Geoscience Reference
In-Depth Information
9.4.1 Pre-kinematic features
Early porphyroblasts are commonly wrapped by later tectonic
foliations (Figure 9.9), but larger bodies (e.g. boudins; Figure
8.16) may also show this relationship, preserving earlier fabrics
and/or minerals in the boudin. They repay careful observation,
recording and perhaps sampling so that the full history of the
rock can be deciphered. Pre-kinematic grains or clasts may be
cracked, bent or even pulled apart; this texture is common in
mylonitic rocks. Inclusion trails in porphyroblasts should be
sketched, if possible, as their form may reveal not only the
relative timing of mineral growth, but potentially also
kinematic information (Figure 9.10).
9
3 cm
1 cm
Figure 9.9
Pre-kinematic
staurolite crystals strongly wrapped
by a strong schistosity (orientated
vertically). Some porphyroblasts
(e.g. large crystal in centre) are
fl anked by strain shadows, mainly
fi lled by a pale mineral. View
about 7 cm across. (Tom W.
Argles, The Open University, UK.)
(a)
(b)
Figure 9.10
Sketches of mineral grains with different relationships to
deformation. (a) Pre-kinematic, broken K-feldspar grains and a boudinaged
tourmaline crystal (top right), wrapped by a high-strain foliation. (b)
Syn-kinematic garnet with curved inclusion trails, showing inferred shear
sense.
9.4.2 Syn-kinematic features
You should always record what mineral(s) defi nes the main
planar tectonic fabric (Section 8.3.1), and any other fabrics too,
especially if there is a distinct timing relationship between
them (e.g. shear bands deforming an earlier foliation).
Identifying the minerals that defi ne different lineations can give
valuable information on the metamorphic conditions during
deformation. Careful observation of the lineation fabrics may
indicate which minerals were stable at the time, as the
following points indicate:
Porphyroblasts commonly
offer the best chance of
obtaining useful textural
information in the fi eld, and
repay careful observation -
and thumbnail sketches
(Figure 9.10)!
mineral grains stretched in one direction (mineral(s) stable
during deformation);
elongate grains that have grown in an alignment (mineral(s)
stable during deformation);
elongate grains that have been passively rotated into an
alignment (mineral(s) not necessarily stable);
mineral X stretched and boudinaged, with mineral Y
infi lling boudin necks (mineral X may be unstable, mineral
Y stable during deformation).
