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5.6 · Microscopic Shear Sense Indicators in Mylonite 133
Fig. 5.20.
Schematic diagram of the prin-
cipal types of objects encoun-
tered in the matrix of mylonites.
This includes large single crystal
shapes such as naked clasts and
mineral fish, single crystal por-
phyroclasts with rims such as
mantles, reaction rims or strain
shadows, and polycrystalline
aggregates such as sigmoids.
A, B, etc. refer to mineral types
of their development (Sect. 5.6.7), and finally their use-
fulness as shear sense indicators is discussed (Sect. 5.6.8).
5.6.5
Mantled Porphyroclasts
Mantled porphyroclasts consist of a central single crys-
tal and a fine-grained mantle of the same mineral. Com-
mon examples are porphyroclasts of feldspar in a ma-
trix of quartz-feldspar-mica, of orthopyroxene in peri-
dotite and of dolomite in a calcite matrix. The fine-
grained soft mantle can be deformed into wings (or trails)
that extend on both sides of the porphyroclast parallel
to the shape preferred orientation in the mylonite
(Passchier and Simpson 1986). Wings are thought to
stretch and change shape while the porphyroclast core
remains rigid or continues to recrystallise along the con-
tact with the rim, shrinking in size (Sect. 5.6.7.2). Wing
shape can be used as a shear sense indicator and con-
tains information on rheology of the matrix and the
matrix-clast coherence (Sect. 9.3.4).
Four types of mantled porphyroclasts have been dis-
tinguished in the literature based on the shape of the
wings (Hanmer 1984b; Passchier and Simpson 1986;
Hooper and Hatcher 1988): φ-type, σ-type, δ-type, and
complex mantled clasts (Figs. 5.21, 5.25, ×Photos 5.9b,
5.23, ×Videos 5.22, 5.23). Θ-type mantled clasts lack
wings but have a mantle with orthorhombic symmetry
(Passchier 1994). σ-type-, δ-type and complex mantled
clasts have monoclinic shape symmetry. σ-type mantled
clasts have wide mantles near the porphyroclast with two
Fig. 5.21. Classification of mantled porphyroclasts. Dextral sense
of shear planar faces and two curved faces that define an internal
