Page 70 - Microtectonics

58 3 · Deformation Mechanisms
Dolomite behaves differently from calcite (Barber and and matrix, or core-and-mantle structures are absent. BLG
Wenk 2001). It deforms by basal <a> slip at low to mod- recrystallisation may occur (Shigematsu 1999). Flame-
erate temperatures and deformation twinning on f-planes perthite (Sect. 9.5.4), a perthite with tapering ‘flame-shaped’
at moderate to high temperatures. Twinning apparently albite lamellae may be present in K-feldspar, especially at
does not develop below 300 °C, in contrast to calcite, which grain boundaries and high stress sites (Figs. 3.37, 7.28; Spry
can even twin at room temperature. Notice that twinning 1969; Augustithitis 1973; Debat et al. 1978; Passchier 1982a;
occurs on different planes in calcite and dolomite. At low- Pryer 1993; Pryer and Robin 1995). Such perthite is thought
grade conditions, dolomite is usually stronger than cal- to develop by albite replacement of K-feldspar driven by
cite, which causes commonly observed boudinage of dolo- breakdown of plagioclase and sericite growth (Pryer and
mite layers in a calcite matrix. Robin 1995); replacement proceeds preferentially at sites
of intracrystalline deformation such as where two feld-
3.12.4 spar grains are touching (Passchier 1982a; Pryer and
Feldspars Robin 1996). ‘Bookshelf’ microfracturing in feldspar is
common at low-grade conditions, splitting the grains up
Deformation behaviour of plagioclase and K-feldspar is into elongate ‘book-shaped’ fragments (Passchier 1982a;
rather similar and therefore the feldspars are treated to- Pryer 1993; Sect. 5.6.12). Pryer (1993) claims that anti-
gether. Laboratory experiments and observation of natu- thetic fracture sets are more common in the low tempera-
rally deformed feldspar have shown that feldspar defor- ture range, and synthetic fractures at higher temperature.
mation is strongly dependent on metamorphic conditions. At medium-grade conditions (450–600 °C) dislocation
The behaviour as observed by several authors (Tullis and climb becomes possible in feldspars and recrystallisation
Yund 1980, 1985, 1987, 1991, 1998; Hanmer 1982; Tullis 1983; starts to be important, especially along the edge of feld-
Dell’Angelo and Tullis 1989; Tullis et al. 1990; Pryer 1993; spar grains. Recrystallisation is mainly BLG by nuclea-
Lafrance et al. 1996; Rybacki and Dresen 2000; Rosenberg tion and growth of new grains (cf. Borges and White 1980;
and Stünitz 2003) is described below, according to increas- Gapais 1989; Gates and Glover 1989; Tullis and Yund 1991).
ing temperature and decreasing strain rate. Indicated tem- This is visible in thin section by the development of man-
peratures are for average crustal strain rates. Notice, how- tles of fine-grained feldspar with a sharp boundary around
ever, that these temperatures are only valid in case of cores of old grains, without transitional zones with sub-
chemical equilibrium between old and new grains; if new grain structures; typical core-and-mantle structures de-
grains have another composition than old grains, e.g. more velop (Fig. 5.20) and micro-shear zones of recrystallised
albite rich, other temperatures will apply (Vernon 1975; grains may occur inside the feldspar cores (Passchier 1982a).
White 1975; Stünitz 1998; Rosenberg and Stünitz 2003). Fracturing in feldspar becomes less prominent under
At low metamorphic grade (below 400 °C) feldspar these conditions but microkinking is abundant, probably
deforms mainly by brittle fracturing and cataclastic flow. associated with cataclastic failure at sites of dislocation
Characteristic structures in the resulting cataclasite are tangles (Tullis and Yund 1987; Altenberger and Wilhelm
angular grain fragments with a wide range of grain size. 2000). If large kink-bands occur, they have unsharp
The grain fragments show strong intracrystalline defor- boundaries (Pryer 1993). Fine-grained recrystallised ma-
mation including grain scale faults and bent cleavage terial may resemble feldspar cataclasite described above,
planes and twins. Patchy undulose extinction and sub- but has a uniform grain size and polygonal grains. Grain
grains with vague boundaries are normally present. TEM boundary sliding has been proposed as a deformation
study of such structures has shown that they are not due mechanism in this fine-grained feldspar (Vernon and
to dislocation tangles or networks, but to very small-scale Flood 1987; Tullis et al. 1990), but this is difficult to assess
brittle fractures (Tullis and Yund 1987). In plagioclase, by optical means, and even by TEM. Optically, the only
deformation twinning on albite and pericline law planes useful criteria are lack of a lattice-preferred orientation and
is important (Seifert 1964; Vernon 1965; Borg and Heard unusual homogeneous mixing of feldspar grains and
1969, 1970; Lawrence 1970; Kronenberg and Shelton 1980; other minerals in the fine-grained aggregates. According
Passchier 1982a; Jensen and Starkey 1985; Smith and to Tullis et al. (1990), microscopic gouge zones can undergo
Brown 1988; Egydio-Silva and Mainprice 1999). Albite recrystallisation and develop into small ductile shear zones,
twins may form at the tips of microfaults and vice versa destroying most evidence for earlier brittle faulting.
(McLaren and Pryer 2001). Towards higher temperature, deformation twinning
At low-medium grade conditions (400–500 °C) feldspar is less abundant. Myrmekite growth becomes impor-
still deforms mainly by internal microfracturing but is tant along the boundaries of K-feldspar porphyroclasts
assisted by minor dislocation glide. Tapering deforma- (Sects. 5.6.9, 7.8.3). Myrmekite occurs mainly along crys-
tion twins, bent twins, undulose extinction, deformation tal faces parallel to the foliation (Simpson 1985; Simpson
bands and kink bands with sharp boundaries may be and Wintsch 1989). Flame-perthite is abundant in K-feld-
present (Pryer 1993; Ji 1998a,b). Clearly separable augen spar (Pryer 1993).