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Precambrian Research 105 (2001) 129–141 www.elsevier.com/locate/precamres

Protoliths of the 3.8–3.7 Ga Isua , West

John S. Myers *

Department of Earth Sciences, Memorial Uni6ersity of Newfoundland, St. John’s, Nfld , A1B 3X5

Received 15 July 1999; accepted 18 October 1999

Abstract

The (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 , but both the deformation and metasomatism were heterogeneous. Transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to 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 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 -bounded rock packages, mainly derived from basaltic and high-Mg basaltic pillow and , –BIF, and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks. A previously mapped, extensive, unit of volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of . © 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 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, 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 (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 oratories indicate protolith ages between 3.8 and and banded formation, intruded by ultra- 3.7 Ga (Moorbath et al., 1997; Nutman et al., mafic sills and sheets of tonalite–, 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 , 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 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 , 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 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 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); (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 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’’. This unit was inter- Isua belt was remapped by Nutman in 1980–82 at preted as a ‘gabbroic, possibly -like intru- a scale of 1:10000, and the stratigraphy presented sion(s)’. The ultramafic rocks were described as by Nutman et al. (1984) and Nutman (1986) has being located ‘at most stratigraphic levels in the remained the basis for all subsequent research, Isua supracrustal belt’ and as being ‘slightly dis- and was recently reiterated with new geochronol- cordant to layering of the supracrustal sequences’. ogy by Nutman (1997) and Nutman et al. (1997), They were interpreted as being derived from in- (1998). trusions of mainly and .

