Geology Reference
In-Depth Information
layers, and is termed
rhythmic layering
. These were originally described from
the Skaergaard intrusion, which is characterised by the occurrence of modally
graded units, 5 - 50 cm in thickness. Finally, many cumulate rocks are also
associated with coarse-grained segregations, such as
pegmatite
(for example,
Section 2.5.2, Figure 2.15) which are potentially important as they can contain
minerals that can be used to date the intrusions. Later minor intrusions, such as
dykes and sills (Chapter 6), often cross cut layered intrusions and are useful in
determining the relative ages of intrusion where clear cross cutting relationships
can be established (for example, Chapter 6, Figure 6.7).
On a broad scale, in most of the large-scale intrusions, the proportion of
felsic material increases upwards, in some cases giving way to granitic rocks
in the uppermost layers. Individual layers vary upwards from a few centimetres
(most typical) to metres and perhaps tens of metres (exceptionally in the upper
regions of an intrusion where the contrast between layers tends to be much
less than near the base).
Accurate observations of band orientations, widths and
their variation normal to the layering therefore provide important criteria for
later interpretation of fieldwork
.
Other mafic-ultramafic dykes, sills and plutons of all sizes occur in the con-
tinental crust. They may have been emplaced tectonically during metamorphism
and thrusting, in which case they will be associated with sheared, deformed and
high-grade metamorphic rocks. More simply, they may have been intruded as
part of a larger igneous province (for example, the Cuillin gabbros of Skye). Var-
ious combinations of the mafic-ultramafic rock-types described earlier are known:
some intrusions are homogeneous gabbros, peridotites or even dunites, others
show igneous lamination, and many are complex, multiple intrusions with map-
pable internal boundaries. Small intrusions of dunite are sometimes found in high
grade gneiss zones where they may be only a few tens of metres to a few kilome-
tres in size, elongated parallel to the surrounding foliation and may, themselves,
show syn-tectonic folding and foliation. They may be extensively serpentinised
(Figure 8.2), particularly around their margins and, as well as serpentine, may
also contain a range of secondary minerals such as chlorite, talc and magnesite.
Finally, it should be noted that the coarse-grained micaceous peridotites,
known as kimberlites, that occur in deeply-eroded volcanic diatremes are clas-
sified as ultramafic intrusive rocks. These are considered in detail in Chapter 6.
8.3 Ophiolite Complexes
Many tectonically-emplaced mafic-ultramafic bodies have an origin as slices
of oceanic lithosphere. Such bodies are termed
ophiolites
and are preserved
as a result of thrusting and/or uplift associated with plate tectonic processes
(cf. Chapter 1). Since these ophiolite complexes have assumed great impor-
tance in interpreting the tectonic history of a region, it is useful to explain
