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.
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.
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).
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 (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
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 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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