Geoscience Reference
In-Depth Information
Figure 8.17
Gneissic banding: compositional layering on a centimetre
scale in a coarse-grained gneiss, Bhutan. It is diffi cult to tell whether this
banding has a sedimentary, tectonic or metamorphic origin without
detailed petrographic or geochemical analysis (Tom W. Argles, The Open
University, UK.)
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Figure 8.18
Stretching lineations. (a) Stretching lineation defi ned by stretched grains of quartz and biotite in
a mylonitic quartzite (5 cm across). (b) Weakly aligned orthopyroxene grains roughly defi ne the stretching
direction in a mantle peridotite (base of sample is 10 cm across). (c) Stretching lineation defi ned by elongate
amphibole crystals in an amphibolite that lacks foliation (base of sample is 8 cm across). (d) High-strain gneiss
with orthogonal faces cut parallel (left face) and perpendicular (right face) to stretching lineation, which is faintly
visible as colour streaking on the top surface (weathered foliation). Note that the left face (parallel to the
lineation) appears much more sheared than the other cut face: this can be a useful feature to look for on an
irregular exposure when searching for stretching lineations. Line on top surface is part of the marking originally
used to orientate the specimen. (Right face is 5 cm across.)
