Geology Reference
In-Depth Information
upper mafic margin
mafic
inclusions
flow banding
lower mafic margin
Figure 6.9
Diagrammatic section of a composite intrusion (grey
=
mafic,
pink
=
felsic).
example basalt or andesite inclusions within dacite or rhyolite, which indicate
magma mixing (see also Chapter 9).
In addition to compositional variation in the main rock-type(s) within minor
intrusions, variations in the type and distribution of phenocryst phases can be
important. For example, phenocrysts may be concentrated in the centre of a
minor intrusion which has fine-grained aphyric margins (Figure 6.10a), reflect-
ing flowage differentiation in which phenocrysts concentrated in the centre of
the intrusion reflect more rapid magma flow compared with that at the margins.
However, such cases might also reflect composite intrusion of aphyric and por-
phyritic magma (Figure 6.10c), so it is important to examine carefully the texture
of possible contact zones in such heterogeneous intrusions in order to distinguish
between these possibilities. Evidence for a sharp (or chilled) contact between the
inner (porphyritic) intrusion would favour interpretation by multiple intrusion,
although the magmas may have been related by fractional crystallisation below
the depth of emplacement. Where vesicles existed in shallow emplacement,
their deformation can also be used to help work out emplacement direction (for
example, Figure 6.10b, cf. Figure 6.4).
