Page 96 - Microtectonics

84 4 · Foliations, Lineations and Lattice Preferred Orientation
4.2.7.4 In the case of quartz and feldspars, recovery may lead to
Crystalplastic Deformation subdivision of equant grains into elongate subgrains
(e.g. Fig. 4.26). If further deformation leads to SGR re-
Mineral grains that deform by crystalplastic processes crystallisation (Sect. 3.7.3), the subgrains may become
such as dislocation creep, pressure solution and solution new independent grains that, by their shape, define a
transfer (Sects. 3.3, 3.4) or solid state diffusion (Sect. 3.8) foliation (subgrain shape preferred orientation; Box 4.2).
may obtain a flattened and/or elongate shape with maxi- Recrystallisation is associated with reequilibration of
mum extension along the XY-plane of finite strain known the chemical composition of minerals in the rock to
as a shape preferred orientation. This process is described metamorphic conditions during cleavage development
in detail in Box 4.2. (White and Knipe 1978; Gray 1981; Knipe 1981; White
and Johnston 1981; Ishii 1988; Williams et al. 2001). In
4.2.7.5 many cases the minerals in cleavage domains reflect
Dynamic Recrystallisation and the Orientation metamorphic conditions during cleavage development,
of New Grains and Subgrains and those in the microlithons older, even diagenetic con-
ditions (Knipe 1979, 1981; White and Johnston 1981; Lee
Dynamic recrystallisation (Sect. 3.7) and oriented new et al. 1984, 1986).
growth of, e.g. mica are important mechanisms of folia-
tion development (White and Johnston 1981; Ishii 1988;
Kanagawa 1991). Dynamic recrystallisation is driven by
the tendency to decrease free energy, such as stored strain
energy in deformed grains and interfacial free energy.
Kinking or tight folding of existing mica grains may ac-
cumulate sufficient strain energy to enhance bulging re-
crystallisation (Sect. 3.7.2). Little deformed fragments
of old mica grains or strain-free nuclei can grow into
the damaged crystal lattice with a preferred orientation
that contributes to the secondary foliation (Fig. 4.18b).

Fig. 4.18. Inferred range of stages in crenulation cleavage devel-
opment with increasing deformation (vertical axis) and tempera-
ture (horizontal axis; cf. Bell and Rubenach 1983). Figure 4.19
illustrates this same sequence with photographs. At low tempera-
ture (up to lower greenschist facies), a, the main mechanisms for
crenulation cleavage formation are thought to be differentiation
by solution transfer and rotation, whereas at higher temperatures, b,
recrystallisation and grain growth (including new minerals) are
probably dominant factors. At stage 1 gentle crenulations have
formed in the original foliation S , but no S cleavage is apparent
2
1
yet. Some recrystallisation may occur in D fold hinges. At stage 2
2
the crenulations are somewhat tighter and a discrete S 2 crenulation
cleavage is visible. S is still the dominant fabric. At stage 3 the
1
new cleavage has developed to such an extent that S and S are
1
2
of approximately equivalent importance in the rock. Recrystal-
lised microfolds known as polygonal arcs may be visible at the
higher temperature range, especially in b3. At stage 4 S clearly
2
predominates and S 1 is only recognisable in some relic fold hinges.
In stage b4 new grains grown along S dominate the fabric. Finally,
2
stage 5 shows the end product of the process where S is completely
1
transposed and not recognisable any more. Most rocks will fol-
low some path from the upper left to the lower right corner of the
diagram during development of a crenulation cleavage (compare
Fig. 4.28). Other factors that influence the development of crenu-
lation cleavage are the presence and activity of a fluid phase,
the presence of soluble minerals and the growth of new minerals.
The step to complete transposition at low temperature (a4–a5)
seems to be difficult without recrystallisation and grain growth.
This may be the reason that old foliations are often better preserved
in low-grade rocks