Geoscience Reference
In-Depth Information
'Enclaves'
Sometimes within an igneous rock you may fi nd patches (or
'enclaves') with a different texture or mineralogy, but which are
not greatly coarser (in fact they may be fi ner) and so are not
pegmatite. These enclaves may be so obviously different (and
large enough) that you can spot them from a distance. Examine
them carefully in order to decide between competing
possibilities that could explain them.
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•
Xenoliths?
These would be fragments of country rock that
were engulfed in the igneous magma (e.g. by stoping: Figure
7.2) and transported within it. If so, they may not be
igneous in origin. The most exotic xenoliths come from the
mantle. Edges of xenoliths tend to be well-defi ned, and
their shape can be angular. At its smallest, a xenolith may
consist of no more than a single exotic crystal, in which
case it is known as a xenocryst. Beware of misidentifying
an aggregation of phenocrysts (known as a 'glomerocryst')
as a xenolith.
•
Magma of different composition that has somehow
avoided total assimilation into the main magma?
These,
sometimes referred to as autoliths, may be blobs that were
injected into the main host rock while it was still partly
molten (Figure 7.22). They tend to have a rounded shape,
and their edges may be diffuse. This is really just a small-
scale variety of the incomplete magma mixing described in
Section 7.2.2.
Figure 7.22
Autolith, 60 cm
long, in the Shap Granite,
Cumbria, UK. On close
examination, it turns out that the
autolith is not mafi c (as would be
implied by its colour). It is a
microgranite, with a higher
proportion of biotite than the
coarse-grained granite that
encloses it. (David A. Rothery, The
Open University, UK.)
