Page 152 - Microtectonics

5.6 · Microscopic Shear Sense Indicators in Mylonite 141
Fig. 5.27a–f.
Idealised development of man-
tled porphyroclasts around
spherical core objects which do
not shrink during progressive
deformation for eye-shaped
(top), and bow-tie shaped (bot-
tom) separatrices and different
initial mantle thickness. Note that
δ-type mantled objects start
their development as σ-type
mantled objects

can be added from the porphyroclast to the wing; the 5.6.7.3
material in the wing nearest to the porphyroclast is mov- Development of Sigmoids
ing towards it, not away from it; if new recrystallised
material is produced, new wings develop. Sigmoids lack a rigid central clast, and usually show signs
An important consequence of the model discussed of internal deformation and recrystallisation in the en-
above is that for exceptionally wide mantles and an eye- tire object. They may have formed by boudinage and
shaped separatrix, φ-type clasts with approximately separation of σ-type asymmetric boudins of layers or
orthorhombic symmetry will develop at high strain and veins (Sect. 5.6.12), by ductile deformation of rectangu-
high recrystallisation rate, even in simple shear. φ-type lar grains which recrystallised completely (Treagus and
clasts therefore are not diagnostic for coaxial flow. Lan 2003, 2004), by complete recrystallisation of the core
In general flow, elongate porphyroclasts can obtain a of a σ-type mantled clast, or inhomogeneous flow in the
stable position, and formation and stretching of mantle edge of a lens shaped aggregate.
material can continue in these cases. Such stable porphy-
roclasts have a “forward tilted” orientation and σ - or 5.6.7.4
a
φ-type mantles (Fig. B.5.5). δ-type or complex mantle Development of Mineral Fish
shapes are only possible around permanently rotating
porphyroclasts (Passchier et al. 1993; Passchier 1994). Characteristic of all mineral fish is the typical elongate
If the mantle is softer than the matrix, the behaviour lozenge or lens shape, a strong preferred orientation and
of porphyroclasts changes. In the case of equidimen- lack of evidence for rotation. The lozenge shape may
sional clasts, the central clast may rotate faster than ex- have developed by internal deformation (Treagus and
pected for passive mantles, and δ-objects can form. If Lan 2003), erosion by recrystallisation or pressure so-
objects are elongate, they may show the behaviour of fish, lution, and lateral growth by precipitation of dissolved
as explained below. material.