Geology of the Skaergaard Intrusion


The Skaergaard Intrusion is the type example of a layered intrusion and displays a wide diversity of structures. The intrusion displays a succession of rocks evolving from troctolitic and gabbroic compositions to ferrodiorites with minor amounts of anorthosite and granophyre. The rocks show systematic trends in mineral assemblages and compositions consistent with the undisturbed crystallisation from a cooling magma in a closed system. For a scientific discipline that relies on many parameters that cannot be directly measured, the observed variations are remarkably consistent with experimentally derived theories on crystallisation and magmatic differentiation.

Cumulates and the evolution of layered intrusions

Layered intrusions are plutonic complexes that show internal stratification, and they are believed to represent the crystalline products of fractional crystallisation. The intrusions show concentrations of minerals implying that mechanisms of mineral sorting operated during their crystallisation. The resulting rocks have textures indicative of crystal sorting

Regional tectonic and magmatic framework

On a regional scale, the Skaergaard intrusion is related to the magmatic province associated with the mantle hot-spot that is presently located beneath Iceland. This hot-spot is believed to be related to a narrow zone of upwelling of mantle material below in what is called "a mantle plume". The zone results in an anomalously high production of basaltic magma. The products are exposed in the extensive flood-basalts on West and East Greenland, the Faeroe Islands, the Vøring Plateau off Norway, and the basalts and intrusive centres in NW Scotland and Northern Ireland. The accumulation of basalt has generated an extensive ridge of material extending from the British Isles via Iceland to East Greenland.

Geological setting

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), and a succession of at least 6.5 km of Palaeogene (Palaeocene to Eocene) flood basalts (to the east and south).

General geology of the Skaergaard intrusion

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) occupies the outermost parts along the walls of the intrusion; the Layered series (LS) accumulated on the magma chamber floor; and the Upper Border series (UBS) crystallised and accumulated against the magma chamber roof.

The Marginal Border Series

The Marginal Border series (MBS) forms the outermost units of the Skaergaard intrusion towards its host rocks. It is believed to have formed from material that became frozen to the magma chamber walls during crystallisation. It's thickness varies according to the structural height in the intrusion with the most extensive exposures found towards the position where the LS is in contact with the UBS. The MBS shows a transitional zone to the LS which displays abundant unconformities and magmatically healed normal faults. Good exposures can be found along the shoreline of Uttental Plateau, on the southern edge of Kraemer Ø, and on the small islands of Mellemø and Ivnarmiut.

The Layered Series

The Layered series (LS) is the most voluminous of the primary Skaergaard subdivisions and dominates the exposed parts of the intrusion. It is believed to have formed by crystallisation and crystal accumulation on the magma chamber floor. The LS displays a multitude of magmatic structures related to the cumulus and postcumulus stages of the evolution. The magmatic layering, which is the most prominent feature of the LS, displays strong similarities to sedimentary layering and defines the stratigraphy of the series. A suite of included blocks (autoliths and xenoliths) fallen from the roof of the intrusion during crystallisation locally disturb and disrupt the layering with abundant impact structures. Locally the layering is disrupted by unconformities or displays evidence of slumping and redeposition. In places, the layering is transgressed by mafic pegmatites and pods and streaks of anorthosite that apparently postdate its deposition. These structures appear to have partially replaced the original cumulates. Good exposures of the LS are found on Uttental Plateau, on Kraemer Island, and on the plateau to the west of Basistoppen.

The Upper Border Series

The Upper Border Series is exposed towards the south-east of the intrusion. It occurs mostly in an area with inaccessible mountainous terrain, but a few exposures can be examined along the shore of Skaergaardsbugt.

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