The Layered Series
Introduction
The Layered series (LS) is the most voluminous of the primary Skaergaard
subdivisions and dominates the exposed parts of the intrusion. It 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.
Subdivision and nomenclature
The LS forms a stratigraphic cumulate succession of around 2500 metres
developed on the magma chamber floor. It evolves systematically from high-temperature
cumulates at it's base towards progressively lower temperature cumulates
towards it's top. The uppermost stratigraphic level where the LS meets
the Upper Border series is known as the Sandwich horizon, which is believed
to represent the level where the last residual melt crystallised.
[more]
The LS is subdivided according to changes in the cumulus mineral assemblage,
which represents the liquidus assemblages of the Skaergaard magma. As such,
it reflects directly the evolution of the Skaergaard magma. The primary
subdivision of the LS relates to the presence of olivine in the cumulates.
The Lower zone (LZ) has olivine as a fractionating phase, the Middle zone
(MZ) has no olivine but locally pigeonite, and the Upper Zone (UZ) is marked
by the return of olivine.
Stratigraphic relations
Layering is developed throughout most parts of the LS from the deepest
exposed levels up to the base of UZc. Several distinct styles can be recognised
that obviously formed by distinctly different processes. Where some types
are clearly distinguishable by differences in their style and occurrence,
others appear to be variations of one of the major types. Commonly distinct
types of layering are superimposed on the cumulates. Common for all styles
of layering is their orientation parallel to the assumed crystallisation
front. The development of layering appears largely to depend on the shape
of the magma chamber and the cumulus mineral assemblage, and consequently
there are stratigraphic variations in the different major types throughout
the LS. Layering is developed on three distinctly different scales that
also appear to have different origins.
Rhythmic layering consists of modally graded planar sheets of
material (layers) of typically 5-50 cm thickness interbedded with variable
amounts of modally uniform (isomodal) gabbro. The layers display strong
density stratification and a weaker grain size stratification in their
mineral constituents consistent with gravity sorting in suspension currents
(Irvine, 1987; Irvine et al., 1998). The individual layers have iron-titanium
oxides at their bases followed upwards by olivine, pyroxenes, and finally
plagioclase towards their tops. Individual layers can be followed for a
maximum of some 300 m along strike after which they taper out and end with
slightly upturned edges in ridges of isomodal gabbro (fig. ). Both the
upper and lower boundaries of the individual layers are sharp to within
millimetres. The rhythmic layering displays abundant structures around
included blocks that resemble structures found in clastic sedimentary rocks
(described below).
Microrhytmic layering defines a modal stratification on the scale
of the crystal size (centimetre scale) within the cumulates. The layering
appears mostly as a banding of cumulus mineral crystals within isomodal
or modally graded rhythmic layers. The layering is mostly faint, but prominent
examples have been found in LZa (McBirney and Noyes, 1979) and LZb (fig.
XX) on Uttental Plateau, in MZ below Pukugaqryggen, and in UZb below Basistoppen.
The microrhythmic layering is somewhat similar to the inch-scale layering
in the Stillwater complex (Hess, 1960). For the Stillwater complex, it
has been suggested that this type of layering formed during postcumulus
coarsening by a process similar to Ostwald ripening (Boudreau, 1987).
Macrorhythmic layering consists of stratigraphic units of typically
0.5 – 5 m thickness. The layers have broadly isomodal compositions and
are distinguished by different proportions of cumulus minerals. The layers
are prominently developed in two successions, the first in the upper half
of the LZb, the other in the upper half of the MZ (the so called zebra
banding of Wager and Deer, 1939). Individual macrorhythmic layers can be
traced for more than a kilometre along strike in the cliff face of Pukugaqryggen,
their upper and lower contacts are gradational over decimetres, and the
layering relations are easiest to observe from a distance. The individual
macrorhythmic layers display no evidence of gravity sorting, but include
frequent gravity stratified rhythmic layers. However, the relations of
macrorhythmic layers around large blocks are similar to those of the rhythmic
layers (depressed below, lapping up against the sides, and get streamlined
over the top of large blocks) strongly suggesting that they too were produced
during the primary cumulus stage of crystallisation (Irvine et al., 1998).
