Page 154 - Microtectonics

5.6 · Microscopic Shear Sense Indicators in Mylonite 143

Fig. 5.29. Photomicrographs of different types of mica fish. a Lenticular mica fish with slightly inclined tips showing undulose extinc-
tion, transitional between groups 1 and 2; b lenticular fish with internal discontinuity in the right tip of the fish; c rhomboidal shaped
fish with (001) parallel to longest side of the fish, group 3; d rhomboidal shaped fish with (001) parallel to the shortest side of the fish,
group 4; e fish with small aspect ratio, group 5; f mica fish with high aspect ratio belonging to group 6. Samples are from Conceição do
Rio Verde, Brazil. Shear sense in all photographs is dextral. Width of view a 3 mm, b 0.75 mm, c, d and e 3 mm, f 6 mm. CPL
Biotite fish (Fig. 5.33a; ten Grotenhuis et al. 2003) dence of internal deformation and were interpreted to
probably form in the same way as white mica fish but are have formed essentially by grain size reducing mecha-
less common. In most mylonites, biotite recrystallises nisms. By contrast, Ishii and Sawaguchi (2002) described
more readily than white mica, possibly leading to the a case where orthopyroxene porphyroclasts embedded
early destruction of biotite fish. K-feldspar fish in a quartz in a fine-grained olivine matrix suffered strong crystal-
feldspar matrix (ten Grotenhuis et al. 2003) in high-grade plastic deformation. Tourmaline fish (ten Grotenhuis
rocks may also form by internal deformation (Fig. 5.33d), et al. 2003) show little or no intracrystalline deformation
possibly assisted by dynamic recrystallisation along the but seem to change shape exclusively by dissolution and
rim. Hyperstene fish in a quartz-feldspar matrix investi- precipitation, cutting the internal zoning patterns (Fig. 5.34).
gated by ten Grotenhuis et al. (2003) did not show evi- The same may apply to some feldspar fish, because of