late Phanerozoic tectonic evolution of Japan, and attributed most of the structural evolution of the belt to this episode of
intraoceanic accretion. However, this interpretation does not appear to recognize the multiple
episodes of intense regional ductile deformation,
metamorphism and metasomatism, described by
most other investigators such as Bridgwater and
McGregor (1974), James (1976), Nutman et al.
(1984), Rosing et al. (1996) and Myers (2000),
that converted most of the Isua belt to schists,
and formed and deformed the schistosity in the
adjacent tonalitic gneiss.
The new mapping by Komiya et al. (1999) and
Myers (2000) indicates that a major component
of the Isua greenstone belt was derived from
basaltic pillow lava, in contrast to previous detailed accounts by Nutman et al. (1984) and
Nutman (1986), who described many of these
rocks in terms of metamorphic textures (such as
‘garbenschiefer unit’, ib. cit., p. 15) and interpreted the protoliths as mafic intrusions (such as
metagabbro, Nutman, 1997).
Another controversial major component of the
Isua greenstone belt is quartzo-feldspathic schist
and mylonite described as felsic formations A6
and B1 by Nutman et al. (1984) and Nutman
(1986), and dated as \ 3790 Ma and 3710 Ma
respectively by Nutman et al. (1997b). Parts of
this schist contain ellipsoidal fragments of granitoid rocks that have been interpreted as of volcanic
pyroclastic
origin
(Allaart,
1976;
Bridgwater et al., 1976; Dimroth, 1982). However, Nutman et al. (1984) and Nutman (1986)
attributed the fragmentation to tectonic processes
in a felsic volcanic rock. They described how the
ellipsoidal fragments were located in fold hinges,
and that away from fold hinges the fragments
passed into tabular bodies and then continuous
layers. Rosing et al. (1996) reinterpreted the supposed felsic volcanic rocks as products of ‘metasomatic alteration of intrusive Amı̂tsoq
tonalite-granite gneiss’ (p. 44). Myers (2000) described how some layers with supposed pyroclas-
tic fragments were derived from metasomatically
altered basaltic pillow lavas and tectonically disrupted intrusive sheets of tonalite. In other parts
of the felsic formation A6 of Nutman et al.
(1984), Myers (2000) found large scale, alongstrike gradations between quartzo-feldspathic
schist and mylonite, and tonalitic gneiss.
Frei et al. (1999) discovered heterogeneity in
late Archean metamorphic recrystallization of the
Isua greenstone belt by Pb stepwise leaching of
magnetite from banded iron formations. They
recognized that the amphibolite facies metamorphic mineral assemblages of banded iron formations in the northeastern part of the belt
developed at 3.69 Ga. These rocks were cut by
magnetite and sulphide-bearing veins associated
with metasomatic alteration at 3.63 Ga. In
contrast, in the western part of the Isua greenstone belt, step leaching of magnetite from similar banded iron formations yielded a PbPb
isochron age of 2.84 Ma, interpreted as the age
of peak amphibolite facies metamorphism in this
region.
Carbon isotopic ratios, considered to be of
biogenic origin, were determined by Rosing
(1999) from graphite in schistose rocks interpreted as metagraywackes in the Isua greenstone
belt. These metasedimentary rocks were recognized in a lenticular zone of relatively low strain
within a thick sequence of deformed amphibolite
schist, mainly derived from basaltic volcanic
rocks. This interpretation, and the \ 3700 Ma
age of the graphite, remains unchallenged.
4. Numerous magmatic events between 3870
and 3625 Ma?
Nutman et al. (1993) proposed on the basis of
SHRIMP UPb zircon data that ‘the Amı̂tsoq
gneisses comprise 3870, 3820 – 3810, 3760, 3730,
3700, and 3625 Ma TTG and 3660 – 3650 and
3625 Ma granites, and their inclusions belong to
several supracrustal sequences with a similar
spread of ages’ (ib. cit., p. 415). They concluded
that ‘these results show that the complex grew by
episodic addition of new TTG and welding to-
107
gether of rocks of different ages’ (p. 415) and
proposed a plate tectonic scenario.
This interpretation has been challenged in a
number of papers by Kamber, Moorbath and
Whitehouse. Kamber and Moorbath (1998) discussed new and published Pb isotope data for
Amı̂tsoq gneiss feldspars and whole rocks and
concluded ‘that the magmatic precursors of the
Amı̂tsoq orthogneisses separated from a depleted
mantle-like source at ca. 3.65 Ga’ (p. 19). They
proposed that zircons described by Nutman et al.
(1993, 1996) with ages significantly older than
3.65 Ga were inherited from older rocks. This
conclusion was supported by cathodoluminescence imaging and ion-microprobe UThPb dating of zircons from Amı̂tsoq gneisses by
Whitehouse et al. (1999), who found 3.8 Ga
cores mantled by overgrowths at 3.65 Ga.
They interpreted the cores as inherited grains and
the overgrowths as indicating the magmatic emplacement age, in contrast to Nutman et al.
