Geology Reference
In-Depth Information
microgranitic globule
nebulous granitic patch
Figure 9.13
Scale section cut through contact between microdioritic enclave
and host granite, Ross of Mull, Scotland. The enclave has been 'contaminated'
during emplacement into the unconsolidated granitic magma by mechanical
incorporation of alkali feldspar and other granitic material.
9.8 Summary
Enclaves are a common feature of plutonic rocks and occur to some degree in
most igneous intrusions irrespective of composition. Large screens of country
rock are well preserved in mafic intrusions (for example, Skeargaard), and
magma mixing and mingling are described also from alkali igneous complexes
and in carbonatite magmas. However, they are often most spectacularly dis-
played in granitic plutons and for this reason we have focused attention primarily
on these.
Microgranular enclaves are common features of calc-alkaline granitic plutons
that form during incomplete mixing of mafic to intermediate melts intruded into
cooler granitic magma. The rheological behaviour of magmas of different chem-
ical composition during interaction is governed by their physical properties (den-
sity and viscosity), temperature difference and strain rate. Factors such as degree
of host and enclave crystallinity exert a significant affect on magma rheology and
in particular their ability to mix together to form a hydrid composition (magma
mixing) or retain their integrity as discrete blobs or enclaves (magma mingling).