The layers are sometimes compared with the megacyclic units of the Eastern
Layered Series on the Isle of Rum, but appear to have closer resemblance
to the kakortokite layering in the Ilimaussaq intrusion. No detailed studies
have been carried out to clarify the origin of the macrorhythmic layering,
but their lateral extent suggest that they formed in response to changes
in the precipitation of cumulus crystals. Their origin must be related
to fluctuations in the cotectic proportions of cumulus minerals – unless
a vary large scale process of crystal sorting was active. The layers have
been attributed to rhythmic crystal nucleation (Maaløe, 1978), differences
in the convective pattern of the magma (Naslund et al., 1991), and repeated
gravitational collapse of crystal loaded suspensions from below the magma
chamber roof (Brandeis and Jaupart, personal communication cited by Irvine,
1987).
The Triple Group is a prominent succession of very large scale
layering occupying the uppermost 100 m of the MZ. The group terminates
the succession of macrorhythmic layering in the upper half of the MZ, but
has a much larger scale and more extreme modal variations. Furthermore,
the individual layers can be recognised across the entire exposure of the
LS, and diamond drilling has confirmed that they extend below the surface
across the entire southern part of the intrusion. The succession has recently
attracted attention because it hosts a potentially economic occurrence
of gold and palladium (the Platinova Reefs, described below). The Triple
Group members appear to consist of pairs of lower leucocratic and upper
melanocratic units, each unit being some 5 – 15 m thick. The members are
separated by variable amounts of mesocratic gabbro. A close examination
revealed that the Triple Group members all host modally graded rhythmic
layers, and that the interbedded mesogabbro displays macrorhythmic layering
on a 5 m scale (Andersen, 1996).
Included blocks
The LS includes numerous blocks and fragments of material that appear to
be of exotic origin. Abundant blocks of anortosite, gabbroic troctolite,
and anorthositic gabbro are spread from the LZa to UZb and appear to be
cognate xenoliths fallen from the UBS during crystallisation. The stoping
appears to have been very intense in periods, where the blocks make up
locally half of the exposure. On the whole, it has been estimated that
some 15% of the entire MZ is made up of exotic material (McBirney, 1989).
A few hornfelsed metabasaltic inclusions are exposed along the south coast
of Kraemer Ø demonstrating that at that particular stage, the stoping
had locally scaled away the entire roof section over that area.
The autolithic inclusions are distributed in large swarms with typically
several tens or hundreds of blocks of various sizes and shapes. The swarms
are stratigraphically controlled and occur at distinct stratigraphic levels
in the cumulates. The lowermost swarm is exposed at the boundary between
the LZa and LZb on Uttental Plateau, extensive swarms occur in the middle
of LZb, in LZc, and throughout the lower half of the MZ. In the MZ, the
stoping activity ceases, but solitary blocks are found scattered up until
the base of the UZb. Apparently the blocks were dislodged from the UBS
at distinct times when the roof of the intrusion became unstable. This
could for example have occurred as a consequence of earthquake activity,
but it is likely that the incorporation of mafic material would add enough
weight to periodically destabilise the roof.
The blocks display a variety of sizes, shapes, and internal structures.
The smallest fragments are only a few centimetres across and sometimes
appear just as clusters of a few mineral grains barely distinguishable
from crystals in their matrix material. Small blocks are commonly distributed
in conglomerate beds (fragmental layers) that locally dominate the magmatic
stratigraphy. The largest blocks are up to ½ kilometre across and
display internal structures such as modal or textural layering, pegmatites,
and even in some places smaller included blocks. In shape, the blocks range
from angular boulders with straight, clear-cut boundaries to their host
cumulates to rounded blocks that appear to have plastically deformed during
impact. Larger blocks are mostly elongate slab-like rafts whereas smaller
blocks may me more equilateral. Internal structures are abundant within
the blocks. Larger blocks display well developed modal or textural layering;
some have smaller blocks included; some have dikes that do not extend into
the surrounding cumulates; some display hydrothermally altered fractures;
and pegmatites and replacement anorthosites are widely developed.
