Precambrian Research 105 2001 129 – 141
Protoliths of the 3.8 – 3.7 Ga Isua greenstone belt, West Greenland
John S. Myers
Department of Earth Sciences, Memorial Uni6ersity of Newfoundland, St. John
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s, Nfld Canada, A
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B
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Received 15 July 1999; accepted 18 October 1999
Abstract
The Isua greenstone belt Fig. 1 contains the oldest known, relatively well preserved, metavolcanic and metasedimentary rocks on Earth. The rocks are all deformed and many were substantially altered by metasomatism,
but both the deformation and metasomatism were heterogeneous. Transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated.
Likewise different kinds, and different episodes, of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths. New geological mapping has
traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents. By this means, the primary nature of the schists that make up most of the Isua greenstone belt was
reinterpreted, and a new map that better portrays the primary nature of the rocks has been produced. The previously mapped stratigraphy was found to be of little value in understanding the geology. Stratigraphic units were defined by
different and diverse criteria, such as current composition, structure, metamorphic texture, and inferred protoliths. Much of this stratigraphy represents a misinterpretation of the primary nature of the rocks. The new work indicates
that most of the Isua greenstone belt consists of fault-bounded rock packages, mainly derived from basaltic and high-Mg basaltic pillow lava and pillow lava breccia, chert – BIF, and a minor component of clastic sedimentary rocks
derived from chert and basaltic volcanic rocks. A previously mapped, extensive, unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of
tonalite. © 2001 Elsevier Science B.V. All rights reserved.
Keywords
:
Early Archaean; Tectonic evolution; Greenstone belt; Protolith interpretation; Greenland www.elsevier.comlocateprecamres
1. Introduction
The Isua greenstone belt also known as Isua supracrustal belt is part of the Archaean gneiss
complex of West Greenland Bridgwater et al., 1976. Most of this gneiss complex consists of
tonalitic gneiss with minor components of granitic gneiss, layered megacrystic anorthosite complexes,
amphibolite derived from basaltic volcanic rocks, metasedimentary rocks and ultramafic rocks, that
all formed between 3.0 and 2.7 Ga. In the middle
Tel.: + 1-709-7378417; fax: + 1-709-7372589. E-mail address
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of this gneiss complex there is a belt 50 – 75 km wide, extending for 200 km northeast through
Godtha˚bsfjord, that contains fragments of early Archaean rocks. Most of the latter are tonalitic
gneisses Amıˆtsoq gneiss: McGregor, 1973 that formed between 3.87 and 3.65 Ga Nutman et al.,
1996 or at ca. 3.65 Ga Kamber and Moorbath, 1998; Whitehouse et al., 1999. These ancient
tonalitic
gneisses contain
fragments of
supracrustal, metavolcanic and metasedimentary rocks. Small fragments of these rocks are known
as the Akilia association McGregor and Mason, 1977 and the largest fragment is called the Isua
supracrustal belt or Isua greenstone belt Appel et al., 1998. The protolith ages of the Akilia associ-
ation are controversial: as old as ca.3.87 Ga Nut- man et al., 1996 or 3.7 – 3.65 Ga Kamber and
Moorbath, 1998; Whitehouse et al., 1999. The precise ages of the diverse components of the Isua
greenstone belt are unknown, but numerous age determinations by diverse methods in several lab-
oratories indicate protolith ages between 3.8 and 3.7 Ga Moorbath et al., 1997; Nutman et al.,
1997.
The Isua greenstone belt Fig. 1 contains the best preserved, oldest known sequence of rocks
that formed on the surface of the Earth. These rocks are therefore of outstanding importance in
recording the oldest known terrestrial environ- ments, and provide the best opportunity for dis-
covering the earliest traces of life on Earth.
The interpretation of these ancient environ- ments, as well as the search for traces of life, rely
upon correct identification of the original nature of the schists that make up the greenstone belt.
This in turn requires that the complex tectonic, metamorphic and metasomatic history of the
rocks be unravelled in order to determine both the original nature and relationships of the compo-
nents of the greenstone belt.
The volcanic and sedimentary rocks from which the greenstone belt was derived were intruded by
sheets of tonalite and several generations of doler- ite dykes. The rocks were repeatedly deformed
and recrystallised in upper greenschist to lower amphibolite facies conditions. All the components
of the greenstone belt are strongly deformed and most are schistose.
Most research on the Isua greenstone belt since 1984 has been described in the context of the
geological map and stratigraphy of Nutman et al. 1984 and Nutman 1986. These authors divided
the greenstone belt into nine formations and two kinds of intrusive rocks: ultramafic rocks and a
major unit called ‘garbenschiefer amphibolite’ that was interpreted as a Mg – Al-rich basic
intrusion.
Rose et al. 1996 made a detailed study of the ‘calc-silicate formation’ of Nutman 1986. In
contrast to Nutman 1986 they concluded that this unit was not derived from calcareous chemi-
cal sediments but by metasomatism ‘‘where fluids flowed across the contacts between ultramafic
bodies and felsic or metabasaltic country rocks at deep crustal levels’’.
The whole stratigraphy of Nutman 1986 was questioned by Rosing et al. 1996 who considered
that the rock sequence was derived from basalt and banded iron formation, intruded by ultra-
mafic sills and sheets of tonalite – granite, and heterogeneously altered by metasomatism. In con-
trast to previous interpretations that the Isua sequence formed in a shallow water, platform
environment, with clastic sedimentary rocks in- cluding conglomerates, and calcareous chemical
precipitates, Rosing et al. 1996 suggested that the Isua sequence could have originated in an
oceanic environment. These authors also reinter- preted the ‘‘garbenschiefer amphibolite’’ as a unit
of mixed volcanic and sedimentary origin rather than an intrusion as suggested by Nutman et al.
1984 and Nutman 1986, 1997.
The new work described here generally sup- ports the reinterpretation of the Isua ‘stratigra-
phy’ by Rosing et al. 1996. Field evidence is presented of the tectonic and metasomatic transi-
tions by which a diversity of metamorphic rocks were generated from a few, relatively uniform,
protoliths. The distribution of these protoliths, and of regionally extensive zones of metasomatic
alteration, are shown on tectonostratigraphic maps of two portions of the Isua greenstone belt
Figs. 2 and 3.
2. Isua greenstone belt — previous work and stratigraphy