(1993), who interpreted similar overgrowths as
metamorphic, resulting from granulite facies
metamorphism first described from the same region south of Nuuk by Griffin et al. (1980).
Subsequently, from a region unaffected by early
Archean granulite facies metamorphism, Nutman
et al. (1999) provided a detailed description of the
3800 Ma age of little deformed metatonalites
and metaquartz-diorites immediately south of the
Isua greenstone belt.
Controversy also surrounds the interpretation
of SmNd isotopic data from the Amı̂tsoq
gneisses. Bennett et al. (1993) interpreted the wide
range in initial oNd values of Amı̂tsoq gneisses as
indicating transient, highly depleted mantle reservoirs, and proposed a major change in processes
between early Archean and later styles of depleted-mantle evolution. These hypotheses were
challenged by Vervoort et al. (1996), Gruau et al.
(1996), Moorbath et al. (1997) and Frei et al.
(1999), who argued that the SmNd systems, assumed to have been closed systems by Bennett et
al. (1993), were modified by subsequent tectonothermal events. This prompted substantial
debate by Bennett and Nutman (1998) and Kamber et al. (1998).
108
5. Terranes and late Archean tectonic evolution
Detailed mapping of the geology between Tre
Brødre and Ameralik (southwestern part of Fig.
1) led to the recognition of three tectonostratigraphic terranes, distinguished by different components and geological histories, that were called
the Færingehavn, Tre Brødre and Tasiusarsuaq
terranes (Friend et al., 1987). Regional reconnaissance studies by Friend et al. (1988) and Nutman
et al. (1989) extended this concept throughout the
Godthåbsfjord region and defined a fourth terrane (called Akia) to the north (Fig. 1). McGregor
et al. (1991) subsequently combined the
Færingehavn and Tre Brødre terranes into a single terrane termed Akulleq. The Akia, Akulleq
and Tasiusarsuaq terranes (Fig. 1) were interpreted as originally independent rafts of continental crust, distinguished by different rock
assemblages with different SHRIMP UPb zircon
ages and metamorphic histories, that were assembled at 2720 Ma (Friend et al., 1996).
The early Archean Amı̂tsoq gneisses and
younger Ameralik dykes are confined to the
Akulleq terrane, where they are tectonically intercalated with younger metavolcanic and metasedimentary rocks and layered anorthosite complexes.
During this tectonic intercalation and associated
amphibolite facies metamorphism, the Akulleq
terrane was intruded by granodiorite (Ikkattoq
gneisses) at 2820 Ma (McGregor et al., 1991;
Nutman, 1997). Nutman et al. (1996) attributed
this event to the development of the Akulleq
terrane as an accretionary complex in which diverse crustal fragments accreted to the margin of
a southern continent marked by the Tasiusarsuaq
terrane. They envisaged that the protoliths of the
Ikkattoq gneisses formed during an early stage of
a 2830 and 2720 Ma episode of accretion and
subduction of the Akulleq terrane beneath the
Tasiusarsuaq terrane. ‘Final juxtaposition of the
Akia, Akulleq and Tasiusarsuaq terranes occurred
after the intrusion of the 2820 Ma Ikkattoq
gneisses (which are truncated by the terrane
boundaries…), but before the intrusion of 2710
Ma granitic sheets that cut all three terranes’ (ib.
cit., p. 8).
The Tasiusarsuaq terrane to the south (Fig. 1)
is dominated by 2920 and 2860–2820 Ma tonalitic
gneisses derived from sheets of tonalite intruded
into metavolcanic rocks (now amphibolite), layered anorthosite complexes, and minor metasedimentary rocks, and was metamorphosed to
granulite facies at 2820 Ma (McGregor et al.,
1991; Nutman, 1997). The Tasiusarsuaq terrane
was thrust northwards over the southern margin
of the Akulleq terrane, causing the development
of new LS fabrics at amphibolite facies conditions
before the intrusion of 2710 Ma granitic sheets
that cut all three terranes (Nutman et al., 1996;
Nutman, 1997). To the north of the Akulleq
terrane, the Akia terrane (Fig. 1) consists of 3230 and 3000 Ma dioritic, tonalitic, trondhjemitic and granitic gneisses, amphibolite largely
derived from volcanic rocks, and intrusions of
leucogabbro and norite, all of which experienced
granulite facies metamorphism at 3000 Ma
(Riciputi et al., 1990; McGregor et al., 1991;
Nutman, 1997).
This subdivision of the Godthåbsfjord region
into terranes led to revision of the previous
nomenclature because rocks formerly called
Malene supracrustals (McGregor, 1969) and Nûk
gneisses (Black et al., 1971) were found to be of
different ages in the different terranes. McGregor
et al. (1991) recommended that the name Malene
supracrustals be discontinued and the name Nûk
gneiss be restricted to rocks in the Akia terrane.
