3. Stratigraphic dilemma
One of the most significant problems in inter- preting the supracrustal succession in the Isua
Greenstone Belt has been the failure to erect a meaningful stratigraphy based on properly inter-
preted protoliths Nutman et al., 1984; Rosing et al., 1996; Appel et al., 1998. Lithologies that
comprise the IGB have been severely deformed, and in many cases also profoundly metasoma-
tized, so that current appearance, mineralogy, and geochemistry rarely resemble the protolith. Al-
though most rocks are highly schistose, locally abundant minerals in the succession, such as
quartz and dolomite, have been dynamically re- crystallized so that internal mineralogic strain is
not congruent with the overall strain state of the rocks Bridgwater et al., 1981.
In one case within the study area, a widespread greenschist unit termed the ‘garbenschiefer amphi-
bolite’ has long been considered an igneous intru- sion Allaart, 1976; Gill et al., 1981; Nutman et
al., 1984; Nutman, 1997, despite the fact that pillow textures had been recognized in the 1970s,
and now are observed throughout the unit occur- ring in a variety of morphologies Rosing and
Rose, 1993; Komiya and Maruyama, 1995; Ros- ing et al., 1996; Appel et al., 1998. Pillow basalt
thus forms a significant and integral part of the stratigraphic development of the IGB, especially
in the domain that contains supposed conglomer- ates. By contrast, Dimroth 1982, who envisioned
that the entirety of the succession at Isua was sedimentary, proposed that much of the iron for-
mation and interlayered quartzite with dolomite represented original primary limestone of shallow
subtidal origin replaced by silica. However, nearly all of the structures seen in these rocks
represent tectonic features, such as boudins, and carbonate forms a major replacement mineral in
many lithologies Rose et al., 1996. Given such examples, it is not surprising that much about the
stratigraphy and surficial conditions during the time of deposition remains highly speculative.
Nutman et al. 1984 and Nutman 1986 erected a stratigraphic hierarchy for the IGB
based on a number of transects perpendicular to foliation, including the area shown in Fig. 1. A
premise of that hierarchy is that the stratigraphy was folded into an upright isoclinal syncline early
in the deformation history. New mapping indi- cates that no such structure exists, nor are litholo-
gies repeated on opposite sides of the belt as would be expected on opposing fold limbs [also
noted by Rosing et al. 1996]. The cornerstone for building a ‘new’ stratigraphy in this part of
the IGB has been the recognition of multiple ductile faults that divide the supracrustal rocks
into tectonic domains whose internal stratigraphic sections are independent of each other and most
likely formed at different times, Fig. 1following on the observations of Rosing et al. 1996, I
propose that the lithologic units and stratigraphic terminology outlined by Nutman et al. 1984 be
entirely abandoned because it no longer accu- rately describes rocks of the IGB. Rocks previ-
ously classified as ‘flat pebble conglomerate structure’ and ‘round pebble conglomerate struc-
ture’ crop out within the central domain of the study area Fig. 1, whereas the ‘conglomeratic
structure in the felsic formation of sequence A’ A6 unit described above is now mapped as a
mylonitized,
boudinaged, and
metasomatized Amıˆtsoq gneiss sheet. Although younging direc-
tion cannot be determined, primary stratigraphic units comprising the central domain are roughly
orthogonal to the main foliation, which strikes NE and dips steeply to the SE Fig. 1; Appel et
al., 1998.
4. Meta-conglomerate of the Central Domain
Kategori