Page 131 - Microtectonics

120 5 · Shear Zones

Fig. 5.8. Quartz-feldspar mylonite. Section parallel to the aggregate lineation and normal to the foliation. Lenses of recrystallised quartz and
feldspar define the mylonitic foliation. The foliation wraps around feldspar porphyroclasts. Minor shear bands (Sect. 5.6.3) define the sense
of shear as dextral. St. Barthélemy, Pyrenees, France. Width of view 10 mm. PPL

Box 5.2 Porphyroclasts and porphyroblasts The planar fabric element of mylonites is known as a
planar shape fabric, shape preferred orientation (Box 4.2)
Porphyroclasts and porphyroblasts are relatively large, sin-
gle crystals in a fine-grained matrix. The word porphyro- or more specifically as a mylonitic foliation (Box 4.4); the
clast is also used for a rounded polycrystalline rock fragment linear fabric element is known as a linear shape fabric or
in a more fine-grained matrix. Porphyroclasts (from ‘clasis’ – aggregate lineation (Sect. 4.3, Box 4.2; Fig. 5.10). Low-
breaking) are inferred to have formed by diminution of strain lenses around which the shape fabric anastomoses
the grain size in the matrix. They are therefore typical for are common in mylonites (Fig. 5.10), from lozenge-shaped
mylonites and cataclasites; they are relic structures of a more single feldspar crystals (Figs. 5.8, 5.12) to km-scale lenses.
coarse-grained original fabric. The word clast is often used
as a short equivalent commonly with the constituent min- Well-developed aggregate lineations are mainly found
eral as a prefix (feldspar clast). Common minerals that form in polymineralic rocks where grain size reduction has
porphyroclasts are feldspar, garnet, muscovite, hornblende taken place (Piazolo and Passchier 2002b; Sect. 4.3). In
and pyroxenes. Quartz forms porphyroclasts only in very originally fine-grained rocks, especially if they were
special cases (Sect. 3.13.3; Figs. 3.9, 3.10). Porphyroclasts monomineralic and no grain size reduction took place,
should not be confused with detrital clasts in sediments.
Porphyroblasts (from ‘blasis’ – growth) are inferred to have aggregate lineations may be absent even if strain is high.
formed by growth of crystals of specific mineral species, At high-grade deformation conditions, grain lineations
while crystals in the matrix did not grow to the same extent dominate.
(cf. Chap. 7). The word blast is commonly used as a short Mylonites commonly contain two or even three foli-
equivalent. They are common in non-mylonitic phyllites and ations, inclined to each other at small angle, that are
schists. In some cases, original porphyroblasts in a schist or thought to have developed contemporaneously (Fig. 5.10).
gneiss may become porphyroclasts when the schist or gneiss
is mylonitised. These are further explained in Sect. 5.6. Quartz, calcite
and feldspar commonly also show evidence of lattice
preferred orientation (Sect. 4.4.5). Foliations in mylonite
same minerals, since rheological properties of minerals are locally subject to tight or isoclinal folding (Figs. 1.2,
depend on metamorphic conditions and initial grain size 5.10, ×Video 1.2). In most cases, the axial planar folia-
(Sect. 3.12). tion in these folds cannot be distinguished from the main