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.