INTRODUCTION AND OCCURRENCE - The Field Description of Igneous Rocks - page 9

bodies with dimensions measured in metres, to enormous masses hundreds of
kilometres in size, which are clearly distinct from smaller minor intrusions.
In studying minor intrusions care should be taken to trace contacts and mea-
sure stratigraphic sections. Most contacts are complex: minor intrusions and
plutons usually show steep contacts and, since the form and areal distribution of
such contacts are used to determine the form of the intrusion in three dimensions,
it is particularly important to determine the attitude and orientation of all planar
contacts in order to determine the overall geometry. For many minor intrusions
it should be possible to locate a contact area with sufficient three-dimensional
exposure to determine dip and strike.
1.3.4 Plutonic intrusions and igneous centres
At the larger end of the igneous spectrum we have the major igneous intrusions/
centres and plutons. Although there is a huge range of sizes and associations that
may be regarded as major plutonic intrusions, in this guide we will be grouping
the large plutonic intrusions into those which are mainly granitic in composition
(Granite complexes - Chapter 7) and those which are mainly mafic/ultra mafic
in composition (mafic complexes - Chapter 8). Additional information about
magma mixing/mingling which is also often found within the large plutonic
intrusions will be dealt with in Chapter 9.
Coarse-grained granitic plutonic rocks occur most frequently within
large, elongate belts of intrusions, 50 - 150 km in width and 500 - 1500 km
in length, that characterise eroded mountain belts. Such elongate plutonic
bodies are termed batholiths . They are usually composed of a large number
of cross-cutting smaller intrusions, including bodies 5 - 50 km in size with
circular outcrop patterns, ring like intrusions and irregular dykes and sills.
Circular intrusions with surface areas of less than 100 km 2 are sometimes
termed stocks . The features of the intrusions in an eroded batholith are shown
in Figure 1.5. This shows that the major contacts are outward dipping but
that where the intrusion has an irregular roof, exposure might leave irregular
downward projections of roof country rocks surrounded by igneous rock. These
are termed roof pendants and can be identified from the outcrop pattern as
shown on Figure 1.5. Although the subsurface form of plutonic intrusions may
be determined from the attitude of contacts, additional clues come from the
extent of the metamorphic aureole and from remote geophysical information
such as gravity and magnetics.
The major igneous centres, which are commonly linked as fossil magma
chambers to extinct volcanoes and isolated large igneous intrusions form a
subset of plutonic igneous rocks. In many instances they have a significant
mafic/ultra mafic compositional component, and can be characterised by multi-
ple injections/inflations of magma at a specific location in the Earth's crust (for
example, Rum and Skye igneous centres, Scotland - Figure 1.6) or in extreme