2.2. Nutman stratigraphy 2.3. Reappraisal of the Nutman stratigraphy

Nutman (1986) defined two stratigraphic se- Doubt was cast over the geological map and quences (A and B). Sequence A formed ‘a coher- stratigraphy of Nutman (1986) by Rose et al. ent stratigraphy…along the whole length of the (1996), following a detailed study of the metacar- Isua supracrustal belt’ whereas part of sequence B bonate rocks. These authors concluded that the was cut out by a fault (Nutman, 1986, p. 10). He calc-silicate formation was metasomatic in origin. defined sequence A as comprising six formations Further reappraisal of the supracrustal rocks by ‘in ascending order’ (Fig. 3a): (A1) amphibolite Rosing et al. (1996) started to demolish the rest of formation of mainly banded amphibolite; (A2) this stratigraphic edifice. The carbonate-rich gar- lower banded iron formation, characterised by net–hornblende– schists that are the main iron formation; (A3) variegated component of the ‘variegated schist formation’ formation consisting of ‘, felsic rocks, were found to be gradational into amphibolite, metapelites and metacherts’; (A4) upper banded and to have similar, basaltic, rare earth element iron formation of ‘magnetite iron formation and content. These schists were reinterpreted as meta- metachert’; (A5) calc-silicate formation compris- somatic alteration products of amphibolite, rather ing ‘carbonates, , calc-silicate rocks and than being of sedimentary origin. Likewise grada- amphibolites’; and (A6) felsic formation of ‘‘pre- tions were described between leucocratic schists, dominantly layered metasediments’’. The accom- mainly –feldspar–muscovite–biotite–car- panying map (Nutman, 1986, plate 1) included a bonate rocks, that comprise the ‘felsic formation’, seventh stratigraphic unit of ‘‘undifferentiated and discordant sheets of tonalitic and granitic variegated schist formation to amphibolite forma- gneiss. Rosing et al. (1996) therefore concluded tion’’, placed beneath the amphibolite formation that the ‘felsic formation’ was derived by metaso- (A1). ‘Sequence B comprises in ascending order: matic alteration of intrusive tonalite and granite, (B1) felsic formation’ of ‘predominantly metasedi- rather than being a sequence of metamorphosed ments’ ‘and (B2) mica schist formation’ ‘predomi- felsic volcanic and epiclastic rocks. In contrast, nantly Fe–Mg-rich mica schists’ (Nutman, 1986). part of the ‘garbenschiefer amphibolite’ unit was He described several alternative explanations of found to contain components of sedimentary the original relationship between sequences A and origin that passed gradually into the dominant B. garbenschiefer-textured magnesian schists. This Nutman (1986) interpreted two additional units unit was reinterpreted as of -sedimentary, as mafic intrusions. ‘garbenschiefer amphibolite’ chiefly basaltic, origin and an integral part of the comprising ‘units up to more than 1 km broad stratigraphy, rather than being an intrusion, re- J.S. Myers / Precambrian Research 105 (2001) 129–141 133 cently described as metagabbro by Nutman served in the central domain, whereas in the (1997). adjacent domains most primary features have A consequence of this reappraisal of the stratig- been obliterated by more intense deformation that raphy was that the simple isoclinal syncline pro- converted most rocks to schists. posed as the dominant structure of the greenstone The southwest part of the greenstone belt (Fig. belt by Nutman et al. (1984) and Nutman et al. 3b) is also divided by ductile faults into a number (1996), Nutman (1997) was also unfounded. Ros- of tectonic slices. Here the deformation was more ing et al. (1996) considered that the dominant heterogeneous, and although most rocks are schis- protoliths of the greenstone belt were basalt and tose, a variety of primary features are locally well banded iron formation, intruded by dunitic sills. preserved in all the tectonic slices. They recognised that the sequence was deformed The main rock types and structures are de- and metamorphosed, including being sliced up by scribed below: faults, before being intruded by tonalite and gran- ite, followed by further metasomatism, deforma- 3.1. Amphibolite deri6ed from pillow la6a and tion and metamorphism. Rosing et al. (1996) related epiclastic rocks concluded that ‘pervasive carbonation and K metasomatism produced a sequence of , This is the most widespread mappable unit and mimicking those found in modern platform de- is marked as ‘‘amphibolite (pillow lava)’’ on Figs. posits. However, the protoliths could have origi- 2 and 3b. The rock mainly consists of – nated in a purely oceanic environment with no chlorite––tremolite schist. The deformation sialic detrital components’. was heterogeneous and the least deformed rocks contain pillow lava (Figs. 4 and 5) or related epiclastic structures. With increasing deformation, 3. Major components of the Isua greenstone belt these rocks were converted to banded amphibolite and their protoliths (Figs. 6 and 7). Some deformed pillow con- tain cooling collapse cavities infilled by quartz The northeast and southwest parts of the Isua (Fig. 4) and some contain occelli (Fig. 5). In some greestone belt (Fig. 1) have been remapped at a cases the occelli are concentrated in concentric scale of 1:8000 and the main features of the zones around the outer parts of individual pil- geology are shown in Figs. 2 and 3b. The green- lows. Some of the occelli are cut by concentric stone belt and adjacent tonalitic gneisses were cracks infilled by quartz that formed during the repeatedly deformed and recrystallised. The crystallisation of the . greenstones were folded into isoclinal structures that were refolded isoclinally before different seg- 3.2. Amphibolite (altered pillow la6a) ments of the greenstone belt were juxtaposed in their current relative positions by ductile faults. This widespread mappable rock type, marked Subsequently the whole package of greenstone as ‘amphibolite9garnet9carbonate (altered pil- slices was deformed and folds were generated on low lava’ on Figs. 2 and 3b, mainly consists of all scales with axial surfaces inclined steeply to the hornblende––biotite–carbonate southeast and fold axes and associated lineations (–ankerite) schist. The rocks were het- plunging moderately to the southeast (Figs. 2 and erogeneously deformed and, as in the previous 3b). unit of amphibolite, with increasing deformation In the northeast (Fig. 2) the greenstone belt is all stages can be seen between amphibolite with composed of three fault-bounded slices, infor- deformed pillow lava structure and banded am- mally described as northwest, central and south- phibolite. There is a spatial association between east tectonic domains by Appel et al. (1998). much of this garnet–carbonate amphibolite and Deformed primary features (depositional struc- chert–BIF. This kind of amphibolite is located tures and macroscopic textures) are widely pre- along the margins of all chert–BIF horizons and 134 J.S. Myers / Precambrian Research 105 (2001) 129–141

