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Protoliths of the 3.8–3.7 Ga Isua Greenstone Belt, West Greenland

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Protoliths of the 3.8–3.7 Ga Isua Greenstone Belt, West Greenland Precambrian Research 105 (2001) 129–141 www.elsevier.com/locate/precamres 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’s, Nfld Canada, A1B 3X5 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 1. Introduction complex of West Greenland (Bridgwater et al., 1976). Most of this gneiss complex consists of The Isua greenstone belt (also known as Isua tonalitic gneiss with minor components of granitic supracrustal belt) is part of the Archaean gneiss gneiss, layered megacrystic anorthosite complexes, amphibolite derived from basaltic volcanic rocks, * Tel.: +1-709-7378417; fax: +1-709-7372589. metasedimentary rocks and ultramafic rocks, that E-mail address: [email protected] (J.S. Myers). all formed between 3.0 and 2.7 Ga. In the middle 0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0301-9268(00)00108-X 130 J.S. Myers / Precambrian Research 105 (2001) 129–141 of this gneiss complex there is a belt 50–75 km of the greenstone belt are strongly deformed and wide, extending for 200 km northeast through most are schistose. Godtha˚bsfjord, that contains fragments of early Most research on the Isua greenstone belt since Archaean rocks. Most of the latter are tonalitic 1984 has been described in the context of the gneisses (Amıˆtsoq gneiss: McGregor, 1973) that geological map and stratigraphy of Nutman et al. formed between 3.87 and 3.65 Ga (Nutman et al., (1984) and Nutman (1986). These authors divided 1996) or at ca. 3.65 Ga (Kamber and Moorbath, the greenstone belt into nine formations and two 1998; Whitehouse et al., 1999). These ancient kinds of intrusive rocks: ultramafic rocks and a tonalitic gneisses contain fragments of major unit called ‘garbenschiefer amphibolite’ supracrustal, metavolcanic and metasedimentary that was interpreted as a Mg–Al-rich basic rocks. Small fragments of these rocks are known intrusion. as the Akilia association (McGregor and Mason, Rose et al. (1996) made a detailed study of the 1977) and the largest fragment is called the Isua ‘calc-silicate formation’ of Nutman (1986). In supracrustal belt or Isua greenstone belt (Appel et contrast to Nutman (1986) they concluded that al., 1998). The protolith ages of the Akilia associ- this unit was not derived from calcareous chemi- ation are controversial: as old as ca.3.87 Ga (Nut- cal sediments but by metasomatism ‘‘where fluids man et al., 1996) or 3.7–3.65 Ga (Kamber and flowed across the contacts between ultramafic Moorbath, 1998; Whitehouse et al., 1999). The bodies and felsic or metabasaltic country rocks at precise ages of the diverse components of the Isua deep crustal levels’’. The whole stratigraphy of Nutman (1986) was greenstone belt are unknown, but numerous age questioned by Rosing et al. (1996) who considered determinations by diverse methods in several lab- that the rock sequence was derived from basalt oratories indicate protolith ages between 3.8 and and banded iron formation, intruded by ultra- 3.7 Ga (Moorbath et al., 1997; Nutman et al., mafic sills and sheets of tonalite–granite, and 1997). heterogeneously altered by metasomatism. In con- The Isua greenstone belt (Fig. 1) contains the trast to previous interpretations that the Isua best preserved, oldest known sequence of rocks sequence formed in a shallow water, platform that formed on the surface of the Earth. These environment, with clastic sedimentary rocks in- rocks are therefore of outstanding importance in cluding conglomerates, and calcareous chemical recording the oldest known terrestrial environ- precipitates, Rosing et al. (1996) suggested that ments, and provide the best opportunity for dis- the Isua sequence could have originated in an covering the earliest traces of life on Earth. oceanic environment. These authors also reinter- The interpretation of these ancient environ- preted the ‘‘garbenschiefer amphibolite’’ as a unit ments, as well as the search for traces