Dynamic Recrystallization of Feldspar:
|
Click image to enlarge |
28. Recrystallization
of feldspar - Deformed feldspars in this image of mylonitic granodiorite from
the Borrego Springs mylonite zone, are predominantly orthoclase. At first glance,
they appear to have deformed by brittle failure, whereas quartz in the lower center
of image has recrystallized to form new, equant, polygonal quartz grains. However,
the very fine-grained, dark material along the "cracks" in the feldspar
are in fact thin zones of recrystallized K-feldspar. For a close-up of these small
grains, see images #29 and #30. Unlike quartz,
feldspar is a relatively strong mineral and the host crystal does not easily bend
under low temperature conditions (deformation here was at middle greenschist facies).
Subgrains do form around the feldspar margins, but they are generally extremely small
and do not extend far into the core of the grain.
|
|
29. Grain boundary recrystallization
of feldspar - An adjacent sample to image #28, but at greater
magnification, shows feldspar porphyroclasts that contain some patchy extinction
which may be the result of microcracking or subgrain formation - without transmission
electron microscopy it is impossible to say which. The newly recrystallized grains
around the clast margins are all the same size, significantly smaller than the patchy
extinction zones, which suggests that the new grains formed by grain boundary recrystallization.
Deformation was at middle greenschist facies.
|
|
| 30. Grain boundary recrystallizationof feldspar
- Same sample as image #29 but at even greater magnification. Recrystallized
grains are elongate, parallel to the opening direction of the two large pieces of
feldspar whose margins are highly irregular. These features suggest that grain boundary
migration recrystallization was the predominant deformation mechanism in the feldspar
grains. FOV 0.36 mm, Nicols Crossed. |
|
