Page 72 - Microtectonics

60 3 · Deformation Mechanisms
3.12.7 Garnet, spinel, plagioclase, hornblende (Fig. 7.29) or
Orthopyroxene quartz can form exsolution lamellae in clinopyroxene.
Exsolution can occur parallel to (100) and (001), but at
In orthopyroxene, dislocation glide is dominant on (100)[001] temperatures above 700–750 °C only along (100).
(Coe and Kirby 1975; McLaren and Etheridge 1976; Mercier
1985; Dornbush et al. 1994). Other slip systems that have 3.12.9
been found are (100)[010] and (010)[001] (Nazé et al. 1987; Garnet
Dornbush et al. 1994). Optically visible subgrain bounda-
ries are usually parallel to (100), (010) and (001). Although garnet behaves as a rigid mineral at low grade
(100)[001] dislocations in orthopyroxene are usually split metamorphic conditions, several studies have presented
into partial dislocations, separated by a stacking fault along evidence for ductile deformation of garnets such as lattice
which the crystal lattice is transformed into that of a clino- bending (Dalziel and Bailey 1968; Ross 1973) and disloca-
pyroxene (Coe and Kirby 1975; McLaren and Etheridge tion substructures revealed by etching (Carstens 1969, 1971)
1976); as a consequence, exsolution lamellae of clinopyro- and electron microscope studies (Allen et al. 1987; Ando
xene can easily develop parallel to (100) and are therefore et al. 1993; Doukhan et al. 1994; Ji and Martignole 1994; Chen
common in deformed orthopyroxene (Suhr 1993). et al. 1996; Voegelé et al. 1998b; Kleinschrodt and McGrew
Under upper mantle conditions (up to 1000 °C) ortho- 2000; Prior et al. 2000, 2002). Elongate lensoid and folded
pyroxene may form ribbon grains with aspect ratios up to shapes of garnet crystals parallel to the deformation fabric
100:1 (Etheridge 1975; Nicolas and Poirier 1976; Mainprice (Kleinschrodt and Duyster 2002; Ji and Martignole 1994),
and Nicolas 1989; Suhr 1993; Ishii and Sawaguchi 2002; subgrain structures and a LPO are found in some garnets
Sawaguchi and Ishii 2003), or equidimensional porphyro- and can also be used as evidence for crystalplastic defor-
clasts if grains had an orientation that was unsuitable for mation (Prior et al. 2000, 2002; Kleinschrodt and McGrew
slip (Etchecopar and Vasseur 1987). The old grains may be 2000; Mainprice et al. 2004). The transition from brittle to
surrounded by a mantle of fine recrystallised orthopyrox- crystalplastic deformation seems to lie at 600–800 °C
ene. Ribbon grains probably form due to the dominant op- (Voegelé et al. 1998b; Wang and Ji 1999). At low and me-
eration of the (100)[001] slip system (Dornbush et al. 1994). dium grade conditions, garnet is much stronger than quartz
Garnet, spinel, plagioclase or quartz can form exsolution and feldspar and does not deform when isolated in a
lamellae in orthopyroxene. quartzo-feldspathic matrix. At higher temperatures, the dif-
ference in strength decreases to an extent that all three min-
3.12.8 erals can deform together (Ji and Martignole 1996; den Brok
Clinopyroxene and Kruhl 1996; Kleinschrodt and McGrew 2000). TEM stud-
ies give evidence for dislocation slip (Ando et al. 1993;
In clinopyroxene, the unit cell is half the length of that of Doukhan et al. 1994; Voegelé et al. 1998a,b; Ji et al. 2003).
orthopyroxene in the a-direction. Burgers vectors in that Since garnet has a cubic crystal structure, many slip sys-
direction are therefore shorter, and since the activation tems can theoretically be activated. Dislocation glide of
energy of a dislocation is proportional to the length of <100> dislocations in {011} and {010} planes and 1/2<111>
the Burgers vector, more active slip systems can be ex- dislocations that glide in {110}, {112} and {123} planes have
pected in clinopyroxene than in orthopyroxene. been observed (Voegelé et al. 1998a), providing 66 possible
At low temperature and/or high strain rate, deformation slip systems. Of these, slip on the 1/2<111>{110} system
occurs by (100) and (001) twinning in combination with seems to dominate. However, microstructures in garnet
(100)[001] slip, but in nature this is mainly restricted to me- which are interpreted as an effect of crystalplastic defor-
teorites due to the breakdown of clinopyroxene at low tem- mation may also have formed by other, so far little investi-
perature (Avé Lallemant 1978; Ashworth 1980, 1985). At high gated processes such as fracturing (Prior 1993; Austrheim
temperature (>500°C) and/or low strain rate multiple slip et al. 1996) multiple nucleation and growth (Spiess et al.
occurs, mainly on {110}1/2<110>, {110}[001] and (100)[001], 2001), and diffusion mechanisms (Ji and Martignole 1994,
and rarely on (010)[100] (van Roermund and Boland 1981; 1996; den Brok and Kruhl 1996; Wang and Ji 1999; Ji et al.
Phillipot and van Roermund 1992; van Roermund 1983; 2003). Ductile deformation of garnet can produce a lattice-
Buatier et al. 1991; Ingrin et al. 1991; Ratterron et al. 1994; preferred orientation but garnet seems to have weak pre-
Godard and van Roermund 1995; Bascou et al. 2001, 2002). ferred orientation in deformed rocks (Mainprice et al. 2004).
Dislocation creep may be assisted by diffusive mass transfer
and dynamic recrystallisation (Godard and van Roermund 3.12.10
1995; Mauler et al. 2000a,b; Bystricky and Mackwell 2001). Op- Amphiboles
tically visible subgrain boundaries are usually parallel to {110},
(100), (010) and (001). Clinopyroxene does not easily form The deformation behaviour of amphiboles is as yet poorly
ribbons such as orthopyroxene at high temperature. understood. In amphiboles, the crystal unit cell in the di-