Page 180 - Microtectonics

6.2 · Veins 169
to be displacement controlled, or to be tracking the open- some curved, optically strain-free fibrous crystals are in
ing direction of the vein or the ISA. Figure 6.7 shows the fact recrystallised deformed fibres, which initially were
geometry of fibres and elongate crystals as they would straight and orthogonal to the vein wall.
appear in veins by the four common types of displace- Urai et al. (1991) presented a model for the bivalent
ment-controlled crystal growth in response to relative growth behaviour of crystals in veins, largely confirmed
changes in movement direction of the wall rock (Sect. 6.2.1). by numerical experiments and observations (Fig. 6.13,
Notice that syntaxial and antitaxial veins have mirror ×Video 6.13; Bons 2001; Hilgers et al. 2001; Hilgers and
symmetry deflections due to their opposed growth di- Urai 2002; Nollet et al. 2005). According to the model,
rections. Composite veins show both senses. Non-local- competition between grains that grow into an open space
ised fracturing in ataxial veins will usually give a ‘mean’ will normally lead to aggregates of equidimensional or
orientation without clear curvature. Unitaxial veins slightly elongate crystals, but if crystals are forced to grow
would only show a single curvature pattern. into a narrow crack and adapt to its shape, highly elon-
In non-coaxial flow, veins and new-grown fibres or gate crystals or fibres may develop. This will occur when-
elongate grains will rotate as material lines with respect ever growth rate of part of the crystals exceeds the mean
to ISA, and fibres that are growing in the (fixed) direc- local opening rate of the crack. If the growth surface of a
tion of the extensional ISA will therefore become curved. vein is irregular in shape, e.g. because the original crack
The curvature of these grains corresponds to certain was not perfectly planar, grain boundaries tend to mi-
clearly defined geometries treated in the next section. grate rapidly towards asperities in the contact which
The orientation of fibres or elongate grains is not in point in the direction of the growing grains. The con-
all cases associated with kinematic directions (Fig. 6.9). tacts will become fixed there during further growth of
Comparison of quartz and calcite fibre and elongate grain the grains into fibres (Figs. 6.13, 6.14, ×Video 6.13). In
orientation with that of trails of phyllosilicates in veins, the case of Taber-growth from a fluid in the wall rock
or with off-set markers in the vein wall has shown that (Means and Li 2001), or if opening and growth occurs by
not all track the opening vector of the vein (Cox and Eth- small steps where a narrow fluid-filled void is filled be-
eridge 1983; Cox 1987; van der Pluijm 1984; Williams and tween opening steps, the fibres will follow changes in
Urai 1989; Bons 2001). In many cases, grains simply grow displacement direction of the vein wall. This is the ori-
normal to the wall rock of the vein in which they nucle- gin of displacement-controlled fibres (Figs. 6.13, 6.14,
ate, and fill the available void without change of growth ×Video 6.13). However, if the vein opens more rapidly,
direction. Williams and Urai (1989) have also shown that or growth is slower, or if the irregularities on the growth

Fig. 6.13.
a If a crack with irregular shape
opens and crystals grow isotrop-
ically and at equal rate to fill the
crack, the grain boundary be-
tween them will be displaced
normal to the crystal face till an
asperity in the growth surface
is reached that points towards
the wall-rock and the growing
grains; subsequently, this asper-
ity will be followed by the grain
boundary until the crack is filled.
b This mechanism allows elon-
gate grains and fibres to track
the opening direction of the vein
if growth and opening occur
intermittently by small steps