Nutman et al. (1996) introduced a term ‘Itsaq
Gneiss Complex’ to include all the early Archean
rocks in the Godthåbsfjord region: Amı̂tsoq
gneisses, Isua supracrustal belt, and Akilia association. These rocks are restricted to, and partly
define, the Akulleq terrane (McGregor et al.,
1991).
6. Life on Akilia ] 3850 Ma?
Here we re-examine fundamental evidence relating to the discovery claimed by Mojzsis et al.
(1996) of the oldest trace of terrestrial life in
carbonaceous inclusions in apatite crystals from a
thin layer of quartz-rich rock on the small island
of Akilia south of Nuuk (Figs. 1 and 2). We focus
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
Fig. 2. Geologic map showing the location of Akilia within the
regional tectonic framework, after Chadwick and Nutman
(1979) and Chadwick and Coe (1983).
on new field evidence of the nature of the rocks,
their geological history and relative ages. We do
not discuss whether the isotopic composition of
the carbon described by Mojzsis et al. (1996) is of
unequivocal organic origin or ‘why the apatite
should act as a safe-deposit vault’ (Hayes, 1996,
p. 22). Such questions were raised by Hayes
(1996) in his Nature review that accompanied the
publication of Mojzsis et al. (1996).
The age of ] 3850 Ma ‘for the Akilia island
sediments is… significant because it would place
their deposition simultaneous with the Late
Heavy Bombardment of the Moon at 3800 – 3900
Ma’ (Mojzsis and Harrison, 2000, p. 3). During
this event, the Earth is thought to have suffered
more severely from frequent and large meteorite
impacts that repeatedly vapourized the oceans
The small island of Akilia south of Nuuk (Figs.
1 and 2) consists of early Archean Amı̂tsoq
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
Amı̂tsoq 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 Qilángârsuit
by Berthelsen (1955) and around Nuuk by McGregor (1973). In the region south and southeast
of Nuuk, the Amı̂tsoq 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 granite and are characterized by megacrysts of microcline and by a high iron content (Chadwick and
Coe, 1983). Amı̂tsoq gneisses contain fragments
of diverse high grade metamorphic rocks, called
Akilia association by McGregor and Mason
(1977), that were thought to be mainly of
supracrustal origin and were interpreted as the
host rocks into which the igneous precursors of
the Amı̂tsoq gneisses were intruded.
On the southwestern peninsula of Akilia (Fig.
3), the type locality of the Akilia association
(McGregor and Mason, 1977) comprises disrupted layers of mafic and ultramafic gneiss
(mainly hornblende-diopside amphibolite, hornblende-pyroxenite and minor serpentinite) and a
thin layer of quartz-rich rock interpreted by Nutman et al. (1997a) as banded iron formation and
the oldest terrestrial record of water-lain sedimentary deposits.
The adjacent quartzo-feldspathic gneiss is heterogeneous and comprises numerous phases of
110
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 pegmatitebanded tonalitic and dioritic Amı̂tsoq gneiss (grey), parallel
layers and boudins of amphibolite (black) derived from Ameralik dykes, and pegmatite veins (white). The superficial simplicity 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.
Fig. 3. Simplified geologic map of southwestern Akilia.
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
Precambrian Research, this volume).
The amphibolite layers interpreted as deformed
remnants of dykes (Ameralik dykes) are generally
more homogeneous than the Amı̂tsoq gneisses
and mafic and ultramafic gneiss, do not contain
abundant thin layers of pegmatite that are an
integral part of the gneiss, and can locally be seen
cutting across heterogeneity in the gneiss and
folded gneissosity. The Ameralik dykes were intruded into the Amı̂tsoq gneisses and mafic and
ultramafic gneiss after the protoliths of these
rocks had already been converted into gneisses by
deformation and metamorphism. This early gneissosity was substantially modified during late
Archean episodes of deformation when it was
Fig. 5. Heterogeneous tonalitic and dioritic gneiss (Amı̂tsoq
gneiss) with parallel layers of pegmatite-veined mafic and
ultramafic gneiss (Akilia association) and thin layers of amphibolite. View to the south on southwestern Akilia. Located on
Fig. 3.
Fig. 6. Amphibolite boudins derived from an Ameralik dyke in
tonalitic gneiss (Amı̂tsoq gneiss). The sampled pegmatite
(270493 Ma UPb age) formed during the development of
the boudins. Located on Fig. 3. The hammer to the right of
the arrow tip is 40 cm long.
111
rotated into parallelism with the Ameralik dykes.
Thus, although the gneissosity of both quartzofeldspathic Amı̂tsoq gneisses and mafic and ultramafic gneiss of the Akilia association is parallel, it
did not all form at the same time and reflects a
long history of development, reworking and transposition. Recognition of the complete history of
development of the gneissosity on southwestern
Akilia is rendered difficult, and probably impossible, because the late Archean tectonic modifications were so intense that the present structure is
superficially simple. Except for a few relatively
little-deformed pegmatites that mostly strike eastwest or northwest-southeast, the rock layers are
generally parallel, subvertical and strike northsouth, 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 Amı̂tsoq gneisses and Ameralik dykes. Isoclinally folded remnants of Ameralik dykes were
disrupted into trains of boudins (Figs. 4 and 6,
and cover of Precambrian Research, this volume)
and then deformed by tight upright southerly
plunging folds (Fig. 7). These younger folds are
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 pattern 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
of Fig. 3.
