Page 134 - Microtectonics

5.3 · Mylonite 123
1. A decrease in grain size, which enhances activity of 5.3.5
grain size-dependent deformation mechanisms such Mylonite Development
as diffusion creep and grain boundary sliding at Different Metamorphic Conditions
(Sect. 3.9; Fig. 3.43, ×Video 11.10a; Allison et al. 1979;
White et al. 1980; Schmid et al. 1977; Behrmann and Although the fabric of mylonites is strongly dependent
Mainprice 1987; Fliervoet et al. 1997; Ji et al. 2004). This on the lithotype and original structure of the rock in
decrease in grain size is caused by the fact that the which it develops, a general fabric gradient exists for all
size of new grains formed by dynamic recrystallisa- rock types with increasing metamorphic grade, depend-
tion is a function of differential stress (Sect. 9.6.2). ing on the rheology and melt temperature of constituent
However, de Bresser et al. (1998) suggest that this minerals (e.g. structures in granite mylonite formed at
mechanism may not be very efficient. 400 °C may resemble those in peridotite mylonite formed
2. GBM recrystallisation, which replaces hardened crys- at 800 °C). As an example, consider the effect of meta-
tals by new, easily deformable crystals without dislo- morphic grade on mylonitisation of a bimineralic rock
cation tangles (Fig. 3.26a). Notice that SGR recrystal- with a mineral A that is ‘hard’ and a mineral B that is
lisation (Fig. 3.26b) will not lead directly to softening ‘soft’ at low-grade conditions due to a different number
since new grains have the same dislocation density as of active slip systems with different critical resolved shear
the old ones (Tullis et al. 1990). stress (Sect. 2.3.4; compare feldspar-quartz aggregates in
3. Growth of new minerals, which are more easily de- Sect. 3.13.2).
formable than minerals of the host rock (reaction sof- At very low grade, A and B deform by brittle fractur-
tening; Mitra 1978; White et al. 1980; Hippertt and ing and a brittle fault rock forms.
Hongn 1998).The replacement of feldspars by aggre- At low-grade conditions A deforms in a brittle man-
gates of white mica and quartz is an example. ner and B by dislocation creep (Handy et al. 1999). Dif-
4. Transformation of large grains of the host rock to new ferential stresses are high (Figs. 3.42, 5.2) and mylonites
phases in a fine-grained aggregate such as in symplec- are therefore fine-grained with fragmented, angular por-
tite formation. Such a newly formed aggregate of min- phyroclasts of A embedded in ductilely deformed grains
erals may be softer that the original grains not be- of B that wrap around the porphyroclasts. Foliations and
cause its individual phases are more easily deform- lineations are usually well developed. Mylonite zones tend
able than the old grains, but because it is more fine to be narrow with sharp boundaries.
grained, and therefore favours another deformation At medium grade, A and B both deform by crystal-
mechanism (Furusho and Kanagawa 1999; Kruse and plastic processes, but A is still stronger than B. As a re-
Stünitz 1999; Newman et al. 1999). An example is the sult, well-developed mylonites form with a mylonitic fo-
transformation of large K-feldspar grains to myrmekite liation containing fragments of partly recrystallised por-
(Tsurumi et al. 2003). phyroclasts of A. Most of the shear sense indicators men-
5. Development of a lattice-preferred orientation of min- tioned in Sect. 5.6 may be recognised in mylonites formed
eral grains which places them in a position for easy under such conditions. Foliations and lineations are well
dislocation glide (geometric softening; Ji et al. 2004). developed.
6. Enhanced pressure solution due to decrease in grain size At high grade, shear zones tend to be wider than at
and opening of voids and cracks (Rutter 1976; Stel 1981). lower grade, since softening and localisation mechanisms
7. ‘Hydrolytic’ weakening of minerals due to diffusion of are less efficient than at lower metamorphic grade (Han-
water into the lattice (Sect. 3.12.2; Luan and Paterson mer et al. 1995; Whitmeyer and Simpson 2003). Under
1992; Kronenberg 1994; Post and Tullis 1998). Quartz these conditions, the difference in rheology between A
at high-grade metamorphic conditions contains little and B decreases, diffusion becomes more important and
intragranular water and is relatively strong (e.g. Naka- differential stresses are low (Figs. 3.42, 5.2). At low strain
shima et al. 1995). If such dry quartz is brought to rate, the result can be a layered rock with few porphyro-
amphibolite facies conditions and subject to water in clasts and a relatively coarse grain size. Grains in the
the pore fluid, it may be weakened rapidly by infiltra- matrix may have a reticular shape. Except from the com-
tion of water into the lattice, probably through crystal positional layering, foliations and lineations tend to be
defects (Kronenberg et al. 1990; Post et al. 1996; Post weakly developed. The rock may appear to be weakly
and Tullis 1998). Under greenschist facies conditions, deformed, but isoclinal folds in layering may show the
however, water infiltration into the quartz lattice is slow intensity of strain. Such high-grade mylonites may be rec-
and may only affect quartz rheology if grain size is ognised by elongate recrystallised ribbons of B (Box 4.4)
small, or if aided by fracturing of the grains or by grain and by few large porphyroclasts of A (e.g. Fig. 5.12), which
boundary migration (Post and Tullis 1998). are usually symmetric. They are known as ribbon mylo-
8. Development of shear bands or shear band cleavage nite (McLelland 1984; Hanmer et al. 1995; Hippertt et al.
(Ji et al. 2004). 2001) or, if quartzo-feldspathic and relatively coarse