Fig. 2. Map showing the main features of the northeast part of the Isua greenstone belt (located on Fig. 1). Red lines indicate major faults. Dip and strike symbols relate to schistosity and transposed compositional banding, and arrows indicate the direction and plunge of fold axes and lineations.

passes gradationally away from these margins ite–talc–tremolite amphibolite with similar de- into actinolite-chlorite–talc–tremolite amphibo- formed pillow lava structures, suggests that the lite. (The thin layers of chert–BIF in ‘amphibolite garnet–carbonate amphibolite represents a meta- (pillow lava)’ in Fig. 2 are also bounded by gar- somatically altered equivalent of the actinolite– net-carbonate amphibolite but this is too thin to chlorite–talc–tremolite amphibolite. Such be shown on this map.) alteration occurred before the last episodes of The spatial association with chert–BIF, and the deformation and recrystallisation because both gradation in composition into actinolite–chlor- rock types are equally overprinted by these events. J.S. Myers / Precambrian Research 105 (2001) 129–141 135

3.3. Chert-banded iron formation greenstone belt (Fig. 2) and thin layers within amphibolite (Fig. 3b). The BIF largely consists of Recrystallised chert and banded iron formation alternating layers of quartz and magnetite. There (BIF) form a major unit in the northeast of the are complete gradations between chert and BIF.

Fig. 3. a — Map of the southwest part of the Isua greenstone belt, simplified from Nutman (1986) (located on Fig. 1). b — New map of the southwest part of the Isua greenstone belt (located on Fig. 1). Red lines indicate major faults. Dip and strike symbols relate to schistosity and transposed compositional banding, and arrows indicate the direction and plunge of fold axes and lineations. 136 J.S. Myers / Precambrian Research 105 (2001) 129–141

Fig. 4. Deformed and recrystallised basaltic pillow lava. A deformed, quartz-filled primary collapse structure can be seen in the centre of a pillow to the right of the scale card. The pillow matrix that has been eroded away consisted of biotite and carbonate.

Where the BIF is least deformed the layers of However, most chert–BIF is intensely de- quartz and magnetite are generally 0.2–1.0 cm formed and in the northeast part of the green- thick and these layers reflect deformed primary stone belt (northeast part of Fig. 2) primary layering that developed on or below the ocean layering is extensively transposed into a new tec- floor. However even the least deformed layering is tonic layering (Fig. 9) and much of the chert substantially modified by deformation, and most comprises or recrystallised mylonite. In layering was folded and extended parallel to the the southwest part of the greenstone belt (Fig. 3b) plunge of the regionally dominant folds and asso- many thin layers of banded quartz schist and ciated lineations (Figs. 2, 3b and 8). In some cases mylonite could have been derived from either individual layers of quartz were disrupted into primary chert or from quartz veins. isolated boudins or fracture-bounded tabular seg- ments during deformation that preceded the fold- 3.4. Ultramafic rocks ing (Fig. 8). These structures have previously been Ultramafic rocks are of two kinds: anthophyl- interpreted as sedimentary (‘flat pebble conglom- lite-rich rocks and layered associated eratic structure’; Nutman, 1986, p. 17–18). with metagabbro and metapyroxenite. Each kind Primary conglomeratic structures occur in a of ultramafic rock is confined to different tectonic chert–BIF horizon in the northeastern part of the slices of the greenstone belt. greenstone belt, in the southern part of the central In the southwestern part of Fig. 3b, anthophyl- tectonic domain (Fig. 2). These conglomerates lite-rich ultramafic rocks form thin layers within include both oligomict conglomerate with round actinolite–chlorite–talc–tremolite amphibolite pebbles of quartz in a matrix of mainly quartz, with deformed pillow lava structures, and thicker biotite and garnet (‘round pebble conglomeratic fault-bounded layers. The apparently simple, mas- structure’ of Nutman, 1986, p. 18), and polymict sive field appearance of these rocks results from conglomerate containing pebbles of quartz and coarse radiating clusters of anthophyllite that meta-basalt in a matrix of quartz, biotite and form most of the rocks. However this overprints a garnet (Appel et al., 1998). complex history of older tectonic layering and J.S. Myers / Precambrian Research 105 (2001) 129–141 137 deformed mafic and ultramafic dykes. These ultra- mafic rocks could have originated as either ko- matiite flows or sub-volcanic intrusions. In the northeast part of Fig. 3b, ultramafic rocks comprise spatially associated with compositionally layered metagabbro and metapyroxenite. These ultramafic rocks appear to be derived from layered dunite–peridotite–pyrox- enite– intrusions.