The context of Akilia in the regional tectonic
framework (Fig. 2) indicates that all the folds seen
on Fig. 3 were refolded on a larger scale. The
112
Fig. 7. Upright, gently plunging folds of composite gneissosity
of heterogeneous Amı̂tsoq gneiss on southwestern Akilia.
Fig. 8. UPb concordia plot. Ellipses for analyses represent the
2s uncertainty. See Table 1 for analytical data.
youngest fold axial trace in the southwestern part
of Fig. 3 may equate with D3 structures on Fig. 2,
where it can be seen that D3 structures are folded
by D4. The older two episodes of folding in
southwestern Akilia post-date the Ameralik dykes
and appear to be additional to the D2 thrusts of
Chadwick and Nutman (1979) (Fig. 2) and their
D1 structures that predate the Ameralik dykes.
To obtain precise control on the age of some
late Archean deformation and metamorphism on
Akilia, we determined the UPb age of zircons in
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
five single grains. Routine sample preparation and
analytical procedures were similar to those described in Dubé 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
27049 3 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
intercept that agrees with zircon ages in nearby
tonalitic gneiss (Nutman et al., 1996, 1997a;
Whitehouse et al., 1999).
In summary, the rocks on Akilia indicate a
complex history of repeated intense ductile deformation in which most geological features were
attenuated, rotated and transposed into a new
tectonic fabric (Figs. 4 and 5 and cover of Precambrian Research, this volume). Early Archean
granulite facies metamorphism at 3600 Ma
(Griffin et al., 1980), that predated the Ameralik
dykes, was followed by probably multiple
episodes of late Archean amphibolite facies metamorphism at 2830–2710 Ma (Nutman et al.,
Concentrationc
Wt.b (mg)
Analysisa
U (ppm)
Atomic ratiosd
Pbf (ppm)
Pbc (pg)
206
Pb
208
206
Pb
207
207
206
207
204
Pb
206
238
U
235
206
238
206
13.5169 0.10
13.8049 0.11
13.8059 0.10
13.3229 0.12
13.3669 0.10
0.18742 9 0.03
0.19025 90.03
0.19082 90.04
0.18538 9 0.04
0.18589 9 0.03
JC-001-99. Pegmatite between amphibolite boudins at lat. 63°56%06%%N,
Z1 pk, 5:2
34
97
53.9
35
Z2 pk, 3:1
15
192
107.3
40
Z3 pk, 3:2
8
244
135.4
48
Z4 pk, 4:3
9
223
122.5
34
Z5 co, 4:3
23
123
67.5
31
a
Age (Ma)e
long.
3150
2386
1342
1830
2932
Pb
Pb
51°40%47%%W
0.043
0.523090.09
0.043
0.526290.10
0.040
0.5247 90.09
0.041
0.521290.11
0.035
0.5215 9 0.10
Pb
U
Pb
Pb
Pb
U
2712
2726
2719
2704
2705
Pb
Pb
2719.7 9 1.1
2744.3 9 1.0
2749.2 9 1.2
2701.6 9 1.1
2706.29 0.9
Fraction code, followed by grain colour (pk, pink; co, colourless) and length to width ratio.
Weight of grain estimated visually using a microscope.
c
Concentration uncertainty varies with sample weight: estimated \10% for grains weighing B10 mg, B10% for grains weighing \10 mg.
d
Ratios corrected for spike, fractionation, blank, and initial common Pb, except 206Pb/204Pb ratio corrected for spike and fractionation only. Errors are 1s in
percent.
e
Errors are 2s in Ma. Pb blanks were 10–15 pg and U blanks were B1 pg.
f
Pb, radiogenic Pb; Pbc, total common Pb in analysis corrected for spike and fractionation.
b
Table 1
UPb analytical data
113
114
1996). These episodes of metamorphism accompanied the main deformation features now seen on
Akilia and extended to 2704 Ma (new data
above) and the younger D3 and D4 events described by Chadwick and Nutman (1979). The
rocks on Akilia were also subjected to a major
thermal and magmatic episode at 2550 Ma
during the emplacement of the nearby Qôrqut
granite (Figs. 1 and 2) (Baadsgaard, 1976; Brown
et al., 1981; Moorbath et al., 1981), and some of
the cross-cutting pegmatites on the island could
be of this age. Another major thermal episode of
regional extent is recorded by biotites that suffered extensive loss of radiogenic 87Sr at 1700 –
1600 Ma (Pankhurst et al., 1973).
The possibility of any trace of early Archean
life that may have existed in southwestern Akilia
surviving these multiple episodes of intense ductile
deformation and high grade metamorphism, and
being recognizable as early Archean, appears
miraculous. Yet such a miracle was claimed by
Mojzsis et al. (1996) and received world-wide
publicity.
