Geological setting of the Skaergaard Intrusion
Introduction
The Skaergaard intrusion occupies a stratigraphic position between the
Precambrian basement (to the north and west), a thin succession of Cretaceous
to Early Palaeocene Kangerdlugssuaq sediments (on Mellemø and at
the north-western corner of the intrusion), and a succession of at least
6.5 km of Palaeogene (Palaeocene to Eocene) flood basalts (to the east
and south).
The Precambrian basement
The Precambrian basement (fig. ) consists of intensely deformed migmatitic
orthogneisses, metapelites, and mafic-ultramafic rocks (Kays et al., 1989).
A radiometric age determination yielded a 207Pb/206Pb
single stage age of 2980±20 Ma for gneisses bordering the Skaergaard
intrusion (Leeman and Dasch, 1976). The gneisses mostly form a rugged terrain
of high ground (fig. ), but accessible exposures can be found along the
southern shore of Kraemer Ø at the contact to the Skaergaard intrusion,
and further west in Kraemer Bugt. The orthogneisses have typical tonalite-trondhjemite-gneiss
compositions ranging from diorites and tonalites to granodiorites and tronhjemites.
Two suites have been recognised of which the later shows clear intrusive
relations and contain xenoliths of older gneisses. The gneisses are hornblende
and biotite bearing. The metapelites include alternating bands of biotite-garnet
schist (with variable amounts of quartz, feldspar, sillimanite, and cordierite)
and quartzite (with garnet, hornblende, and orthopyroxene) indicating peak
metamorphic conditions in the upper amphibolite facies. The ultramafic
lithologies appear to be zoned bodies having peridotite cores and pyroxenite
and amphibolite margins. The ultramafic rocks are compositionally similar
to picritic basalts and komatiites. At least four phases of ductile deformation
have been recognised in the rocks with structures ranging from small-scale
folds and the development of foliation to large-scale regional folds. The
latest phase of deformation is cross-cut by later shear planes (all information
from Kays et al., 1989).
The Kangerdlugssuaq sediments
The Kangerdlugssuaq group sediments are exposed in a small outcrop on Mellemø
where it is in direct contact with the Skaergaard intrusion, and further
larger outcrops are accessible in Vandfaldsdalen and Sødalen (to
the west of the intrusion). The sediments rest unconformably on the basement
lithologies and infill a basin extending north-eastwards from the Skaergaard
area. Only a thin succession is exposed near the intrusion, but the sediments
increase in stratigraphic thickness to the Northeast – probably reaching
600-800 m further inland (Hamberg, 1990). The sediments start with fluvial
and estuarine sandy deposits towards the base and gradually evolve to marine
mudstones and turbidites upwards. Sedimentary structures indicate flow-directions
to the E and NE (Nørgaard-Pedersen, 1991; Hamberg, 1990).
The Palaeogene basalts
The sediments succession is unconformably truncated by pebbly arkosic sandstone
that forms the basal conglomerate for the overlying Blosseville group ([Higgins,
1981 #731; Brooks, 1982 #80]). The Blosseville group consists dominantly
of volcanic and volcanoclastic material with thin intercalated units of
sandy sediments towards its base ([Higgins, 1981 #731]). The lowermost
unit, the Vandfaldsdalen formation, consists of submarine picritic lava
flows, hyaloclastites, and pillow breccias.
Tectonic framework
The Palaeogene tectonics of the Kangerlussuaq area is dominated by the
emplacement of the Icelandic hot-spot and the subsequent continental rifting
that led to the opening of the North Atlantic Ocean. The rifting was probably
controlled by older structural lineaments in the area, which is located
at the souternmost expression of the Caledonian fold-belt in Greenland
(Wager and Hamilton, 1964). Sedimentary basins formed already in the Cretaceous
indicating that the arrival of the hot-spot was predated by an episode
of crustal extension.
The rifting was followed by an extensive structural collapse of the
continental margin. This resulted in the seaward rotation of basalts near
the coast (coastal flexure). The collapse was interpreted by Wager and
??? (19??) to be a monoclinal flexure.
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