Page 198 - Microtectonics

6.6 · Microboudinage 187
6.6 6.6
Microboudinage

Boudinage affecting elongate mineral grains at thin sec-
tion scale is often referred to as microboudinage (Misch
1969, 1970; Vernon 1976; Allison and LaTour 1977; Masuda
and Kuriyama 1988; Masuda et al. 1989, 1990, 1995a,b,
2003, 2004; Ji and Zhao 1993; Ji 1998; Figs. 6.32, 6.33,
×Video 11.11c; see also Sect. 5.6.12). The structures cre-
ated in the necks of these boudins may show many simi-
larities with those in strain shadows and extensional veins.
Microboudins can help to establish metamorphic con-
ditions during deformation, indicated by the mineral
assemblage that grew in the necks of the boudins. In some
cases, grain growth accompanies microboudinage to
form zoned grains. Analysis of the zoning in these grains
may also allow establishment of changing metamorphic
circumstances during progressive deformation (Misch
1969, 1970; Figs. 6.33, 6.34, ×Photo 6.33).
Microboudinage can also be used as a strain gauge
(Sect. 9.2; Ferguson 1981, 1987; Masuda et al. 1995a): the
length of a boudinaged grain divided by the sum of the
length of the boudins gives a minimum value for stretch
along the long axis of the boudinaged grain. However,
this axis does not necessarily coincide with the direction
of maximum extension in the rock. It is also possible to
use the length of boudin fragments to make estimates of Fig. 6.34. Schematic deformation-growth sequence involved in
differential stress (Lloyd et al. 1982; Masuda et al. 1990, microboudinage of zoned crystals, showing a single site of rup-
ture. Although represented graphically as finite steps, both stretch-
1995a,b, 2003; Ji and Zhao 1993) and aspects of the rhe- ing and accretion are thought to proceed essentially simultaneously.
ology of the matrix (Masuda and Kimura 2004). (After Misch 1969, p 48)