6.2. Host rock of ‘oldest life’
Mojzsis et al. (1996) measured ‘the carbon-isotope composition of carbonaceous inclusions
within grains of apatite’ (p. 55) and found compositions that they interpreted to be the result of
‘biological activity’ ‘at least 3800 Myr before
present’ (p. 56).
The apatite grains were extracted from a thin
layer of highly deformed and metamorphosed
quartz-rich rock within mafic and ultramafic
gneiss on the southwestern tip of Akilia (Fig. 3).
This quartz-rich rock was interpreted as a banded
iron formation and is described in detail by Nutman et al. (1997a). The rock is claimed to be ‘the
oldest known sediment … and the oldest known
rock with evidence of biological processes active
during the time of deposition’ (Mojzsis and Harrison, 2000).
We found that this layer of rock is more than
twice as extensive as previously indicated by Nutman et al. (1997a, their Fig. 1) and Mojzsis and
Harrison (2000, their Fig. 4). The layer extends
north and north-northeast from the locality near
the southern shore sampled by Mojzsis et al.
(1996) (Fig. 3), and defines the eastern limb of a
third-generation fold of the thickest layer of mafic
and ultramafic gneiss. Thinner layers of this
quartz-rich rock were also seen to the northnorthwest in the mafic and ultramafic gneiss on
the west coast in the vicinity of the word ‘sea’ on
Fig. 3, but are too thin to be shown on this map.
The rock mainly consists of quartz with a few
thin layers and lenses rich in diopside and magnetite, and thin cross-cutting layers of diopside
and quartz of similar thickness. Smaller amounts
of hornblende, orthopyroxene and garnet occur
with the magnetite and diopside. These thin
melanocratic layers are unevenly distributed
within the layer of quartz-rich rock. The rock has
a post-tectonic, very coarse grained metamorphic
texture. The large grain size suggests substantial
diffusion during metamorphism.
The age of the recrystallization is unknown, but
in view of the tectonic history, the rock must have
undergone substantial ductile deformation and recrystallization during the late Archean. Sano et al.
(1999) determined UPb and PbPb isotopic ages
of apatites from this locality to be 1500 Ma
and concluded that ‘either the apatites in the BIFs
grew about 1500 Myr ago, or that they grew
earlier than that but were substantially affected by
recrystallization, and/or diffusive exchange with
the environment, which reset the UPb system of
the samples’. As noted by Sano et al. (1999), this
metamorphic event, which they estimated reached
600°C, could have been part of the regional thermal event recognized by Pankhurst et al. (1973).
In reply to the discussion of Sano et al. (1999),
Mojzsis et al. (1999), p. 128) asserted that ‘this
rock, which is essentially a quartzite, has no intrinsic capacity to form apatite by metamorphic
reactions following diagenesis. Thus, the apatite
present in this rock must be original (formed
before 3850 Myr) unless phosphate-bearing fluids
were subsequently introduced metasomatically.’
The latter possibility cannot be lightly dismissed
in view of the very coarse metamorphic grain size
of the rock and the presence of syn- and/or premetamorphic veins of quartz and quartz-diopside,
with the same coarse grain size and texture as the
host rock. Nor can the rock with certainty be
classified as ‘essentially quartzite’ (or metamorphosed banded iron formation) with apatites that
have suffered little more than diagenesis. A sedimentary origin is not impossible, but alternative
origins or multiple origins, such as all or part of
the rock being derived from vein quartz, need to
be considered and discounted. The apparent absence of ‘apatites with graphite inclusions in the
immediately adjacent, encompassing gneisses’
noted by Mojzsis et al. (1999), p. 128) as an
indication that apatite was not derived from the
adjacent rocks does not preclude metasomatic
transfer along the quartz-rich layer. If all or part
of the quartz-rich layer was derived from vein
quartz then mechanical transport of mineral
grains along a ductile fault zone is also a possibility to be considered.
Thus the age of crystallization, and primary
source, of the carbonaceous inclusions in the apatite crystals are uncertain. The rock may be
derived from a banded iron formation, but it
appears to have been so substantially modified by
deformational and metamorphic processes that
this interpretation is a hypothesis. This hypothesis
needs to be substantiated, and alternative hypotheses, such as that some or all of this layer
could have been derived from vein quartz that
was repeatedly deformed and metamorphosed,
need to be dismissed before a sedimentary origin
can unequivocally be established.
6.3. Age of host rock
Nutman et al. (1997a) extracted zircons from
the quartz-rich rock (Fig. 3) ‘in the hope that
either a few volcanogenic or extremely old detrital
grains might be found’ (p. 2481). However, all
zircons yielded ages of 2700 – 2590 Ma that
were interpreted as growing ‘during one or more
episodes of late Archaean high grade amphibolite
facies metamorphism…and provide no information on the deposition of the sediment’ (p. 2481).
