Page 90 - Microtectonics

78 4 · Foliations, Lineations and Lattice Preferred Orientation
4.2.6 foliation (Fig. 4.14) or, if the spacing is sufficiently nar-
Spaced Foliation row, domainal slaty cleavage. Other morphological fea-
tures of spaced foliations that may be considered in their
Rocks with spaced foliation consist of two types of do- description are (Fig. 4.7):
mains, cleavage domains (also known as cleavage lamel-
lae) and microlithons (e.g. Fig. 4.5). As an alternative, the The spacing of the cleavage domains.
terms M domain (mica-rich), or P domain (phyllosilicate- The shape of cleavage domains: rough (Gray 1978),
rich) and Q domain (quartz-rich) have been used in mi- smooth (e.g. Fig. 4.5), wriggly or stylolytic.
caschist or phyllite (Shelley 1993). Cleavage domains are The percentage of cleavage domains in the rock; if this
planar and contain fabric elements subparallel to the trend is higher than 30%, the term zonal foliation may be
of the domains. In metapelites, cleavage domains are usu- applied (Fig. 4.13). At 100% the foliation becomes con-
ally rich in mica and in minerals such as ilmenite, graph- tinuous.
ite, rutile, apatite and zircon. The spatial relation between cleavage domains: paral-
Microlithons lie between cleavage domains and con- lel, anastomosing or conjugate (two intersecting direc-
tain fabric elements that have a weak or no preferred ori- tions without signs of overprinting).
entation, or which contain fabric elements oblique to the The transition from cleavage domain to microlithon.
cleavage domains. Spaced foliations may be further sub- This may be gradational (Figs. 4.12, 4.13, ×Video 4.12,
divided according to the structure in the microlithons. If ×Photos 4.12, 4.13) or discrete (e.g. Figs. 4.14, 4.15,
these contain microfolds of an earlier foliation (e.g. 4.20). Note, however, some discrepancy in the litera-
Figs. 4.5, 4.12, ×Video 4.12, ×Photo 4.12) the term crenu- ture about this use of gradational; some authors (Gray
lation cleavage is applied (Rickard 1961). If not, the struc- 1977; Powell 1979; Kisch 1998) use the term zonal
ture is known as disjunctive foliation (or disjunctive cleav- crenulation cleavage to describe these gradational
age if fine-grained, e.g. Fig. 4.11). The more general terms structures. Our use follows Borradaile et al. (1982).
spaced cleavage and spaced schistosity are also used to The shape of microfolds in crenulation cleavage.
describe fine-grained and coarse-grained disjunctive fo- This may be symmetric (e.g. Fig. 4.12, ×Video 4.12,
liation (Fig. 4.7). Some spaced foliations contain lens- ×Photo 4.12), asymmetric (e.g. Fig. 4.13, ×Photo 4.13),
shaped microlithons and may be called domainal spaced tight, open, etc.

Fig. 4.11. Disjunctive cleavage in quartz-mica phyllite, defined by subhorizontal biotite-rich layers (cleavage domains) and quartz-mica
layers (microlithons). Leiden Collection. Width of view 4 mm. PPL