Page 86 - Microtectonics

74 4 · Foliations, Lineations and Lattice Preferred Orientation
4.2.4 ferred orientation. Most common are minerals such as
Morphology of Foliations mica or amphibole (Fig. 4.8), but quartz (Fig. 3.31) or
other minerals (Fig. 3.39) may also define a continuous
Powell (1979) and Borradaile et al. (1982) have proposed foliation. Fine-grained rocks such as slates, which show a
a descriptive classification of foliations, independent of continuous cleavage in thin section (Fig. 4.10), may re-
their primary or secondary origin, using only morpho- veal a spaced foliation if studied at stronger magnification,
logical features. This classification is based on the fabric e.g. by scanning electron microscope (SEM – Sect. 10.2.4).
elements that define the foliation such as elongate or platy Normally, the terminology used for a specific foliation
grains, compositional layers or lenses, or planar disconti- is based on the geometry observed in thin section. If
nuities. The distribution of these fabric elements in the field or SEM observations are discussed, the scale of ob-
rock defines whether the foliation is spaced or continu- servation (field observation, thin section, SEM) should
ous. In the first case, the fabric elements are not homoge- be mentioned in descriptions of foliation morphology.
neously distributed and the rock is divided into lenses or Continuous foliation may be further described through
layers of different composition. Continuous foliation is the fabric elements, e.g. grain shape and size. If the grains
used for rocks in which the fabric elements are homoge- that define the foliation are visible by the unaided eye,
neously distributed, normally down to the scale of the the foliation is called a continuous schistosity or simply
individual minerals. Figure 4.7 summarises the classifi- schistosity (Figs. 4.8, 4.9). If the grain size is finer, the
cation used in this book. It is a simplified version of the structure is known as a continuous cleavage or slaty cleav-
classification proposed by Powell (1979) and Borradaile age (Figs. 4.10, 10.4). Since the continuous nature of a
et al. (1982) with minor modification. Durney and Kisch foliation is scale-dependent, we include finely spaced
(1994) proposed a different field classification and inten- cleavage with a spacing up to 0.01 mm (Powell 1979, p 333)
sity scale for first generation cleavages, but this classifi- in the definition of slaty cleavage in order to remain in
cation is less suitable for microstructures. line with the current use of this concept in the literature
(cf. Fig. 10.4). Continuous foliations can be subdivided
4.2.5 into mineral foliations, defined by the preferred orienta-
Continuous Foliation tion of platy but undeformed mineral grains such as mi-
cas or amphiboles (Fig. 4.8), and a grain shape preferred
A continuous foliation consists of a non-layered homo- orientation (Box 4.2) defined by flattened crystals such
geneous distribution of platy mineral grains with a pre- as quartz or calcite (Figs. 3.31, 3.39, 4.25).

Fig. 4.8. Continuous schistosity defined by parallel crystals of biotite, muscovite and quartz. Mt Isa, Australia. Width of view 1.8 mm. PPL