Consequently, Nutman et al. (1997a) concluded
that only a minimum age of the quartz-rich rock
could be determined by dating granitoid rocks
that intruded the quartz-rich rock or the spatially
associated mafic and ultramafic gneiss.
115
The tonalitic gneiss adjacent to the eastern margin of the mafic and ultramafic gneiss, near the
southern shore of Akilia (Fig. 3), was dated by
Nutman et al. (1996) at 38729 10 Ma (SHRIMP
UPb zircon date, sample G88-66). Mojzsis et al.
(1996), referring to this locality, stated that the
mafic and ultramafic gneiss was ‘cut by a deformed quartz-dioritic sheet’ (p. 57), the same
gneiss dated by Nutman et al. (1996). The age of
the ‘sheet’ was considered to be the minimum age
of the quartz-rich rock hosting the apatites with
carbonaceous inclusions (Mojzsis et al., 1996).
Nutman et al. (1997a) contended that the
‘sheet’ should not be used to constrain the age of
the mafic and ultramafic gneiss and associated
quartz-rich layer because ‘given the strong deformation at the contact, it cannot be totally ruled
out that the gneisses and supracrustal rocks have
been tectonically juxtaposed’ (p. 2477). Therefore,
the age of another layer of quartz-dioritic gneiss
in contact with the mafic and ultramafic gneiss
was investigated by Nutman et al. (1997a) (sample
G93-05, located by the arrow marked (Figs. 9–11
in Fig. 3). They described this layer as a ‘quartzdioritic sheet within amphibolites of the
supracrustal body… Where sampled, this sheet is
1 m wide and its contacts are up to 10°
discordant to the compositional layering in host
layered amphibolites’ (p. 2477). They stated that
‘apart from the mafic inclusions and the sparse
pegmatite veins, the quartz diorite sheet is homogeneous—it is not a composite banded gneiss. On
the basis of the field relations, the quartz-diorite
sheet definitely cuts the supracrustal rocks’ (p.
2477). The age for this sheet, and thus the minimum age for the mafic and ultramafic gneiss, was
interpreted from SHRIMP UPb zircon dating as
being ‘definitely 3840, and possibly 3865 9 11
Ma’ (pp. 2481–2482).
Whitehouse et al. (1999) reinvestigated the age
of this ‘sheet’ by performing UPb zircon ion-microprobe dating in conjunction with cathodoluminescence imaging on a sample (SM/GR/97/7)
collected from the same locality as sample G93-05.
The imaging revealed cores and overgrowths of
zircon, and in contrast to Nutman et al. (1997a)
little evidence for zircon older than 3650
Ma was found (only one core with an age of
116
3800 Ma was discovered). Whitehouse et al.
(1999) concluded that the rock ‘shows unambiguous 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’
Fig. 9. Simplified map of a composite layer of granitoid gneiss,
the ‘quartz-diorite sheet’ interpreted by Nutman et al. (1997a)
as an intrusion into the mafic and ultramafic gneiss of the
Akilia association. Simplified after mapping at 1:12 scale.
Located by arrows on Figs. 3 and 5, on the shore of southwestern Akilia. The layer is too thin to be represented on Fig.
3. The outcrop slopes gently northwards except for a small
vertical portion marked by the thick line. Therefore the geology of this vertical rock face cannot be seen on this map. A
photograph and sketch of this vertical face are shown in Fig.
11.
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
fold (Fig. 3; outcrop located by the arrow marked
Figs. 9–11 on Figs. 3 and 5). Our map is shown
in Fig. 9 and a photograph of the outcrop is
shown in Fig. 10. Part of this outcrop (shown in
detail in our Fig. 11) was previously portrayed by
Nutman et al. (1997a) as evidence of discordance
between the ‘quartz-diorite sheet’ and the mafic
and ultramafic gneiss. Their figure (ib. cit., Fig. 2,
p. 2478), a sketch from a photograph, was unfor-
tunately drawn in reverse image and the scale
indicated was incorrect (the scale bar should be
40% longer).
Figs. 9–11 show that geologic relationships in
this outcrop are considerably more complex than
depicted by Nutman et al. (1997a). The ‘sheet’ is a
strongly deformed composite layer, 0.6 – 0.8 m
wide, consisting of discontinuous layers of: heterogeneous gneiss (thinly banded mafic and felsic
layers), relatively homogeneous tonalitic gneiss,
leucocratic pegmatite, hornblendite, and vein
quartz (Figs. 9–11). These rocks are cut by an
undeformed, gently north-dipping pegmatite dyke
(probably late Archean). Compositional layering
in the heterogeneous gneiss parallels the foliation
in the tonalitic gneiss, the margins of the composite layer, and the gneissosity in the adjacent
mafic and ultramafic gneiss (Figs. 9 – 11). The ‘up
to 10°’ discordance between the composite
layer and gneissosity in the adjacent rocks that is
117
claimed by Nutman et al. (1997a), p. 2477) does
not exist; apparent discordance seen in Fig. 11 is
due to the perspective of the photograph and the
three-dimensional outcrop surface. The gneissosity in the mafic and ultramafic gneiss is also
parallel to the contact with, and foliation in, the
tonalitic gneiss across strike to the east as can be
seen in Fig. 5 (outcrop located on Fig. 3 by the
arrow marked Fig. 5).