3.5. Quartzo-feldspathic schist

Prominent layers of quartzo–feldspathic schist (Figs. 2 and 3) have previously been interpreted as derived from felsic volcanic rocks (Allaart, 1976; Nutman, 1986). Most of these rocks are intensely deformed schists or , but the amount of

Fig. 6. Banded amphibolite derived from strongly deformed recrystallised pillow lava with flattened and extended dark pillow cores and pale pillow rims.

deformation in the thickest layer in Fig. 3b de- creases towards the east and the schist passes gradationally into tonalitic gneiss. To the north- west in Fig. 3b, thin attenuated and boudinaged layers of quartzo–feldspathic schist and mylonite occur within garnet–biotite–carbonate amphibo- lite with discoidal bodies of quartz (Fig. 10). These rocks were previously interpreted as pyro- clastic felsic volcanic rocks (Allaart, 1976; Nut- man, 1986), but are here interpreted as deformed and recrystallised pillow lavas (with discoidal quartz representing pillow matrices) and de- formed, tectonically disrupted, sheets of recrys- tallised tonalite, derived from veins intruded into the pillow lavas, as these features can be observed Fig. 5. Deformed and recrystallised high-Mg basalt pillow lava with inter-pillow quartz. The pale spots within the pillows are in the least strained parts of this unit. In other deformed occelli. places where the pillow lavas suffered a greater 138 J.S. Myers / Precambrian Research 105 (2001) 129–141

Fig. 7. Banded amphibolite derived from strongly deformed recrystallised pillow lava with flattened and extended dark pillow cores and pale pillow rims and inter-pillow quartz. degree of alteration, flattened quartz in pillow and ankerite) and biotite (Fig. 11). These rocks matrices, and tectonically disrupted sheets and were also formerly interpreted as pyroclastic felsic veins of recrystallised tonalite and quartz are set volcanic rocks and included in the ‘felsic forma- in a matrix dominated by carbonate (dolomite tion’ of Nutman (1986).

Fig. 8. Deformed and recrystallised banded iron formation. A layer of folded quartz boudins can be seen near the top of the photograph. Similar layers were previously interpreted as depositional conglomeratic structures (‘‘flat pebble conglomeratic structure’’of Nutman (1986), and Nutman et al. (1984). J.S. Myers / Precambrian Research 105 (2001) 129–141 139

4. Conclusions

This study, based on new field investigations and mapping, supports the general conclusions of Rosing et al. (1996) and strengthens them by presenting new geological maps of portions of the Isua greenstone belt. These maps reveal the loca- tion, structure and relationships of the main pro- toliths and their altered equivalents. The field investigations indicate that a major part of the actinolite–chlorite–talc–tremolite amphibolites and hornblende–garnet–biotite–carbonate am- phibolites were derived from basaltic pillow lava and related epiclastic rocks, because deformed volcanic structures, including pillows and pillow , are widespread in all these rocks. All stages of deformation can be observed between well preserved pillow lava and pillow lava breccia structures and compositionally banded amphibo- lite schists. Likewise transitions can be followed, on a regional scale, between tonalitic gneiss with relict igneous textures to quartzo–feldspathic schists and mylonites that were previously inter- preted to be of felsic volcanic origin. The previ- ously mapped stratigraphy Nutman (1986) is of Fig. 9. Intensely deformed recrystallised chert and metadolerite little value in understanding the geology of the dyke, tectonically transposed into the regional tectonic fabric. Isua greenstone belt and is best abandoned. This

