Page 74 - Microtectonics

62 3 · Deformation Mechanisms

Fig. 3.42. Changes in the deformation behaviour of quartz-feldspar aggregates with depth. At right, a depth-strength graph with brittle
(straight line) and ductile (curved line) segments of quartz and feldspar is shown. At very low grade, both quartz and feldspar are brittle, but
feldspar is the weaker mineral. At low to medium-grade conditions, quartz deforms by dislocation creep and feldspar is the stronger min-
eral, developing core-and-mantle structure and mantled porphyroclasts (Figs. 5.12, 5.22–5.25). At high grade, feldspar and quartz deform
by dislocation creep and have similar strength (Fig. 5.11)

Under low-grade conditions, feldspar is still brittle single crystals or crystal aggregates, but also by coales-
while quartz deforms ductilely by dislocation glide and cence of grains (Hippertt et al. 2001). Such high tempera-
creep (Fig. 3.42; Tullis and Yund 1977; Simpson 1985; ture ribbons tend to have grain and subgrain boundaries
Gapais 1989; Gates and Glover 1989; Fitz Gerald and oblique to the long axis of the ribbons. Both have sub-
Stünitz 1993; Stünitz and Fitz Gerald 1993). However, the grains in old grain cores and a gradual transition from
strength contrast is now reversed, and quartz is the weaker the core to a recrystallised mantle. Feldspar augen are rare.
mineral; feldspar porphyroclasts deform by fracturing Feldspar and quartz show similar deformation intensity
and may develop to core-and-mantle structures as a re- and seem to have a relatively small contrast in strength.
sult of neocrystallisation due to compositional disequi- At high-grade conditions, grain boundaries between
librium. Cores show abundant evidence of brittle faulting quartz and feldspars are commonly strongly curved, with
and patchy undulose extinction. Stretched mantled por- lobate and cuspate and even amoeboid shapes (Fig. 3.33;
phyroclasts may form elongated wings that eventually Passchier 1982a; Gower and Simpson 1992). This geom-
define a compositional layering (Fig. 3.42). Quartz aggre- etry may be due to deformation at high-grade conditions,
gates are elongate to ribbon-shaped and may consist of possibly with a large component of solid-state diffusive
tightly folded crystals which have recrystallised to some mass transfer such as Coble or Nabarro-Herring creep
extent (Passchier 1985; Hongn and Hippertt 2001). These (Gower and Simpson 1992).
low temperature ribbons tend to have grain boundaries One of the characteristic differences in behaviour of
and subgrain boundaries parallel to the long axis of the feldspar and quartz at low temperature and high strain
ribbons. They usually wrap around feldspar aggregates rate is the development of core-and-mantle structures in
and deform much more homogeneously; cores of old feldspar, and more homogeneous deformation in quartz.
quartz grains show abundant subgrains that laterally pass This has been explained by Tullis et al. (1990) as a result
into recrystallised (new) grains. At high strain, ‘augen’ of different deformation mechanisms of feldspar and
(German for eyes) of feldspar develop, separated by finely quartz at these conditions; in feldspar dislocation climb
laminated aggregates of fine-grained quartz and feldspar. is difficult and deformation occurs by BLG recrystallisa-
At medium to high-grade conditions, both feldspar and tion-accommodated dislocation creep (Dell’Angelo and
quartz deform by dislocation creep assisted by diffusion Tullis 1989). The newly produced grains of feldspar are
and recrystallisation. Both minerals may form monomin- free of dislocations and relatively soft, and grain bound-
eralic and polymineralic ribbons that give the rock a ary migration can easily replace them by new grains once
banded appearance (Culshaw and Fyson 1984; McLelland they develop dislocation tangles. As a result, the mantle
1984; Mackinnon et al. 1997; Hippertt et al. 2001; Box 4.2; of recrystallised feldspar grains surrounding feldspar
Fig. 3.42). These ribbon grains may form by stretching of cores is much softer than the core, and deformation is