Structures associated with included blocks
The layering is commonly greatly affected by the presence of included blocks
and displays abundant structures relating to their occurrence. It has been
claimed (McBirney, 1989) that the layering is generally best developed
in successions with included blocks, and indeed the many structures associated
with the blocks are central for the investigation of the physical conditions
during crystallisation.
Layers below blocks are generally depressed The abundance of small
blocks in some layers
Structures associated with block impact
Block interactions with layering
Impact structures
Slumping
Replacement anorthosite
The LS locally displays transgressive bodies of anorthosite and leucocratic
gabbro (commonly known as replacement anorthosites) that appear to have
partially replaced the primary cumulates (cf., McBirney and Sonnenthal,
1990; Irvine et al., 1998). These replacement anorthosites are particularly
abundant in the LZa and LZb on Uttental Plateau, where they form small
local patches in the cumulates or up to 20 meter sized transgressive or
diapiric bodies. Locally, the anorthosites appear to spread laterally with
dendritic growth patterns along primary layering planes, and in places
they include residual in-situ structures from their protolith (such as
layering). The replacement anorthosites are commonly associated with mafic
bodies that appear to be most commonly situated at their bases.
The replacement anorthosite bodies can be explained in the context
of melting in a crystalline material fluxed by water (Yoder, 1970). Fluxing
with hydrous fluids in the system diopside-anorthite-SiO2 results
in a shift in the eutectic composition towards anorthite. Partial melting
produces liquids rich in plagioclase, and the association between mafic
bases and the anorthosites material can possibly be considered as zones
of dissolution and reprecipitation of plagioclase (Irvine et al., 1998).
Mafic pegmatite
Mafic pegmatite appears to be related to the replacement anorthosites but
have a much wider distribution in the LS. They still, however, only form
a quantitatively minor rock unit in the intrusion – amounting to less than
a percent of the exposure. The pegmatites form podiform bodies; stringers
and veins associated with the margins of included blocks; and semiconformable
sheets following layer boundaries. There seem to be a morphological evolution
of the pegmatites with stratigraphy in the LS, from dominantly podiform
bodies in the lower parts to more conformable sheets towards the top. Crystal
sizes are mostly around 10 cm, although in rare places they can exceed
½ meter. In composition the pegmatites are broadly gabbroic and
generally display lower temperature mineral compositions than their host
cumulates. The pegmatites are commonly strongly fractionated with plagioclase-rich
margins, hydrous gabbroic zones, and granophyric cores; and commonly they
are roofed by gabbro showing variable degrees of anorthositic replacement
(Larsen and Brooks, 1994).
Podiform and vein-type pegmatites are from a few centimetres to some
10 m across and can have sharp or diffuse boundaries towards their host
rocks. Some display well defined semi-circular bodies – in places having
smaller satellite bodies, others appear as diffuse sprays of coarse grained
material. Semiconformable pegmatites are typically 10-20 cm thick and several
tens to hundreds of meters long. They have generally sharp boundaries to
their host rocks. Some appear to have replaced the felsic parts of modally
graded layers, but others display evidence of having intruded with brittle
failure along the layering planes. Commonly these pegmatites jump between
adjacent layers having the occasional included blocks of their host material.
The pegmatites display equilibrium temperatures from 1006 to 766 ºC
consistent with a formation at the late magmatic postcumulus stage (Larsen
and Brooks, 1994), and fluid inclusions indicate the involvment of methane-bearing
saline hydrous fluids (Larsen et al., 1992).
© 2003 skaergaard.org