Precambrian Research 103 2000 101 – 124
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 A
1
B
3
X
5
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 interpreta-
tions, 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 www.elsevier.comlocateprecamres
1. Introduction
The Godtha˚bsfjord 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 sedimentary rocks during the late Archean. These
rocks were repeatedly deformed and metamor- phosed, 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 Godtha˚bsfjord 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 metasedi-
mentary schists and gneisses.
Corresponding author. Tel.: + 1-709-7378417; fax: + 1- 709-7372589.
E-mail address
:
jmyerssparky2.esd.mun.ca J.S. Myers. 0301-926800 - 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
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 out-
lining current controversies. Most of the early 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.
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 Precam-
brian Research, this volume. Therefore a brief outline of the late Archean tectonic evolution is
included in this review Section 5, indicating cur- rent interpretations of how the early Archean
rocks were intruded by, and tectonically inter- leaved with, younger Archean rocks.
The second part of this article Section 6 re-ex- amines 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 Godtha˚bsfjord
