Page 147 - Microtectonics

136 5 · Shear Zones
asymmetry. The tips of the wings lie at different eleva- Grotenhuis et al. 2003), leucoxene (Oliver and Goodge
tion on both sides. This difference in elevation is referred 1996), sillimanite (Fig. 5.36; Pennacchioni et al. 2001;
to as stair-stepping (Figs. 5.21–5.25, ×Photos 5.9b, 5.23, Mancktelow et al. 2002), olivine (Mancktelow et al. 2002)
×Video 5.22; Lister and Snoke 1984). Passchier and and quartz (Fig. 5.35; Bestmann et al. 2000, 2004; ten
Simpson (1986) distinguished σ - and σ -type mantled Grotenhuis et al. 2003).
a
b
clasts; the former occur isolated in a mylonitic matrix; Besides normal mineral fish, described as Type 1 by
the second as part of developing C/S fabrics. δ-type man- Mancktelow et al. (2002), Pennacchioni et al. (2001) de-
tled clasts have narrow wings and characteristic bends scribed sillimanite fish of an unusual truncated shape in
in the wings adjacent to the porphyroclast. As a result, an amphibolite facies ultramylonite (Type 2 of Manck-
two embayments of matrix material occur adjacent to telow et al. 2002). These sillimanite fish have the same
the porphyroclast. Not all δ-type mantled clasts have backward inclination as normal fish with respect to the
stair-stepping (Fig. 5.21). Complex mantled clasts have foliation, but with a mirror image lozenge-geometry to
more than one set of wings (Fig. 5.21). normal fish (Fig. 5.36, ×Photo 5.36a,b). The geometry is
δ-type and complex mantled clasts mainly occur in defined by short shear bands that trail off the truncated
high strain mylonites, while σ-type mantled clasts oc- end of these sillimanite fish.
cur also at lower strain. Naked clasts occur commonly in Polycrystalline mica layers in quartzite or marble can
ultramylonites, especially at high grade (Whitmeyer and also have a lozenge shape and are known as foliation fish.
Simpson 2003; Pennacchioni et al. 2001). φ-type mantled Since they are commonly bordered by shear bands this
clasts are most common in high-grade relatively coarse- structure grades into C/S fabric (Sect. 5.6.3). It can also
grained mylonites. σ-type mantled clasts should not be be used as a reliable shear sense indicator.
confused with fish or sigmoids, or with asymmetric strain
shadows and strain fringes treated in Chap. 6 since each 5.6.7
category is formed by different mechanisms (see Fig. 5.20 The Development of Porphyroclast Systems
for differences).
5.6.7.1
5.6.6 Introduction
Mineral Fish
It is as yet not completely clear how porphyroclast sys-
Mineral fish are elongate lozenge or lens-shaped single tems develop but it is clear that most of the scenarios as
crystals, which are common in mylonites. They char- sketched in Box 5.4 can apply in nature. Real porphyro-
acteristically lie with their longest dimension at a small clasts have a complex 3D shape, while most of the mod-
angle to the mylonitic foliation (Figs. 5.30, 5.31, 5.33, elling described in Sect. 5.6.7.2 is two-dimensional. How-
5.37). Most common are mineral fish of large single white ever, 2D cross-sections along the vorticity profile plane
mica crystals known as mica fish in micaceous quartzitic through porphyroclast systems and mica-fish usually
mylonites (Figs. 5.10c, 5.28, 5.30, 5.31, ×Photos 5.29a–c, give a good approximation of the 3D shape (Jezek et al.
5.31; Eisbacher 1970; Choukroune and Lagarde 1977; 1994). Porphyroclasts may be rigid, deformable or weaker
Simpson and Schmid 1983; Lister and Snoke 1984; ten than their surrounding matrix material; they may have a
Grotenhuis et al. 2003; Sawaguchi and Ishii 2003). mantle with similar or lower viscosity than the matrix,
Commonly, trails of small mica fragments extend into or bonding between the porphyroclast and the matrix
the matrix from the tips of isolated mica fish (Figs. 5.30, may be reduced due to crystallographic properties or
5.31; Lister and Snoke 1984) with well-defined stair high fluid pressure, which is comparable to the effect of
stepping (Sect. 5.6.5). Ten Grotenhuis et al. (2003) pro- a thin weak zone close to a rigid object.
posed a morphological subdivision of mica fish into six An isolated rigid porphyroclast without a mantle
groups (Fig. 5.28, ×Photos 5.29a–c, 5.31), based on a which has perfect bonding with the matrix and no re-
study of crystals from an upper greenschist facies mylo- crystallisation nor erosion will rotate as a rigid object,
nite from Brazil. or may become stationary in general non-coaxial flow
Besides white mica, a number of other minerals can de- (Figs. B.5.2, B.5.5), but this behaviour will be difficult to
velop as mineral fish with a similar orientation with respect recognise in thin section. If differential stresses are high
to the mylonitic foliation, as observed in mylonites from a in the rim of the porphyroclast local crystalplastic de-
variety of locations and metamorphic grade. Presently formation and storage of dislocation tangles in the rim
known are examples of biotite (Fig. 5.33a), tourmaline of a porphyroclast can lead to dynamic recrystallisation
(Fig. 5.34), K-feldspar (Fig. 5.33d), garnet (Fig. 5.33e), pla- in the rim of the porphyroclast to form a core-and-man-
gioclase (Fig. 5.33c, ×Photo 5.33c), staurolite, kyanite, am- tle structure (Box 3.8; White 1976). Once a mantle of re-
phibole (Fig. 5.33h), hypersthene, diopside (Fig. 5. 33f), apa- crystallised material exists, this will normally be weaker
tite, rutile, hematite, prehnite (Mancktelow et al. 2002; ten than the remaining porphyroclast and will deform with