Page 175 - Microtectonics

164 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins

Fig. 6.5. Five types of fibre growth in veins common in nature. Youngest parts of the fibres are shown in the lightest colour. Growth
surface is indicated by a bold dark line (g). a Syntaxial growth. Fibres are in continuity with the wall rock crystals; the oldest part of the
fibres lies along the edge of the vein. A median line marks the final position of the growth surface if growth is from both vein walls. If
growth is from one vein wall only, growth is unitaxial and a median line is lacking. b Antitaxial growth; fibres grow from the centre
outwards. The vein usually consists of other material then the wall rock. The oldest part of the vein is along the median line. Two growth
surfaces are localised along the contact with the wall rock. c Composite growth, with a syntaxial and antitaxial component. Two growth
surfaces are present at the contact between the vein segments where the youngest parts of the vein are situated. d Ataxial growth. Fibres
are in continuity with crystals in the wall rock but lack a localised growth surface; growth is by repetitive fracturing at different sites.
Young and old parts of the fibres can be mixed throughout the vein. No median line is present
vein boundary, this leads to development of synthetic, stretched crystals (Figs. 6.5–6.7; Durney and Ramsay
antithetic or composite veins with median lines as dis- 1973; Bons 2000, 2001) and are relatively common in na-
cussed above (Fig. 6.5a–c, ×Videos 6.5ab, 6.5cd). Alter- ture (Hilgers and Urai 2002). We use the terms ataxial
natively, veins may form by repeated fracturing and veins for veins with such crystals (Figs. 6.5d, 6.6, 6.7,
growth at alternating different sites in the vein (Fig. 6.5d, ×Video 6.5cd). Note, that lack of a median line is a char-
×Video 6.5cd). Such non-localised, ‘ataxial’ cracking and acteristic of both ataxial and unitaxial veins.
growth produces veins with jogged or stepped and rarely Some veins consist of pockets of elongate crystals, or
smooth elongate grains without a median line that are columns of crystalline material with inclusion bands, and
in continuity with fragments of single crystals on both sections where the wall rock material collapsed into the
sides of the vein. Such elongate grains are known as veins; such structures may have formed where water