Geology Reference
In-Depth Information
(a) (b) (c)
Figure 6.10
(a) Concentration of phenocrysts in the centre of a dyke as a result
of flowage differentiation. (b) Vesicle deformation with flow (cf. Figure 6.4). (c)
A composite dyke with an aphyric margin sharply separated from a porphyritic
centre.
Besides the variations described above, sills, in particular, show other types of
mineralogical variation as a result of their horizontal tabular form. These include
variation as a result of crystal settling (cf. Chapters 2 and 8) which occurs to a
noticeable extent within some thick sills, generally exceeding 50 m in thickness.
Such sills may have a layer or layers rich in dense minerals such as olivine,
pyroxene and Fe-Ti oxides near to the base (cf. Chapter 8). These layers overlie the
fine-grained lower marginal layer and may be complemented by layers relatively
poor in these minerals towards the top of the sill, below the upper chilled margin.
However, it may be extremely difficult to distinguish between such mineralogical
differentiation of a single magma, and that resulting from multiple or composite
intrusion. For example, fine-grained olivine and/or pyroxene-rich dolerite may
intrude aphyric dolerite. Distinction of such origins may require detailed chemical
and mineralogical study of the sill. In other examples, the intrusion of two or
more distinct magmas can result in quenched (blocky) textures of the hotter
mafic magma against the cooler felsic (for example, Figure 6.11). Sometimes
one finds that a new generation of magma, perhaps due to renewed magmatism
at the site of an old intrusion, has cut slightly older but related igneous rocks.
A particular example of this might be where a granitic magma has invaded a
rectangular joint set in a previous mafic (that is, generally dark-coloured) igneous
body. This is known as
net-veining
. As mafic magmas generally crystallise at
higher temperatures than felsic magmas, a stage in the crystallisation of a mafic
magma body may be reached at which it causes local melting of the host rocks to
produce granitic liquid. This liquid may then penetrate joints within the cooling
mafic body and its surrounding, a process known as
back-veining
.
As a result of fractional crystallisation and volatile concentration during crys-
tallisation, many sills are characterised by segregations of coarser grained and/or
more felsic material in the upper part. These include sharp-sided veins, irregular
