Shear Sense Indicators:
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Click image to enlarge |
97. Compositional banding in sheared
adamellite - This sheared adamellite from the Maggia Nappe, Switzerland deformed
at epidote-amphibolite facies. Recrystallized quartz and feldspar grains are in distinct
compositional bands. Clinozoisite (elongate, dark blue) and epidote (high birefringence)
grains are concentrated in feldspar-rich domains and along foliation-parallel zones
within quartz-rich areas (one-fourth way up image). Grain growth in the feldspathic
domain is inhibited by the small accessory grains (see also image
# 27).
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98. Grain boundary pinning - Biotite grains in this
deformed metaquartzite from the Needles Mountains, southern Colorado have a strong
alignment that defines foliation (lower left to top right). Deformation was at lower
amphibolite facies. Quartz grains are polygonal and show little or no internal strain.
Image analysis of this specimen shows that there is a strong preferred alignment
of quartz grain boundaries parallel to foliation, caused by the effective 'pinning'
of the highly mobile quartz grain boundaries by the biotite flakes. Evidence in favour
of syntectonic grain boundary pinning, rather than post-tectonic biotite growth,
includes: preferred alignment of biotite (001) planes; preferred alignment of quartz
grain boundaries parallel to foliation; and a secondary preferred alignment of quartz
grain boundaries perpendicular to the biotite (001) planes.
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| 99. Grain boundary pinning - Biotite granite from
the alpine Maggia Nappe, Ticino, Switzerland was deformed at epidote-amphibolite
facies into this biotite gneiss. As in image #98, biotite grains have 'pinned' the
quartz grain boundaries, effectively preventing grain growth (compare grain size
of quartz in this specimen with that in image # 97 from a sample
only a few meters away). FOV 1.5 mm, Nicols Crossed. |
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