Page 132 - Microtectonics

5.3 · Mylonite 121

Fig. 5.9. Zone of ultramylonite with straight internal layering (left) in a coarse-grained host rock composed of quartz, feldspar and biotite.
Section parallel to the aggregate lineation and normal to the foliation. A minor mylonite zone transects the centre of the photograph. Along
this zone deflection of the foliation in the wall rock indicates dextral sense of shear. Pernambuco, Brazil. Width of view 8 mm. CPL

foliation in the mylonite; this indicates that such folds the core of the shear zone (Fig. 5.10, top left). The overall
should not be regarded as the effect of a separate phase monoclinic symmetry of mylonite zones and of fabric
of deformation affecting an older mylonitic fabric, but elements in them reflects the monoclinic symmetry of
as the result of a local distortion in the flow field during non-coaxial flow in a shear zone.
mylonite genesis (Sect. 2.5; Cobbold and Quinquis 1980). It is sometimes difficult to decide if a certain strongly
Some of these folds are sheath folds, that is, they have a deformed rock in an isolated outcrop should be called a
tubular shape parallel to the aggregate or grain lineation mylonite or not. In such cases it is important to use good
(Fig. 5.10; Cobbold and Quinquis 1980; Lacassin and illustrations in publications.
Mattauer 1985; Alsop and Holdsworth 2004). Others are
non-tubular but cylindrical with a straight fold axis par- 5.3.3
allel to the lineation; these are known as curtain folds Mylonite Classification
(Fig. 5.10; Hartwig 1925; Lotze 1957; Passchier 1986a).
Curtain folds commonly decrease in amplitude and fade Mylonites are classified according to the metamorphic
out laterally (Fig. 5.10). grade at which deformation took place (e.g. high-grade
An important characteristic of many mylonites is a mylonite) or according to the lithotype or mineralogy in
clear difference in geometry of structures in thin sec- which they are developed (e.g. quartzite-mylonite, grano-
tions cut normal and parallel to the aggregate or grain diorite-mylonite, quartz-feldspar mylonite). If mylonite
lineation (Fig. 5.10). In sections normal to the lineation, develops in a monomineralic rock it is referred to as cal-
the rock may seem relatively undeformed or structures cite-mylonite, quartz-mylonite etc. (Burlini and Kunze
have orthorhombic symmetry (Fig. 3.29); in sections 2000). Another commonly used classification of mylonites
parallel to the lineation, the deformation fabric is usu- is based on the percentage of matrix as compared to por-
ally much stronger, and structures with monoclinic sym- phyroclasts (e.g. Spry 1969; Sibson 1977b; Scholz 1990;
metry that may be used as shear sense indicators appear Schmid and Handy 1991). Rocks with 10–50% matrix are
(Sects. 5.5, 5.6). Characteristic is the curved shape and classified as protomylonites (Fig. 5.9, right hand side);
decreasing intensity of mylonitic foliations away from with 50–90% matrix as mylonites (or mesomylonites; e.g.