Geology Reference
In-Depth Information
or may create new sets with different orientations. Joints in minor intrusions
and extrusive igneous bodies that have not been buried significantly and have
not experienced deformation or faulting will usually relate simply to the tension
created by cooling and contraction (see Chapter 4 for details).
Faults
are surfaces, sometimes utilising pre-existing joints, upon which rel-
ative motion of opposite sides has occurred, and may not be easily identified in
homogeneous igneous rocks where the two displaced sides cannot be 'matched'.
However, the presence of
slickensides
(grooves or striations on the sides of
a fault, parallel to the direction of movement) and of crushed or fragmented
brecciated rock along the fault should be noted, as both are evidence of fault-
ing. Such brecciation may result in formation of a cemented
fault-breccia
and
in some cases, especially where softer rocks are involved, the rocks may be
crushed to a soft, uncemented clay-like material, called a
fault-gouge
.
If deformation occurs when the igneous rocks are still hot or partially molten
it can result in
shear zones
. These are characterised by deformation structures
that are typical of ductile deformation. In such zones a careful investigation of
the fabrics present can help to elucidate the direction of motion through the use
of
shear sense indicators
(see the mapping of geological structures by McClay
in this topic series for detailed examples of shear indicators). Contacts between
major batches of magma in multiple intrusions may be characterised by sheared
contacts, also the deformation of igneous rocks in deeper crustal settings will
result in such ductile deformation, for example in subduction zone settings and
collision zones.
2.5.2 Veins, aplites and pegmatites
Veins are sheet-like, or tabular bodies that occur within both igneous and country
rocks. The formation of veins and associated structures are usually due to late
stages in magmatic activity, the cooling of magma bodies and the development
of hydrothermal systems associated with igneous rocks. Here we shall briefly
look at the main examples that we find in igneous terrains.
2.5.2.1 Hydrothermal veins
Hydrothermal veins
are commonly white and usually of quartz or calcite. They
vary in width from a few millimetres to a metre or more and are very common in
igneous terrains. The mineral constituents may be distinguished as quartz or cal-
cite using the hardness and cleavage tests described later (Chapter 3). Such vein
materials may be precipitated either in pre-existing joints or in newly developed
joints or faults. If the igneous host is cold, the veins will tend to be straight sided,
whereas reaction between vein material and hot, recently-crystallised igneous
rocks results in veins with irregular and diffuse edges. Occasionally, complex
veins may be found. These contain suites of different minerals arranged in bands
with a definite sequence from the walls to the centre of the vein. The variations