Nutman et al. (1997a) and Whitehouse et al.
(1999) did not indicate where their UPb samples
were collected, but V.R. McGregor and M.J.
Whitehouse (personal communications, 1999) indicated the samples were taken from the tonalitic
gneiss at location 3 in Fig. 11b. The tonalitic
gneiss appears to have a less complex history than
the heterogeneous gneiss (Figs. 9–11), but there is
no field evidence suggesting that it intruded into
the heterogeneous gneiss or the adjacent mafic
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
nature of the hornblendite lenses in the composite
layer probably reflects deformational rather than
intrusive processes.
This outcrop on southwestern Akilia, which
forms the basis for the age attributed to the oldest
life claimed by Mojzsis et al. (1996), comprises
highly strained rocks. These rocks underwent
multiple deformation and high grade metamorphic events that obliterated primary relationships
between the tonalitic gneiss and the mafic and
ultramafic gneiss (Figs. 9 – 11). Although our observations do not rule out intrusion of part of the
protolith of the tonalitic gneiss into the protolith
of the mafic and ultramafic gneiss, they do suggest
that other primary relationships are equally plausible. These relationships include (i) deposition of
the protoliths of the mafic and ultramafic gneiss
as lava flows and/or clastic or pyroclastic fragments onto the tonalitic gneiss; (ii) intrusion of
the protoliths of the mafic and ultramafic gneiss
into the tonalitic gneiss; (iii) intrusion of the protolith of a younger component of the tonalitic
gneiss (carrying xenoliths of older heterogeneous
gneiss) into the mafic and ultramafic gneiss or its
protoliths, and (iv) tectonic juxtaposition, such as
the D2 thrusting of Chadwick and Nutman
(1979).
Nutman et al. (1997a) stated that ‘extreme caution 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
Itsaq Gneiss Complex (Nutman et al., 1996), (p.
2477). Need for caution was further stressed by
Nutman et al. (2000): ‘bearing in mind that the
older components of the geology on southern
Akilia underwent granulite-facies metamorphism
and some partial melting at ca. 3650 Ma and that
most of them were highly deformed in both the
early and late Archaean, a degree of caution may
be warranted regarding whether the dates from
samples G88-66 and G93-05 give a minimum age
for the unit of banded iron formation… that is
interpreted as the oldest evidence of a hydrosphere on Earth’.
We agree that the evidence needs to be rigorously evaluated and that even greater care than
that exhibited by Nutman et al. (1997a) must be
taken in order to prove that any components of
the Akilia association on southwestern Akilia
were intruded by any of the protoliths of the
adjacent tonalitic gneiss and may therefore be
older than 3.85 Ga (Nutman et al., 1997a) or 3.65
Ga (Whitehouse et al., 1999). The layer of gneiss
(Figs. 9–11) is heterogeneous and clearly contains
components of different composition and age.
The current controversy between Nutman et al.
(2000) and Whitehouse et al. (2000) over the
interpretation of disparate ages of zircon populations in this rock could reflect differences in the
components, or different mixtures of components,
that were studied from within this composite
layer.
The southwestern tip of Akilia is the type locality of the Akilia association (McGregor and Mason, 1977) and therefore it would be important to
obtain unequivocal protolith ages directly from
the mafic and ultramafic gneiss at this location.
However, in order to constrain the age of any
early life on Akilia, it would also be necessary to
determine the primary age relationships between
the mafic and ultramafic gneiss, the quartz-rich
rock, and the apatite with carbonaceous inclusions (Mojzsis et al., 1996). At present these crucial relationships are unknown and appear to be
extremely difficult to resolve in the intensely deformed and metamorphosed rocks on Akilia.
7. Conclusions
What do we know about the Earth’s oldest
continental crust in the Godthåbsfjord region?
The early Archean gneisses were mainly derived
from plutonic igneous rocks, tonalite and granodiorite and a smaller amount of quartz-diorite,
trondhjemite, diorite, ferrodiorite and granite.
The precise ages of intrusion as magmas is controversial. Largely on the basis of SHRIMP UPb
data on zircons, Nutman et al. (1996) considered
that the magmas that formed these early Archean
rocks were intruded over a protracted period between 3870 and 3625 Ma. In contrast, Kamber
and Moorbath (1998) used PbPb data from
feldspars and whole rocks to conclude that ‘the
magmatic precursors of the Amı̂tsoq orthogneisses separated from a depleted mantle-like
source at ca. 3.65 Ga’ (p. 19). A broadly similar
conclusion was reached by Whitehouse et al.
(1999), based on UPb zircon data and cathodoluminescence imaging. They found that the Amı̂tsoq gneisses they studied were derived from rocks
intruded at 3.65 Ga, and older zircons are
xenocrysts.
