Page 62 - Microtectonics

50 3 · Deformation Mechanisms

Box 3.8 Evidence for dynamic recrystallisation
Evidence for dynamic recrystallisation is usually more difficult grain boundaries can be seen to change laterally into grain
to find than evidence for deformation or recovery. Two types of boundaries. In the TEM, BLG recrystallisation is characterised
characteristic microstructures can be distinguished: partially by grains with a strongly variable dislocation density, while for
and completely recrystallised fabrics. SGR recrystallisation all grains have approximately similar dis-
In partially recrystallised fabrics a bimodal grain size distri- location density (Fig. 3.26; Tullis et al. 1990). A special lattice-
bution is characteristic, with aggregates of small new grains of preferred orientation may occur in recrystallised aggregates in
approximately uniform size between large old grains with the form of orientation families of grains, which may derive from
undulose extinction (Figs. 3.27–3.29, 3.37, ×Video 3.28a,b, large single parent grains that were completely substituted by
×Photo 3.23). The uniform size of new grains is due to defor- SGR recrystallisation (see also domain shape preferred orienta-
mation and recrystallisation at a specific differential stress tion, Box 4.2).
(Sect. 9.6.2). The three mechanisms of dynamic recrystallisation In the case of high-temperature grain boundary migration
can be distinguished as follows. (GBM) recrystallisation, the distinction between old and new
In the case of bulging (BLG) recrystallisation old grains can grains is difficult. Characteristic are large new grains with in-
have patchy undulose extinction, kinks, deformation lamellae terlobate to amoeboid grain boundaries, internally subdivided
and evidence for brittle fracturing while grain boundaries are in smaller subgrains (Figs. 3.32, 3.33). In quartz, chessboard-type
irregular and loboid with lobes on the scale of the new grains. subgrains are typical. Jessell (1987) proposed microstructures
New grains form at the expense of old grains along grain bounda- that can be used to recognise GBM and to establish the migra-
ries, and therefore form aggregates of small equally sized grains tion direction of a grain boundary. Grains of a second mineral
between the old grains (Figs. 3.27–3.29). such as micas can pin a grain boundary and cause ‘pinning’, ‘win-
In the case of subgrain rotation (SGR) recrystallisation the dow’ or ‘dragging’ microstructures (Fig. 3.34, ×Video 3.34a,b,c).
transition from old to new grain is less abrupt. Old grains are If a grain is almost completely replaced by a neighbour, ‘left-
flattened, show sweeping undulose extinction and contain sub- over’ grains with identical orientation may indicate the presence
grains the size of new grains, and gradual transitions in orien- of an originally larger grain (Urai 1983; Jessell 1986; Fig. 3.34,
tation from subgrains to new grains occur (Fig. 3.30, 3.31). Sub- ×Video 3.34d).

Fig. 3.37. Layer of fine-grained K-feldspar in quartz, both dynamically recrystallised. A perthitic fragment of a K-feldspar porphyroclast
with flame-shaped albite lamellae is present in the recrystallised feldspar layer. Notice the difference in grain size of recrystallised quartz
(coarse) and feldspar (fine). Granite mylonite. Qin Ling Mountains, China. Width of view 0.8 mm. CPL