Page 98 - Microtectonics

86 4 · Foliations, Lineations and Lattice Preferred Orientation
4.2.7.6 existing minerals inheriting their shape (Fig. 4.27a); they
Static Recrystallisation and Mimetic Growth may have nucleated and grown within a fabric with strong
preferred orientation, following to some extent this ori-
Foliations can be modified in several ways after defor- entation (Figs. 4.16(7), 4.27b); or they may have grown
mation ceases. If low-grade foliation is subjected to con- along layers rich in components necessary for their
siderable temperature increase in the absence of defor- growth, in this way mimicking the layered structure in
mation, as in contact aureoles, the strength of this folia- their shape fabric (Sect. 7.3; Fig. 4.27c). Some mono-
tion normally decreases due to nucleation and growth of crystalline ribbons may develop in this way. Mimetic
new minerals over the foliated fabric in random orienta- growth is probably an important process in the later stages
tion, changing a foliated rock into a hornfels. Limited heat- of foliation development, especially at medium to high-
ing, however, without a change in mineral paragenesis, grade metamorphic conditions. Since micas grow fastest
can also strengthen a foliation by growth of micas that in the (001) direction, grain growth catalysed by reduc-
are approximately parallel to the foliation and preferred tion of interfacial grain energy can lead to strengthening
dissolution of grains in unfavourable orientations (Ho of an existing preferred orientation (Figs. 4.16(5), 4.28,
et al. 2001). The latter probably occurs because of stored ×Photo 4.28; Etheridge et al. 1974; Ishii 1988). Crenula-
strain energy in grains with (001) planes oblique to the tion cleavage may be progressively destroyed by this proc-
original shortening direction. ess transforming itself into an irregular schistosity
In some rocks, elongate crystals that help define a sec- (Fig. 4.28, ×Photo 4.28). Partly recrystallised relicts of
ondary foliation may actually have grown in the direc- crenulation cleavage microfolds as in Fig. 4.28c are known
tion of the foliation after the deformation phase respon- as polygonal arcs.
sible for that foliation ceased. This process is known as An effect similar to mimetic growth is growth of nor-
mimetic growth. The elongate crystals may have replaced mally equidimensional minerals such as quartz or cal-
cite between micas or other elongate crystals with a pre-
Box 4.4 Mylonitic foliation and monocrystalline ribbons ferred orientation (Fig. 4.16(8)). Due to restriction in their
growth direction imposed by the micas, such grains may
A foliation in mylonite is usually referred to as mylonitic fo- obtain an elongate shape that strengthens the pre-exist-
liation; it is generally a spaced foliation composed of alter-
nating layers and lenses with different mineral composition ing foliation.
or grain size, in which more or less strongly deformed por-
phyroclasts are embedded; the mylonitic foliation wraps 4.2.7.7
around these porphyroclasts (Sect. 5.3). Some lenses are sin- Oriented Growth in a Differential Stress Field
gle crystals with an unusual planar or linear shape that de-
fine or strengthen a foliation in the rock. Such lenses are
known as monocrystalline ribbons (Sect. 5.3.5). Common ex- The possibility of oriented nucleation and growth of
amples are quartz ribbons, but ribbons of mica, feldspar and metamorphic minerals in a differential stress field
orthopyroxene are also known (Sect. 3.12). In low to medium- (Fig. 4.16(6)) was suggested by Kamb (1959) and is ther-
grade mylonites, quartz ribbons are strongly elongate and modynamically possible; it may produce a strong pre-
show strong undulose extinction, deformation lamellae, sub- ferred orientation of both shape and crystal habit with-
grain structures and dynamic recrystallisation, mainly along
the rim of the ribbons. Commonly, such ribbons show extinc- out necessarily being associated with high strain. How-
tion banding parallel to their long axis, which may be due to ever, rocks subject to high differential stress are usually
folding of the crystal lattice (Boullier and Bouchez 1978; deformed, and it is difficult to prove that a mineral-pre-
Passchier 1982a). Most ribbons probably form by extreme flat- ferred orientation did not develop by one of the proc-
tening and/or stretching of large single crystals. esses outlined above. Some well developed schistosities
In high-grade gneiss, quartz ribbons consist of single crys-
tals with an elongate shape, which lack intracrystalline defor- in medium to high-grade rocks with undeformed crys-
mation structures (Figs. 5.11, 5.12). Such monocrystalline tal habit and straight grain boundaries may be a result
quartz ribbons are also known as platy quartz (Behr 1965; of this process, but static recrystallisation and mimetic
Frejvald 1970; Boullier and Bouchez 1978) and commonly in- growth of grains which obtained their preferred orien-
clude equidimensional or elongate feldspar grains. The quartz tation by rotation may form a similar fabric (Fig. 4.28).
may contain rutile needles that have a preferred orientation
or show boudinage, indicating that these ribbons have been
subject to strong deformation. Monocrystalline quartz rib- 4.2.7.8
bons in high-grade gneiss are probably formed by strong de- Microfolding
formation followed by recovery and significant grain bound-
ary migration that removed most older grain boundaries and If an older planar fabric is present in the rock, the associ-
intracrystalline deformation structures (Sect. 3.12.2). In this ated mechanical anisotropy may give rise to a harmonic,
case, static recrystallisation leads to elongate single crystals
of quartz because other minerals hamper grain growth in di- regularly spaced folding which produces some of the most
rections normal to the ribbons. intriguing structures in rocks, crenulation cleavage. The
limbs of the folds may line up to form a crude foliation,