of lithological similarity and strong deformation it was not realised until the late 1980’s that they
belong to two age groups; previously all or- thogneisses were thought to represent reworked
Archaean rocks e.g. Hickman and Glassley, 1984. The presence of juvenile Palaeoproterozoic
rocks was documented by Kalsbeek et al. 1984, 1987 with the help of isotope data, but even then
it was not possible to distinguish Archaean and Palaeoproterozoic rocks in the field, and chemi-
cally the dated Archaean and Proterozoic rocks appeared to be very similar Kalsbeek et al.,
1987.
Recently, parts of the Nagssugtoqidian orogen have been re-investigated by the Danish Litho-
sphere Centre Marker et al., 1995; van Gool et al., 1996; Mengel et al., 1998. To get an insight
into the regional distribution of Archaean and Palaeoproterozoic rocks, zircons from a large
number of samples, scattered over the whole oro- gen, were analysed in a geochronological recon-
naissance programme Kalsbeek and Nutman, 1996, and in nearly all cases a distinction could
be made between reworked Archaean and Palaeoproterozoic rocks. The objective of this
work is in more detail to compare the geochem- istry of dated Archaean and Palaeoproterozoic
meta-igneous rocks from a part of the orogen where they occur in close spatial association, and
to see to what extent the chemical distinctions described by Martin 1986 can be recognised for
these rocks.
2. Nagssugtoqidian orogen and regional setting of the investigated rocks
The Nagssugtoqidian orogen of West Green- land Fig. 1; Ramberg, 1949; Escher et al., 1976;
Korstga˚rd, 1979; Marker et al., 1995; van Gool et al., 1996; Mengel et al., 1998 is a 250 km-wide
ENE-trending belt north of the ‘Archaean craton’ of southern Greenland, within which Archaean
rocks
were strongly
reworked during
the Palaeoproterozoic, 1850 – 1750 Ma ago. The
orogen is believed to be part of a major orogenic belt, running from Canada the Torngat orogen,
over West and East Greenland, to northern Scot- land and the northern part of the Baltic Shield
e.g. Bridgwater et al., 1990, but details of the correlation between these areas are still uncertain.
The central part of the Nagssugtoqidian orogen Fig. 1 is dominated by Archaean rocks in
Palaeoproterozoic granulite
facies. Juvenile
Palaeoproterozoic rocks are also present, al- though in much smaller proportions than re-
worked Archaean rocks. Palaeoproterozoic units comprise two major meta-igneous suites of similar
age, 1920 Ma: 1 the Sisimiut charnockite complex in the southwest; and 2 the Arfersiorfik
association in the northeast Fig. 1. Within the Sisimiut complex syenitic rocks with very high Ba,
Sr, LREE and P have been found, suggesting participation of a strongly enriched mantle source
in the petrogenesis of these rocks Steenfelt, 1997.
In those parts of the orogen where the rocks are in granulite facies, no lithological differences have
been found to distinguish between Archaean and Proterozoic units, but in the easternmost central
part of the orogen around the head of Nordre Strømfjord Fig. 2, the rocks are in amphibolite
facies, and original lithological differences are bet- ter
preserved. In
this area
Archaean and
Palaeoproterozoic rocks are in tectonic contact, commonly separated by thin slivers of strongly
deformed marble or calc-silicate rocks that pro- vided glide planes during tectonic imbrication,
and the Proterozoic rocks are interpreted as an allochthon Kalsbeek and Nutman, 1996; van
Gool et al., 1999. Palaeoproterozoic units com- prise both metasedimentary and meta-igneous
rocks. Detrital zircons in two samples of metased- iments are mainly of Palaeoproterozoic age
2200 – 2000 Ma, and indicate that the original sediments were not derived from the Archaean
complexes with which they are now in tectonic contact Nutman et al., 1999. In the area east of
inner Ussuit Fig. 2 one metasedimentary unit contains numerous lenses of komatiitic metavol-
canic rocks Kalsbeek and Manatschal, 1999.
The Archaean basement consists of polyphase gneiss complexes. Felsic grey orthogneisses, with
biotite as the main mafic mineral, are dominant; they contain local rafts of more mafic tonalitic to
dioritic hornblende-bearing rocks, with which they have locally intrusive relationships. Younger
granitic sheets are also present. A more detailed description of the Archaean basement, illustrated
with colour photographs, is presented by Mengel et al. 1998. Connelly and Mengel 1996, 2000
have obtained precise U-Pb ages between 2810 and 2870 Ma on igneous zircons from samples of
the regional felsic gneisses, in broad accordance with the reconnaissance data presented by Kals-
beek and Nutman 1996.
Palaeoproterozoic meta-igneous
lithologies comprise quartz-dioritic and tonalitic rocks, grad-
ing into more felsic varieties; locally they have been intruded into metasedimentary rocks. The
igneous rocks are collectively referred to as the Arfersiorfik association Kalsbeek and Nutman,
1996. Their age has been determined at 1920 – 1900 Ma, and they consist mainly of juvenile
components Kalsbeek et al., 1987; Whitehouse et al., 1998, although minor contamination with
older crustal components is suggested by isotope data. The largest outcrop of these rocks is the
Arfersiorfik quartz diorite Fig. 2; Henderson, 1969; Kalsbeek et al., 1987, within which igneous
textures and mineralogy as well as igneous layer- ing are locally preserved.
After initial imbrication Archaean and Protero- zoic rocks were complexly folded; sheets of
Palaeoproterozoic hornblende-biotite gneiss in eastern Nordre Strømfjord now occur as folded
layers, often only a few hundreds of metres wide, alternating with more felsic Archaean granitoids
Fig. 2; van Gool et al., 1999. From this area
Fig. 2. Geological map of the eastern Nordre Strømfjord – Arfersiorfik area outlined in Fig. 1 with sample localities modified after van Gool et al., 1999.
zircons from 22 samples for location see Fig. 2 were investigated by SHRIMP to determine their
age Kalsbeek and Nutman, 1996; some of the samples were collected in pairs, on both sides of a
thin marble layer or mylonite zone. Eleven sam- ples represent Archaean rocks; 10 were analysed
for this study one Archaean sample contained pegmatic veins and is not considered. Age esti-
mates for these samples Table 1 are unprecise because of the disturbance of the U-Pb zircon
systems during high-grade Proterozoic metamor- phism, together with the reconnaissance nature of
the age determinations. However, there is no doubt that the analysed samples represent Ar-
chaean rocks. The remaining 11 samples are Palaeoproterozoic; all of these were analysed.
Ages for these samples Table 2 are better con- strained and fall between 1900 and 1950 Ma.
While this approach using dated samples only limits the number of analyses in this study, mak-
ing detailed statistical comparisons impossible, classification of the samples as Archaean vs.
Proterozoic is unquestionable.
3. Sample description and classification
