General geology

Introduction

Geological map of the Skaergaard intrusion

The Skaergaard intrusion formed 55.7 ± 0.3 Ma ago (⁴⁰Ar/³⁹Ar, Hirschmann et al., 1997) during the magmatic activity that makes up the Palaeogene North Atlantic Igneous Province. Precise dating suggests that it formed during a second phase of magmatic activity coincident with the continental breakup (Tegner et al., 1998). On that account it differs from most other intrusions along the coast that are considerably younger (notably the PGE-bearing Kap Edvard Holm and Kruuse Fjord complexes that have both been dated to 48 – 47 Ma, Tegner et al., 1998). The chilled margin composition indicates that the parental magma was an evolved high-titanium tholeiitic basalt (Hoover, 1989) similar to basalts found in the main East Greenland flood basalt succession (Larsen et al., 1989).

Emplacement and crystallisation

The Skaergaard intrusion was emplaced at the base of the East Greenland flood basalts, and is in contact with the Precambrian basement (pink), a thin succession of Cretaceous sediments (light yellow), and the Palaeogene basalts (light grey). The shape of the intrusion indicates that the emplacement was related to a N-S extensional regime with movement along normal faults and fault-block rotation. At present, the intrusion is rotated some 20-30º to the south. Consequently the deepest stratigraphic levels are exposed towards the north and the uppermost towards the south. After crystallisation, the intrusion was fractured and made way for the emplacement of the Vandfaldsdalen macrodike and the Basistoppen sheet (dark grey).

Internal structure of the intrusion

The Skaergaard intrusion was originally envisaged by Wager and Deer (1939) to be wedge-shaped (or funnel-shaped) extending to a considerable depth below the surface. This implied the existence of an extensive unexposed “Hidden Layered Series” below the present day erosion surface. This model was based on an extrapolation of the convergent orientations of the intrusive contacts combined with a reconstruction of the dips of the layering prior to coastal flexure. A gravimetric survey by Blank and Gettings (1973), however indicated that the intrusion only extends to a relatively shallow level in the crust, and consequently the model was revised by McBirney (1975). According to this (and later studies) almost the entire cumulate succession is exposed.

The Skaergaard intrusion was recognised by Wager and Deer (1939) to subdivide naturally into three lithological series that evolve along roughly similar fractionation trends towards a common centre where the last remaining liquid is believed to have crystallised. The Marginal Border series (MBS, green) occupies the outermost parts along the walls of the intrusion and builds a succession of up to 300 m of material frozen to the magma chamber wall; the Layered series (LS) builds a succession of more than 2500 m that accumulated on the magma chamber floor (here subdivided into lower [LZ], middle [MZ] and upper zones [UZ], blue colours); and the Upper Border series (UBS, green) – despite mostly eroded away – displays a succession of some 960 m of material that crystallised and accumulated against the magma chamber roof. The Platinova Au-Pd reefs are located towards the top of the MZ (dark yellow).