The Amı̂tsoq gneisses are diverse, extensive,
and heterogeneous, and although the amount of
geochronology carried out on these rocks is probably greater than on any other region of early
Archean rocks, it is still far from adequate to
define the whole temporal evolution of the gneiss
complex. However, it is probably significant that
the age determinations all fall in the range of
3850–3600 Ma and that no significantly older
zircon xenocrysts have been found. This contrasts
with the Acasta gneiss complex in the Slave
Province of Canada and the Narryer gneiss complex in the Yilgarn Craton of Australia that have
both yielded much older zircons. Tonalitic and
granodioritic gneisses from the Acasta region contain zircons with SHRIMP UPb zircon ages
between 4.0–4.03 Ga (Bowring et al., 1989;
Stern and Bleeker, 1998; Bowring and Williams,
1999), and in the Narryer region, late Archean
granites contain \4.0 Ga zircon xenocrysts (Nelson et al., 1997), and detrital zircons in late
Archean metasedimentary rocks range back, beyond the oldest known 3.73 Ga rocks of the
region, from 3.9– 4.3 Ga (Froude et al., 1983;
Compston and Pidgeon, 1986; Maas et al., 1992).
The wide range of extremely ancient zircons in the
Narryer gneiss complex indicates the presence, or
former presence, of substantially older and more
complex continental crust in this region than appears likely in the Godthåbsfjord region.
Small fragments of diverse composition (Akilia
association) are widespread in the Amı̂tsoq
gneisses. They are generally considered to be
derived from highly metamorphosed mafic and
ultramafic intrusions and sedimentary rocks, and
to be older than the Amı̂tsoq gneisses. Nutman et
al. (1996, 1997a) considered that the age of the
Akilia association ranges back to over 3850
Ma, whereas Kamber and Moorbath (1998) argued that these rocks formed at 3700 – 3650
Ma.
119
The Isua greenstone belt is generally considered
to be a better preserved equivalent of the Akilia
association at lower metamorphic grade. Nevertheless, these rocks are also intensely deformed
and suffered numerous episodes of deformation
and metamorphism to amphibolite facies. The
ages of these rocks are poorly constrained but
thought to be 3.8–3.7 Ga (Nutman et al., 1996,
1997b; Moorbath and Whitehouse, 1996).
Is there any direct trace of life in the early
Archean rocks of the Godthåbsfjord region? The
answer appears to be very little. The age and
significance of the graphite described from apatite
crystals on Akilia by Mojzsis et al. (1996) are
controversial. The age and origin of the host rock
are unknown. Previous claims that the rock is
\ 3850 Ma are unsupported by reinvestigation of
the field evidence. The discovery by Rosing (1999)
of carbon of possible biogenic origin in \3700
Ma schists interpreted as metasedimentary rocks
of turbiditic and pelagic origin in the Isua greenstone belt is more plausible. These rocks are from
a zone of relatively low strain where primary
features can be recognized, but the tectonostratigraphy of the greenstone belt is complicated and
there is insufficient precise geochronology to
define the ages of all the tectonostratigraphic
units. However, existing data suggest that much
of the greenstone belt is \ 3700 Ma, and life, if it
then existed, is more likely to be preserved in
these rocks than in any other early Archean rocks
in the region.
Banded iron formations are widely considered
to reflect the precipitation of insoluble ferric iron
as a result of oxidation of soluble ferrous iron in
seawater (e.g. Awramik, 1981, p. 357). The required oxygen is generally thought to have been
generated by organic photosynthetic processes.
Thus the presence of banded iron formation
within the \ 3700 Ma Isua greenstone belt, and
possibly within the Akilia association, could be
macroscopic indications of photosynthetic
processes.
Although these ancient rocks of the
Godthåbsfjord region have been dead and cold
for a long time, there is still plenty of heat and life
left in the current research and controversies surrounding their interpretation. The geology around
120
Godthåbsfjord is exceptionally well exposed in a
rugged glaciated terrain with vertical relief of
2000 m. Access and infrastructure are, for such
an Arctic region, very good due to the fjord
complex and the location of the main Greenland
town of Nuuk at the mouth of the fjord (Fig. 1).
There remains plenty of scope for substantial
research into the early history of the Earth, for
better constrained mineral exploration, and for
lively geological debate well into the 21st century
and beyond.
Acknowledgements
Recent work at Isua was supported by the Isua
Multidisciplinary Research Project (IMRP)
funded by the Danish National Science Research
Council, the Commission for Scientific Research
in Greenland, the Greenland Bureau of Minerals
and Petroleum, and the Geological Survey of
Denmark and Greenland (GEUS). Further support was provided by the Voisey’s Bay Nickel –
Paterson research chair held by JSM at Memorial
University of Newfoundland. G.R. Dunning is
thanked for facilitating the geochronology, and
J.A. Percival and M.T. Rosing are thanked for
constructive reviews.
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Vestiges of life in the oldest Greenland rocks? A

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