tions Allaart, 1976; Bridgwater et al., 1979; Dim- roth, 1982; Nutman et al., 1984. Despite the
apparent recognition of such coarse rocks, there consistently has been concern that some of them
could in fact represent severely tectonized rocks of uncertain protolith Bridgwater et al., 1979; Nut-
man et al., 1984, whose current outcrop appear- ance
merely resembles
metamorphosed ‘conglomerate’. Lack of consensus on this very
basic matter still exists, and leads to significant confusion when discussing and interpreting the
setting in which these historically problematic rocks were formed.
One problem in assessing possible origins for supposed clastic sedimentary rocks at Isua is the
occurrence of several types of coarse-grained rocks, especially those termed ‘conglomeratic
structures’ by Nutman et al. 1984, who recog- nized three specific types, namely 1 ‘flat pebble
conglomerate structure’; 2 ‘round pebble con- glomerate structure’ and 3 ‘conglomeratic struc-
ture in the felsic formation of sequence A’. The three different types of conglomerate are exposed
in ‘Sequence A’ Nutman et al., 1984; Nutman, 1986, which represents a poly-deformed and
lithologically diverse set of rocks that comprise most of the supracrustal succession of the IGB.
Based on new detailed stratigraphic by C.M. Fedo and regional structural by J.S. Myers
mapping in the main study area Fig. 1, it has become clear that the stratigraphy and protolith
lithologies as outlined by Nutman et al. 1984 needed significant revision Rosing et al., 1996;
Appel et al., 1998 in order to better interpret the paleogeography. Utilizing new data collected dur-
ing three field seasons 1997 – 1999, the purpose of this letter is to 1 re-examine the stratigraphic
setting; 2 critically evaluate whether or not coarse detrital sediments occur within the belt,
and 3 discuss the paleogeographic implications.
2. Geologic framework
Detailed outlines of the controversies and geo- logic events that have shaped the Early Archean
terrane that includes the IGB region have been presented elsewhere Nutman et al., 1996; White-
house et al., 1999. Tonalite – trondjhemite – gran- odiorite-suite rocks representing the Amıˆtsoq
gneisses intruded the terrane that includes the IGB between about 3650 Ma and \ 3800 Ma
Nutman et al., 1996; Whitehouse et al., 1999, providing an indirect constraint on the formation
age of the belt. Presently, the supracrustal succes- sionAmıˆtsoq gneiss contacts are highly tec-
tonized. The IGB, which forms an arcuate-shaped package
of rocks,
is the
largest of
many supracrustal enclaves that occur within the Amıˆt-
soq gneiss, and that are termed the Akilia associa- tion McGregor and Mason, 1977. Field relations
show that the Amıˆtsoq gneisses and the IGB were repeatedly deformed and regionally metamor-
phosed at amphibolite facies.
Attempts to date the stratigraphy directly have encountered problems concerning protoliths of
the sampled material and lack of proper target rocks to examine. Rocks belonging to the ‘A6
felsic unit’ of Nutman et al. 1984 have yielded consistent single crystal U-Pb zircon ages of
3810 Ma Compston et al., 1986; Nutman et al., 1997, although new mapping shows that the A6
unit is most likely a metasomatized sheet of Amıˆt- soq gneiss also see Nutman et al., 1996; Rosing et
al., 1996. Nutman et al. 1997 also reported a
3710 Ma single crystal U-Pb zircon age from quartzo-feldspathic schist unit B1, interpreted to
be of volcanic origin, west of the main NNE-strik- ing ductile structure that divides part of the IGB
stratigraphy Fig. 1. Kamber et al. 1998 sug- gested a source and depositional age in excess of
3.7 Ga for at least this part of the belt based on a Sm-Nd whole-rock regression age of 3742 9 49
Ma for a collection of 24 schist samples from B1. More recently, Frei et al. 1999 documented a
3691 9 22 Ma date using the PbSL method on magnetite, which was interpreted as the age of
metamorphic recrystallization. Collectively, these studies point to a depositional age in excess of 3.7
Ga.
The northeastern part of the Isua Greenstone Belt is divided into three tectonic panels north-
west, central, and southeast domains, Fig. 1; Ap- pel et al., 1998, each separated by a ductile fault.
Intense deformation of the NW and SE domains has produced linear belts of highly schistose
quartzo-feldspathic rock,
garnet amphibolite,
BIF, chert, and ultramafic rocks, whose primary depositional features have been mostly obliter-
ated. Although still very deformed, the central domain is somewhat less metamorphosed and in a
comparatively lower strain state relative to the NW and SE domains, such that primary features
are locally preserved Appel et al., 1998. All litho-types in the central domain show a promi-
nent foliation in association with a strong, steeply SE-plunging, stretching lineation that formed dur-
ing the last phases of deformation.
Fig. 1. Simplified geologic map of the northeastern part of the Isua Greenstone Belt. Strikedip and lineation measurements relate to the main schistosity folded by the regional synform. Ductile faults segment the area into the northwest NW, central CD, and
southeast SE domains. Outcrops of meta-conglomerate are found in the central domain. Heavy arrows indicate the locations of the eastern and western measured sections shown in Fig. 2.
3. Stratigraphic dilemma
