Geology Reference
In-Depth Information
to represent the residual melt of a crystallised intrusion, but, unlike aplites, the
silicate liquids that form pegmatites are often rich in water and other volatiles
(for example, fluorine and boron) and rare trace elements (hence the exotic
minerals noted). These volatiles are responsible for depressing the freezing tem-
perature of the pegmatite liquids, so allowing the characteristic large crystals to
form (these range from 1 cm to as much as several metres in length). Simple,
homogeneous igneous-textured pegmatites without mineral zonation are termed
simple pegmatites; those with mineral zonation along the length of the vein, and
often with exotic minerals, are complex pegmatites.
Hydrothermal quartz veins, aplites and pegmatites are commonly intimately
associated, sometimes occupying the same complex fracture with multiple infill-
ings and sometimes cutting across one another. They are found most frequently
in granite intrusions because the magmas that form these rocks are usually rich in
the volatiles that concentrate in residual liquids and enhance their mobility. It
should be noted that they can also be found in mafic intrusions, where they
have been used to find exotic magmatic crystals for dating (for example, badde-
leyite and zircon). It will be clear that both aplites and pegmatites may extend
beyond the boundaries of their host intrusion as veins cutting country rocks.
This means that the veins may be sheet-like intrusions which are discordant to
the pre-existing fabric of their surrounding rocks. Such intrusions are termed
dykes
and, where they are about a metre or so in width, they might be termed an
aplite dyke
or
pegmatite dyke
. (NB In cross-section, a dyke is a straight-sided
vein. The more common forms of dyke are described together with other minor
intrusions in Chapter 6.)
2.6 Outcrop Contact Relationships
Most intrusions emplaced into hot country rocks will show little sign of marginal
chilling, but the contact may have undergone plastic deformation, it may be
diffuse due to remelting across the boundary, or it may be disrupted if magma
has penetrated into thermally metamorphosed wall rocks. In such cases, the
material on either side of the contact will be crystalline, although the contact
may be recognised through contrasts of grain size, colour and mineralogy. In
examples where multiple intrusions have occurred, the lack of chilled margins
may result when later intrusions occur when the earlier ones are still hot.
At the opposite extreme, intrusions emplaced into cooler country rocks at
a high level within the crust frequently exhibit chilled margins (Figure 2.16)
of rapidly cooled magma, ranging from a few millimetres to a few metres
in width. The nature of the contact and the width of the chilled margin both
depend on the temperature difference between an intrusive magma and its host
which, in turn, reflects factors such as the original depth of magma emplacement
and the composition of the magma (since basic magmas crystallise at higher
temperatures than acid magmas).
