Page 71 - Microtectonics

3.12 · Deformation of Some Rock-Forming Minerals 59
At high-grade conditions (above 600 °C), dislocation 3.12.5
climb and recovery are relatively easy in feldspar and real sub- Micas
grain structures form (Vidal et al. 1980; Olsen and Kohlstedt
1985; Pryer 1993; Kruse and Stünitz 1999; Altenberger and Micas deform mainly by slip on either (001)<110> or
Wilhelm 2000). Both SGR and BLG recrystallisation occur (001)[100], and therefore show abundant evidence for ac-
(Fig. 5.12). Core-and-mantle structures still occur, but the commodation mechanisms such as pressure solution and
boundary between the core and the mantle is less pronounced fracturing (Kronenberg et al. 1990; Shea and Kronenberg
than at lower temperature. Myrmekite along foliation planes 1992; Mares and Kronenberg 1993), undulose extinction,
is abundant. At low and intermediate pressure, feldspar kinking and folding (Wilson 1980; Lister and Snoke 1984;
grains are strain-free, with isolated micro-kink bands while Bell et al. 1986b). Folds and kinks are particularly common
flame-perthite is absent. Fracturing of grains can still be in mica; commonly, folding occurs on the outside and pres-
common (Berger and Stünitz 1996; Kruse et al. 2001). At sure solution or kinking in the core of a folded crystal. Frac-
high-pressure conditions, Altenberger and Wilhelm (2000) tures are commonly associated with deflection of basal
report microfractures, kinkbands, deformation bands, planes and lead to barrel or fish-shaped boudinaged grains
undulose extinction and flame perthite in K-feldspar and (Sect. 5.6.7). Grain boundary migration recrystallisation
recrystallisation by SGR, or by BLG at high strain rate. becomes important at medium to high grade (Bell 1998).
At ultra high-grade conditions (>850 °C), GBM recrys- In the brittle domain, biotite may show crude kinking or
tallisation has been reported for plagioclase in the pres- layer parallel slip to develop ‘cleavage steps’ or mica fish
ence of a melt phase (Lafrance et al. 1996, 1998; Rosenberg (Sects. 5.6.7, 5.7.3; Kanaori et al. 1991). Biotite behaves
and Stünitz 2003), indicated by strain-free grains with in- ductilely at temperatures above 250 °C (Stesky et al. 1974;
terlobate grain boundaries and left-over grains (Fig. 3.34). Stesky 1978). Muscovite is generally more resistant to de-
However, compositional effects are again very important formation than biotite and therefore commonly forms mica
for such microstructures (Rosenberg and Stünitz 2003). fish in mylonite (Sect. 5.6.7).
Several dislocation slip systems can be active in feld-
spars, especially at high temperature. In plagioclase, slip 3.12.6
on (010)[001] and (001)<110> seems to dominate at me- Olivine
dium to high-grade metamorphic conditions (Olsen and
Kohlstedt 1984, 1985; Montardi and Mainprice 1987; Kruhl Different slip systems operate in olivine at different tempera-
1987a; Ji et al. 1988; Ji and Mainprice 1990; Kruse and tures in the mantle (Nicolas and Christensen 1987; Main-
Stünitz 1999; Heidelbach et al. 2000). Slip on {001}<100>, price and Nicolas 1989; Suhr 1993). At ‘low’ temperature
(010)[100] and {111}<110> is reported as well (Montardi (700–1000 °C), slip systems (010)[001] (Nicolas and Chris-
and Mainprice 1987; Ji and Mainprice 1988; Dornbush ten-sen 1987) or {110}[001] (Carter and Avé Lallemant 1970)
et al. 1994; Ullemeyer et al. 1994; Marshall and McLaren have been reported, and additional slip on several planes that
1977a,b; Olsen and Kohlstedt 1984, 1985; Ji and Mainprice intersect along the [100] direction. The latter is called pencil
1987, 1988, 1990 and Stünitz et al. 2003). For K-feldspar, glide on (0kl)[100]. Old grains of olivine show strong undulose
activity of (010)[100] has also been reported by Gandais extinction and subgrain boundaries. Olivine recrystallises to
and Willaime (1984). At high-grade metamorphic condi- fine-grained crystals that are concentrated in shear zones by
tions, diffusion creep may be important in feldspar de- flow partitioning (Suhr 1993). At medium temperature around
formation (Gower and Simpson 1992; Selverstone 1993; 1000 °C, pencil glide on (0kl)[100] is dominant. At high tem-
Martelat et al. 1999). A deformation mechanism map for perature (T > 1000°C), only (010)[100] dominates and at
feldspar was constructed by Rybacki and Dresen (2004). very high temperature (T > 1250°C), (010)[100] is dominant
The limited number of active slip systems in feldspars and (001)[100] may be active (Nicolas and Christensen 1987;
leads to dynamic recrystallisation and core-and-mantle Mainprice and Nicolas 1989). A polygonal granoblastic fab-
structures. At low temperature, BLG recrystallisation may ric of coarse-grained, strain-free olivine develops. A strong
nucleate on small brittle fragments in crush zones (Stünitz lattice preferred orientation of olivine and trails or bands
et al. 2003). Two types of mantled porphyroclasts of other minerals in olivine might be the only indication
(Sects. 5.6.5, 5.6.6) may develop in plagioclase at high tem- that the rock was deformed. The relatively coarse grain size
perature: relatively little deformed ‘globular’ porphyro- of olivine (0.4–1 mm; Suhr 1993) corresponds to low flow
clasts, similar to those at low temperature, which have stresses in the mantle at these levels (Sect. 9.6.2).
(010)[001] slip systems in an unfavourable orientation for Besides temperature, water may influence slip system
slip, and ribbon plagioclase grains which were in a favour- activity in olivine (Jung and Karato 2001). At high water
able orientation for slip on (010)[001] (Ji and Mainprice content [001] slip may become predominant over [100] slip
1990; Kruse et al. 2001; Brodie 1998; Olesen 1998; Box 4.2). in olivine. Therefore, a LPO with an [001] maximum paral-
Deformation twins, undulose extinction and deformation lel to the stretching lineation could be related to hydration
bands are common in such ribbons. rather than low temperature (Jung and Karato 2001).