Page 196 - Microtectonics

6.5 · Non-Fibrous Strain Shadows and Strain Caps 185
site of strain caps, solution transfer, and redeposition of
material at grain boundaries in the developing strain
shadow without opening of distinct voids. Figure 6.30b
shows schematically some models for the development
of different types of strain shadows.
Mantled porphyroclasts superficially resemble strain
shadows (Sect. 5.6.5) but they have the same mineral
composition as the core object (porphyroclast), form by
different mechanisms and have a different kinematic sig-
nificance as strain shadows (Sect. 5.6.7: Figs. 5.20, 5.21,
6.24). We therefore do not include mantled porphyro-
clasts in the category of strain shadows. It is preferable
to reserve the terms strain shadow and strain fringe for
domains of material that have a different composition
from the core object. However, mantled porphyroclasts
and strain shadows are end members of a range of pos-
sible combinations; some strain shadows may be diffi-
cult to distinguish from mantled porphyroclasts with
wings that have undergone chemical or mineralogical
changes (Robin 1979; Wintsch 1986).

Fig. 6.30. a Geometry of strain shadows and strain caps around a
rigid object in a foliated rock. b Schematic presentation of the de-
velopment of three types of strain shadows, depending on strain
and volume change as indicated by a deformed square. Grey strain
shadows are formed by precipitation from solution. Notice that
strain caps are not necessarily present

Fig. 6.31. Non-fibrous strain shadow around a garnet porphyroblast in micaschist. Orós, NE-Brazil. Notice the presence of strain caps
and the gradual transition between the strain shadows and the matrix. Width of view 3.9 mm. Polars at 45°