Page 103 - Microtectonics

4.2 · Foliations 91
Solution transfer plays probably also a major role in the less competent material. Although the initial shear bands
development of disjunctive cleavages that evolve by pre- may make an angle up to 60° with the XY-plane of finite
ferred dissolution along sets of parallel fractures; the frac- strain, they tend to rotate progressively towards this plane
tures may act as channelways for the fluids with enhanced during subsequent deformation, resulting in an anasto-
dissolution along them, causing accumulation of residual mosing network, roughly parallel to the XY-plane (Jor-
material that results in the formation of cleavage domains. dan 1987; Goodwin and Tikoff 2002). This mechanism is
essentially mechanical and may occur in any bi- or poly-
4.2.8.1 mineralic medium, from poorly lithified sediment under
Development of Spaced Foliation diagenetic conditions up to granulite facies gneisses. Even
without Dissolution-Precipitation in monomineralic rocks the crystal lattice orientation of
individual grains may cause gradients in competency, re-
If the original rock is coarse grained, development of a shape lated to the orientation of slip systems (Fig. 4.24).
fabric may be sufficient to create a spaced cleavage. Alter- This mechanism is capable to produce compositional
natively, a contrast between domains of different mineral- layering at various scales, and is not necessarily accom-
ogy, including individual mineral grains, is likely to pro- panied by dissolution and precipitation or other diffu-
duce mechanical instabilities during deformation that may sional mass transfer mechanisms. However, the change
result in the nucleation of micro shear bands (Goodwin and in shape of the less competent domains must involve
Tikoff 2002). The less competent mineral or material tends cataclasis, grain boundary sliding, crystal plastic defor-
to become elongated along these shear bands, leading to a mation or a combination of these mechanisms, treated
compositional layering defined by subparallel lenses of this above (e.g. Sects. 4.2.7.2, 4.2.7.4).

Fig. 4.27. Three examples to show how mimetic growth may play a role in the formation of secondary foliation. a A foliation defining min-
eral may be substituted, after deformation has ceased, by another mineral that inherits its shape and so continues to define the older
foliation. b A new mineral may grow in a fabric with strong preferred orientation, mimicking this preferred orientation to a certain extent
(e.g. biotite in a muscovite fabric). c Certain minerals may follow pre-existing compositional banding because of limited mobility of ions
3+
(Sect. 7.3; e.g. cordierite or staurolite may follow pelitic bands because of availability of Al ions)

Fig. 4.28. Progressive obliteration of crenulation cleavage structure by grain growth of micas. Many somewhat irregular schistosities may be
the result of such a process (cf. Figs. 4.18, 4.19, 4.22). a Fine-grained phyllite with vertical crenulation cleavage (lower greenschist facies).
Pyrenees, Spain. Width of view 1.2 mm. PPL. b Coarse phyllite with micas that grew at least partially after crenulation, lower amphibolite
facies. Carrancas, Southern Minas Gerais, Brazil. Width of view 3 mm. PPL. c Schist with coarse micas showing a fabric in which ‘ghost’ folds
or polygonal arcs are just recognisable (amphibolite facies). Marsfjällen, Sweden. Width of view 5 mm. CPL