Isua greenstone belt — previous work and stratigraphy

2. Isua greenstone belt — previous work and stratigraphy

The Isua greenstone belt Fig. 1 is located on the edge of the inland ice cap, 150 km northeast of Nuuk. The greenstones form an arcuate belt 35 km long that is truncated to the northwest by the Ataneq fault McGregor, 1979. This fault is linked in the southwest to the Ivinnguit fault that was interpreted by McGregor et al. 1990 as a ca. 2.72 – 2.7 Ga terrane boundary between the Akulleq terrane, containing the Isua greenstone belt and other early Archaean rocks, and the Akia terrane to the northwest dominated by ca. 3.2 – 2.98 Ga tonalitic gneiss. 2 . 1 . Pre6ious work A substantial amount of research has been car- ried out on the Isua greenstone belt since the great antiquity of these rocks was first established by Moorbath et al. 1972, 1973 who obtained a PbPb whole rock age of 3760 9 70 Ma subse- quently recalculated to 3710 9 70 Ma, see Moor- bath and Whitehouse, 1996 on the banded iron formation, and a RbSr whole rock age of 3700 9 140 Ma from the tonalitic gneiss. The main fea- tures of the geology were first described by Bridgwater and McGregor 1974. They com- pared the tonalitic gneiss to the Amıˆtsoq gneiss of the Godtha˚b Nuuk region McGregor, 1973, and the dykes that cut both the gneiss and the supracrustal rocks to Ameralik dykes. The supracrustal rocks were mapped and described by Allaart 1976, and further description and inter- pretation were given by Bridgwater et al. 1976. They discussed various interpretations of the quartzo – feldspathic schists interleaved with the mafic and ultramafic rocks and concluded that they were derived from acid volcanic rocks. There was a surge of research activity during the late 1970s to early 1980s on a variety of topics including: stratigraphy and sedimentology Dim- roth, 1982; Nutman et al., 1984; structure James, 1976; petrology, mineralogy and geochemistry 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. al., 1983; metamorphism Boak and Dymek, 1982; geochronology Moorbath et al., 1975; Baadsgaard, 1976; Michard-Vitrac et al., 1977; Hamilton et al., 1978, oxygen and sulphur iso- tope studies Oehler and Smith, 1977; Oskvarek and Perry, 1976; Perry and Ahmad, 1977 and organic chemistry Nagy et al., 1975, 1977. The Isua belt was remapped by Nutman in 1980 – 82 at a scale of 1:10000, and the stratigraphy presented by Nutman et al. 1984 and Nutman 1986 has remained the basis for all subsequent research, and was recently reiterated with new geochronol- ogy by Nutman 1997 and Nutman et al. 1997, 1998. 2 . 2 . Nutman stratigraphy Nutman 1986 defined two stratigraphic se- quences A and B. Sequence A formed ‘a coher- ent stratigraphy…along the whole length of the Isua supracrustal belt’ whereas part of sequence B was cut out by a fault Nutman, 1986, p. 10. He defined sequence A as comprising six formations ‘in ascending order’ Fig. 3a: A1 amphibolite formation of mainly banded amphibolite; A2 lower banded iron formation, characterised by magnetite iron formation; A3 variegated schist formation consisting of ‘amphibolites, felsic rocks, metapelites and metacherts’; A4 upper banded iron formation of ‘magnetite iron formation and metachert’; A5 calc-silicate formation compris- ing ‘carbonates, quartzites, calc-silicate rocks and amphibolites’; and A6 felsic formation of ‘‘pre- dominantly layered metasediments’’. The accom- panying map Nutman, 1986, plate 1 included a seventh stratigraphic unit of ‘‘undifferentiated variegated schist formation to amphibolite forma- tion’’, placed beneath the amphibolite formation A1. ‘Sequence B comprises in ascending order: B1 felsic formation’ of ‘predominantly metasedi- ments’ ‘and B2 mica schist formation’ ‘predomi- nantly Fe – Mg-rich mica schists’ Nutman, 1986. He described several alternative explanations of the original relationship between sequences A and B. Nutman 1986 interpreted two additional units as mafic intrusions. ‘garbenschiefer amphibolite’ comprising ‘units up to more than 1 km broad that form approx. 25 of the Isua supracrustal belt’ and ultramafic rocks Fig. 3a. The ‘garben- schiefer amphibolite’ was described as being char- acterised by ‘well-developed garbenschiefer texture of amphiboles on its foliation surfaces’, and as being ‘‘slightly discordant to lithological layering in adjacent rocks’’. This unit was inter- preted as a ‘gabbroic, possibly sill-like intru- sions’. The ultramafic rocks were described as being located ‘at most stratigraphic levels in the Isua supracrustal belt’ and as being ‘slightly dis- cordant to layering of the supracrustal sequences’. They were interpreted as being derived from in- trusions of mainly dunite and peridotite. 2 . 3 . Reappraisal of the Nutman stratigraphy Doubt was cast over the geological map and stratigraphy of Nutman 1986 by Rose et al. 1996, following a detailed study of the metacar- bonate rocks. These authors concluded that the calc-silicate formation was metasomatic in origin. Further reappraisal of the supracrustal rocks by Rosing et al. 1996 started to demolish the rest of this stratigraphic edifice. The carbonate-rich gar- net – hornblende – biotite schists that are the main component of the ‘variegated schist formation’ were found to be gradational into amphibolite, and to have similar, basaltic, rare earth element content. These schists were reinterpreted as meta- somatic alteration products of amphibolite, rather than being of sedimentary origin. Likewise grada- tions were described between leucocratic schists, mainly quartz – feldspar – muscovite – biotite – car- bonate rocks, that comprise the ‘felsic formation’, and discordant sheets of tonalitic and granitic gneiss. Rosing et al. 1996 therefore concluded that the ‘felsic formation’ was derived by metaso- matic alteration of intrusive tonalite and granite, rather than being a sequence of metamorphosed felsic volcanic and epiclastic rocks. In contrast, part of the ‘garbenschiefer amphibolite’ unit was found to contain components of sedimentary origin that passed gradually into the dominant garbenschiefer-textured magnesian schists. This unit was reinterpreted as of volcano-sedimentary, chiefly basaltic, origin and an integral part of the stratigraphy, rather than being an intrusion, re- cently described as metagabbro by Nutman 1997. A consequence of this reappraisal of the stratig- raphy was that the simple isoclinal syncline pro- posed as the dominant structure of the greenstone belt by Nutman et al. 1984 and Nutman et al. 1996, Nutman 1997 was also unfounded. Ros- ing et al. 1996 considered that the dominant protoliths of the greenstone belt were basalt and banded iron formation, intruded by dunitic sills. They recognised that the sequence was deformed and metamorphosed, including being sliced up by faults, before being intruded by tonalite and gran- ite, followed by further metasomatism, deforma- tion and metamorphism. Rosing et al. 1996 concluded that ‘pervasive carbonation and K metasomatism produced a sequence of lithologies, mimicking those found in modern platform de- posits. However, the protoliths could have origi- nated in a purely oceanic environment with no sialic detrital components’.

3. Major components of the Isua greenstone belt and their protoliths