Microstructural stability and the kinetics of textural evolution

This thesis is concerned with constraining the processes by which a rock texture may be
altered in the geological history after a rock has been deformed and the rates at which
that microstructural modification may occur. The area which has been chosen to
investigate microstructural change is the Ballachulish aureole in the NW Highlands of
Scotland, using samples from the Appin Quartzite. Rocks in this region were deformed
during the Caledonian orogeny. Post-dating the deformation was the intrusion of the
Ballachulish granite. As a consequence of the thermal perturbation associated with the
granite intrusion, deformation microstructures were gradually altered. This process is
known as annealing. The Ballachulish aureole is essentially a ‘natural laboratory’ which
preserves an annealing experiment over geological time; rocks are gradually annealed
along the traverse towards the intrusive complex where higher temperatures were
attained.
Rocks have been sampled along a traverse from outside the igneous aureole towards the
contact. A variety of microstructural elements have been characterised using image
analysis and crystallographic techniques, some of which have been developed during this
study. The changes in rock fabrics along the traverse are used as a basis for developing
models of annealing for many microstructural features, including dislocations, subgrains,
subgrain walls and grains. The relative stability of each of these microstructures is
assessed to show that dislocations are the least stable, then subgrains and grains. An
interesting ‘mesh’ microstructure is identified, preserving evidence of fracturing on a
grain scale. These mesh-grains are very stable, surviving annealing in some cases
altogether. The presence of impurities on all scales of observation are shown to greatly
influence the mechanisms of microstructural change.
The kinetics of microstructural modification are examined using theoretical models based
on the processes of annealing which have been identified during this investigation. Again
these indicate that impurities can have marked implications on the rates of
microstructural change. Kinetic models for rocks undergoing regional prograde and
retrograde metamorphosis are developed to indicate how this work may be applied on a
more regional tectonic scale.