Page 61 - Microtectonics

3.9 · Competing Processes During Deformation 49
Fig. 3.35.
Pinning structure of quartz
grain boundaries on white mica.
Amphibolite facies micaceous
quartzite. Southern Minas Gerais,
Brazil. Width of view 2.5 mm. CPL

Fig. 3.36. a Low magnification TEM image of equidimensional grains in quartz ultramylonite. Note occurrence of voids (marked V) at triple
points and grain boundary micas (marked M); b TEM image of free dislocations, visible as dark lines, inside small quartz grains from an
ultramylonite. The homogeneous distribution of recovered dislocations indicates that dislocation creep was a significant deformation mecha-
nism, but dominant grain boundary sliding is suggested by a random LPO in the quartz grains, measured by electron diffraction, and the
voids at the grain boundaries. Quartz ultramylonite, Portugal. (Photographs courtesy Martyn Drury)

of recovery, and finally to combined GBM and SGR recrys- indirectly, on deformation parameters such as strain rate
tallisation because of increasing ease of diffusion in the crys- and temperature. In general, a high temperature and the
tal lattice (Hirth and Tullis 1992). The same seems to apply presence of a fluid on grain boundaries promotes recovery
to other minerals (Lafrance et al. 1996) but switches in ac- and recrystallisation processes; high strain rate enhances
commodation mechanism will occur at other temperatures. crystal distortion. These facts have been known from the
At high temperatures, diffusion processes may accom- earliest age of metalworking; a sword or horseshoe can be
pany or take over from dislocation climb and recrystallisa- shaped from a piece of metal by hammering if it is suffi-
tion (Sect. 3.8). ciently heated. In thin section, only structures related to the
The state of affairs during any stage of deformation and last stages of the competing processes are normally pre-
the final result that we observe in deformed rocks depend served, formed shortly before temperature and/or strain rate
on the relative importance of the processes listed above and, fell below a critical value and the structures were ‘frozen in’.