Page 130 - Microtectonics

5.3 · Mylonite 119

Fig. 5.7. Mylonite derived from pelitic gneiss with quartz, feldspar, garnet and micas in a section parallel to the aggregate lineation and
normal to the foliation. Alternating layers rich in quartz (clear) and feldspar (grey), with porphyroclasts of garnet define the mylonitic
foliation. Sense of shear indicators are poorly developed in this section but subtle stair-stepping (Sects. 5.6.5–5.6.7) of wings on porphyro-
clasts and small C'-type shear bands (Sect. 5.6.3) indicate a dextral shear sense. Marsfjällen, Sweden. Width of view 13 mm. PPL

strictly structural term that refers only to the fabric of 5.3.2
the rock and does not give information on the mineral Characteristic Fabric Elements
composition. Mylonite should therefore not be used as a
rock name in a stratigraphic sequence. Mylonites can be recognised in the field by their small
Mylonite occurs in high-strain zones known as mylo- grain size and strongly developed, unusually regular and
nite zones, interpreted as exhumed, ‘fossil’ ductile shear planar foliations (Figs. 5.7–5.9) and straight lineations.
zones. The contact of a mylonite zone and unaffected wall Lenses and layers of fine-grained material that are com-
rock tends to be a gradual fabric transition. Grain size in mon in mylonites are thought to derive from a more
the mylonite is usually smaller than that in the wall rock. coarse-grained parent rock by intracrystalline deforma-
(Fig. 5.9). Mylonite zones can occur in any rock type and tion and recrystallisation. Such deformed lenses usually
have been described from a sub-millimetric scale to zones have a ‘surf-board’ shape defining both a planar and lin-
several km wide. (Bak et al. 1975; Hanmer 1988). The in- ear fabric element. This shape may be explained by com-
tensity of deformation may vary from one mylonite zone mon development of mylonites in an approximately plane
to another but is always high. The word ‘mylonite’ de- strain regime such as in simple shear.
rives from the Greek ‘µυλων’ (a mill) since the original Many mylonites contain porphyroclasts (Figs. 5.6,
opinion on these rocks was that they formed by brittle 5.7; Box 5.2) which are remnants of resistant mineral
‘milling’ of the rock (Lapworth 1885). However, present grains of a size larger than grains in the matrix. The
use of the word mylonite refers to rocks dominantly de- foliation in the matrix wraps around porphyroclasts
formed by ductile flow, while brittle deformation may (Figs. 5.7, 5.8, 5.10). Porphyroclasts develop because
play a minor role in isolated included lenses or grains of a difference in rheology between constituent min-
(Bell and Etheridge 1973; Tullis et al. 1982); in other erals; relatively ‘hard’ minerals will form porphyro-
words, the stress-supporting network is affected by crys- clasts, while relatively soft ones form part of the matrix.
talplastic deformation (Sect. 3.8). However, porphyroclasts do not always form in the