of life, rely of mixed volcanic and sedimentary origin rather upon correct identification of the original nature than an intrusion as suggested by Nutman et al. of the schists that make up the greenstone belt. (1984) and Nutman (1986, 1997). This in turn requires that the complex tectonic, The new work described here generally sup- metamorphic and metasomatic history of the ports the reinterpretation of the Isua ‘stratigra- rocks be unravelled in order to determine both the phy’ by Rosing et al. (1996). Field evidence is original nature and relationships of the compo- presented of the tectonic and metasomatic transi- nents of the greenstone belt. tions by which a diversity of metamorphic rocks The volcanic and sedimentary rocks from which were generated from a few, relatively uniform, the greenstone belt was derived were intruded by protoliths. The distribution of these protoliths, sheets of tonalite and several generations of doler- and of regionally extensive zones of metasomatic ite dykes. The rocks were repeatedly deformed alteration, are shown on tectonostratigraphic and recrystallised in upper greenschist to lower maps of two portions of the Isua greenstone belt amphibolite facies conditions. All the components (Figs. 2 and 3). J.S. Myers / Precambrian Research 105 (2001) 129–141 131 2. Isua greenstone belt — previous work and quently recalculated to 3710970 Ma, see Moor- stratigraphy bath and Whitehouse, 1996) on the banded iron formation, and a Rb/Sr whole rock age of 37009 The Isua greenstone belt (Fig. 1) is located on 140 Ma from the tonalitic gneiss. The main fea- the edge of the inland ice cap, 150 km northeast tures of the geology were first described by of Nuuk. The greenstones form an arcuate belt 35 Bridgwater and McGregor (1974). They com- km long that is truncated to the northwest by the pared the tonalitic gneiss to the Amıˆtsoq gneiss of Ataneq fault (McGregor, 1979). This fault is the Godtha˚b (Nuuk) region (McGregor, 1973), linked in the southwest to the Ivinnguit fault that and the dykes that cut both the gneiss and the was interpreted by McGregor et al. (1990) as a ca. supracrustal rocks to Ameralik dykes. The 2.72–2.7 Ga terrane boundary between the supracrustal rocks were mapped and described by Akulleq terrane, containing the Isua greenstone Allaart (1976), and further description and inter- belt and other early Archaean rocks, and the Akia pretation were given by Bridgwater et al. (1976). terrane to the northwest dominated by ca. 3.2– They discussed various interpretations of the 2.98 Ga tonalitic gneiss. quartzo–feldspathic schists interleaved with the mafic and ultramafic rocks and concluded that 2.1. Pre6ious work they were derived from acid volcanic rocks. There was a surge of research activity during A substantial amount of research has been car- the late 1970s to early 1980s on a variety of topics ried out on the Isua greenstone belt since the great including: stratigraphy and sedimentology (Dim- antiquity of these rocks was first established by roth, 1982; Nutman et al., 1984); structure (James, Moorbath et al. (1972, 1973) who obtained a 1976); petrology, mineralogy and geochemistry Pb/Pb whole rock age of 3760970 Ma (subse- (Schidlowski et al., 1979; Gill et al., 1981; Boak et Fig. 1. Outline map of the Isua greenstone belt and the location of segments A and B shown in Figs. 2 and 3. 132 J.S. Myers / Precambrian Research 105 (2001) 129–141 al., 1983); metamorphism (Boak and Dymek, that form approx. 25% of the Isua supracrustal 1982); geochronology (Moorbath et al., 1975; belt’ and ultramafic rocks (Fig. 3a). The ‘garben- Baadsgaard, 1976; Michard-Vitrac et al., 1977; schiefer amphibolite’ was described as being char- Hamilton et al., 1978), oxygen and sulphur iso- acterised by ‘well-developed garbenschiefer tope studies (Oehler and Smith, 1977; Oskvarek texture of amphiboles on its foliation surfaces’, and Perry, 1976; Perry and Ahmad, 1977) and and as being ‘‘slightly discordant to lithological organic chemistry (Nagy et al., 1975, 1977). The layering in adjacent rocks’’.
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