Fig. 10. Strongly deformed, recrystallised metasomatically altered pillow lava (black garnet–biotite–carbonate amphibolite, with lenses of white inter-pillow quartz), and strongly deformed, disrupted and recrystallised veins of tonalite (more continuous grey layers and lenses). This rock unit was previously interpreted by Nutman (1986) as derived from pyroclastic felsic volcanic rocks. 140 J.S. Myers / Precambrian Research 105 (2001) 129–141

Fig. 11. Strongly deformed, recrystallised metasomatically altered pillow lava (black carbonate with remnants of white inter-pillow quartz and disrupted quartz veins). This rock unit was previously interpreted as being derived from a sedimentary conglomerate by Bridgwater and McGregor (1974) and Dimroth (1982), and as being derived from pyroclastic felsic volcanic rocks by Allaart (1976), and Nutman (1986). oldest known greenstone belt consists mainly of ated occurrence of quartz-banded ironstone. In: Windley, tectonically juxtaposed slices of complexly de- B.F. (Ed.), The Early History of the Earth. Wiley, London, pp. 177–189. formed and recrystallised, basalt, ultramafic rocks Appel, P.W.U., Fedo, C.M., Moorbath, S., Myers, J.S., 1998. and chert–BIF. Recognizable primary volcanic and sedimentary features in a low-strain domain of the highly deformed, oldest known (3.7-3.8 Gyr) Greenstone Belt, West Greenland. Terra Acknowledgements Nova 10, 57–62. Baadsgaard, H., 1976. Further U-Pb dates on from the early Precambrian rocks of the Godthaabsfjord area, West This work is part of the Isua Multidisciplinary Greenland. Earth and Planetary Science Letters 33, 261– Research Project (IMRP) led by Peter Appel and 267. Stephen Moorbath, and supported by the Danish Boak, J.L., Dymek, R.F., 1982. Metamorphism of the ca. 3800 Ma supracrustal rocks at Isua, West Greenland: implica- National Science Research Council, the Commis- tions for early Archaean crustal evolution. Earth and sion for Scientific Research in Greenland, the Planetary Science Letters 59, 155–176. Greenland Bureau of and Petroleum, Boak, J.L., Dymek, R.F., Gromet, L.P., 1983. Petrology and and the Geological Survey of Denmark and rare earth element geochemistry of clastic metasedimentary Greenland. Their support is gratefully acknowl- rocks from the Isua supracrustal belt, West Greenland. Report Grønlands Geologiske Undersøgelse 112, 23–33. edged. The content of the paper was presented at Bridgwater, D., McGregor, V.R., 1974. Field mapping on the the EUG meeting in Strasbourg on 31 March early Precambrian rocks of the Godtha˚bsfjord region, 1999. southern West Greenland. Report Grønlands Geologiske Undersøgelse 65, 39–44. Bridgwater, D., Keto, L., McGregor, V.R., Myers, J.S., 1976. Archaean gneiss complex of Greenland. In: Escher, A., References Watt, W.S. (Eds.), Geology of Greenland. Grønlands Ge- ologiske Undersøgelse, Copenhagen, pp. 18–75. Allaart, J.H., 1976. The pre-3760 m.y. old supracrustal rocks Dimroth, E., 1982. The oldest rocks on Earth: stratigraphy of the Isua area, central West Greenland, and the associ- and sedimentology of the 3.8 billion years old Isua J.S. Myers / Precambrian Research 105 (2001) 129–141 141

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