Page 68 - Microtectonics

56 3 · Deformation Mechanisms
3.11 3.11 by passive heating of a previously deformed material. The
Static Recrystallisation term is also sometimes used for the interpretation of
microstructures in rocks, e.g. in xenoliths (Vernon 1976;
When the deformation of a volume of rock decelerates or Shelley 1993). Occasionally, the term is used (incorrectly)
stops, the polycrystalline material will not be in a state of as a general synonym for static recrystallisation.
minimum internal free energy, not even if recovery and re- During static recrystallisation, unstable minerals are
crystallisation during deformation were important. Crys- replaced by stable ones, dislocation tangles are removed,
tals still contain dislocations, dislocation tangles and sub- grain boundaries become straight and grains tend to grow
grain boundaries. Grain boundaries have an irregular, wavy in size due to GBAR. Nevertheless, a grain aggregate usu-
shape, and some minerals may be unstable. If deformation ally retains cores of material in each grain, which have
was at relatively low temperature or if little free water was not been swept by grain boundaries (×Video 11.10d;
present in the rock, the deformed fabric may be preserved Jessell et al. 2003). Such cores can retain information on
relatively unaltered during subsequent uplift to the surface. the size, shape and chemical composition of the original
This situation allows geologists to observe structures di- grains, and may contain fluid and solid inclusions that
rectly associated with the deformation process. However, if predate static recrystallisation. Characteristic for unswept
temperature was relatively high when deformation stopped cores are sharp boundaries and an irregular shape, which
or if much water was present along grain boundaries, re- is not centred in all cases on the new grain boundaries
covery, recrystallisation and GBAR can continue in absence (Jessell et al. 2003). If dislocation density is high in an
of deformation towards a lower internal energy configura- aggregate and if the temperature is high enough, some
tion. This combined process is known as static recrystalli- grains may grow to a large size and commonly irregular
sation (Evans et al. 2001; Figs. 3.38, 3.41, 11.6, Box 3.10, shape at the expense of others (Fig. 4.9).
×Videos 11.6a,b, 11.10b–d). Dynamic and static recrystal-
3.12 lisation are also known as primary and secondary recrys- 3.12
tallisation, but these terms are not recommended since Deformation of Some Rock-Forming Minerals
they suggest an invariable sequence of events.
Static recrystallisation strongly modifies the geometry 3.12.1
of grain- and subgrain boundaries and can destroy a Introduction
shape-preferred orientation (×Video 11.10c) but can pre-
serve crystallographic preferred orientation (Sect. 4.4). In This section gives examples of specific deformation struc-
quartz, for example, the asymmetry of the crystallographic tures and deformation mechanisms in some common
fabric due to non-coaxial flow can be perfectly preserved rock-forming minerals. Criteria to recognise deformation
after static recrystallisation (Heilbronner and Tullis 2002), mechanisms in thin section are mentioned. Aspects which
allowing determination of shear sense for the last defor- deviate from the general trend as sketched above are
mation stage. stressed. Treatment is from low to high-grade metamor-
In metallurgy, the term annealing is used to indicate phic conditions unless stated otherwise. Most published
processes of recovery and static recrystallisation induced work concentrates on crystalplastic deformation, espe-
cially on dislocation creep and this section is therefore
Box 3.10 Evidence for static recrystallisation somewhat biased in this direction.
Evidence for static recrystallisation and its principal mecha- 3.12.2
nism, grain boundary area reduction (GBAR), is provided by
the presence of crystals with straight or smoothly curved grain Quartz
boundaries (Figs. 3.39, 11.6) which lack undulose extinction
or subgrains in a rock that was strongly deformed as shown by Although quartz is one of the most common minerals in
the presence of folds in layering, relict augen or the presence of the crust, its deformation behaviour is very incompletely
a strong lattice-preferred orientation. Such grains are said to understood. This is mainly due to the complex role that
be strain-free. In a statically recrystallised fabric it is commonly
possible to recognise relicts of a largely destroyed older struc- water plays in the deformation of quartz. The presence of
ture; relicts of a foliation or porphyroclasts may be preserved. water in the crystal lattice influences its strength (Kro-
Static grain growth is indicated by small grains of a second nenberg 1994; Luan and Paterson 1992; Gleason and Tullis
mineral with a preferred orientation that are included in grains 1995; Kohlstedt et al. 1995; Post et al. 1996). There are in-
of the main mineral (Fig. 4.9), and by elongate strain-free crys- dications that with increasing water pressure in the pore
tals that define a foliation; these may have grown in a rock with
an older foliation where they were hampered in their growth space dislocation creep strength of quartz decreases, prob-
by grains of a second mineral (Figs. 3.41, 5.11, 5.12). Static re- ably through an increase in water fugacity in the quartz
crystallisation may be followed once more by deformation in- grains (Luan and Paterson 1992; Post et al. 1996).
ducing undulose extinction and dynamic recrystallisation, At very low-grade conditions (below 300 °C) brittle
starting a new cycle. fracturing, pressure solution and solution transfer of