Geology Reference
In-Depth Information
Where such contacts exist they can be complex if the earlier intrusion was
incompletely crystallised, as in this state the intermingling of two magmatic flu-
ids can result in magma mingling and where compositions are similar magma
mixing or hybridisation across a broad (perhaps tens of metres) contact zone.
Crystallised hybrid magmas may contain early-formed crystals derived from two
compositionally-different magmas, each of which may be available for individ-
ual study on either side of the hybrid zone. If the earlier intrusion was solid but
still warm enough to deform plastically, then diffuse contact zones, often with
interpenetrating or lobate structures, would be found. Also, inclusions of earlier
crystallised rocks within the later intrusion may provide clues about the state
of the earlier magma when the later intrusion was emplaced. These alternative
interpretations of compositional changes may be difficult to distinguish in the
field and, in many cases, may only be resolved by laboratory work (see also
Chapter 9).
7.4 Internal Structures and Textures
The mineral fabric (see also Chapter 2) and texture seen in granitic plutons can
be more complex than that of extrusive associations. Often it is irregular in
form, but regular linear or planar fabrics are often also well developed defined
by alignment of phenocrysts, megacrysts or enclaves (Figure 7.7).
Identification of fabrics in plutonic rocks is a
major goal
of any field study as
their measurement (both lineation and foliation) can provide useful information
on mode, style and shear-sense during emplacement. For example, some align-
ment fabrics may reflect exclusively internal processes such as convective flow,
layering or shearing during magma emplacement. This can result in localised
but sometimes spectacular structures including ladder dykes, cross-layering and
pipes (Figure 7.8).
7.4.1 Magmatic planar fabrics
Rotation and translation of crystals in cooling magma during emplacement either
by local flow or regional deformation can lead to the development of a
magmatic
planar
fabric. Identification in the field is an important but not always easy task.
This is because magmatic fabrics often develop locally as non-pervasive folia-
tions, either at the contact between different magma pulses, or in the vicinity of
enclave swarms (Figure 7.9). They may also result from wider-scale deformation
related to magma emplacement and in this case their orientations may corre-
spond to the regional structural grain. Magmatic fabrics are unique in that by
definition they form during crystallisation from a viscous fluid. However, their
presence means that magma can also behave as an elastic solid, even though it
may contain locally up to 50% melt. For this reason they are also referred to as
pre-full crystallisation fabrics
.
