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| Titel |
Force chain forming quartz in an ultramylonite |
| VerfasserIn |
Rüdiger Kilian, Luiz F. G. Morales, Max Peters |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098954
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| Publikation (Nr.) |
 EGU/EGU2014-14680.pdf |
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| Zusammenfassung |
| Polymineralic ultramylonites often show microstructures indicative of grain size sensitive
creep with dissolution precipitation or diffusion accommodated grain boundary sliding.
Typically phases show an anticorrelated distribution, the grain size is small and a
crystallographic preferred orientation is absent. The latter observation is usually thought to
originate from rigid body rotation of grains because flow dominated by diffusion
creep operates at differential stresses, which are too low to activate crystal-plastic
mechanisms.
Here, we present quartz texture measurements from a natural ultramylonite, deformed
under upper amphibolite facies conditions from the Nordmannvik Nappe, Upper Allochton of
the Norwegian Caledonides. The ultramylonite has a mean grain size < 10 μm (eq. diameter)
and shows a very homogeneous microstructure with an anticorrelated phase distribution with
quartz (50 vol%), separated by a matrix of biotite, white mica, plagioclase and
titanite while garnet forms porphyroclasts. Quartz occurs either as isolated grains or
in “one grain” thick, small clusters. Two types of clusters can be distinguished:
foliation parallel clusters and oblique clusters with a long axis at a small angle to the
inferred shortening direction, the latter being prominent in the most homogeneous
ultramylonite.
Quartz shows a weak but non-random texture. In the foliation parallel clusters a
[c]-axis maximum is elongated around the y-direction towards the normal of the
foliation, -axes form point maxima at a small angle to the lineation, very similar
to textures found in high temperature quartz mylonites (e.g. Pennacchioni et al.,
2010). In the foliation oblique clusters, the [c]-axes form a very broad maximum
around the y-direction and axes show three distinct, close to orthogonal maxima
close to x,y,z-directions, rotated about 10-15° antithetically around the y-direction.
Isolated quartz grains also show a weak texture of this type. Quartz grains contain low
angle boundaries, some of which can be interpreted as subgrain boundaries, mainly
related to prism-a and rhomb-a slip, suggesting the activation of crystal-plastic
processes. Alternative texture forming processes (e.g. growth textures) are also
discussed.
The texture in the foliation parallel clusters is thought to be an inherited texture from
lower strain stages in the ultramylonite, as it is mostly present in the least deformed parts of
the ultramylonite. However, we suggest that the texture formed in the foliation
oblique clusters is related to a dynamic formation of force chains between quartz
grains, where differential stresses become high enough for plastic yielding. The
presence of force chains questions whether ultramylonites necessarily need to possess a
linear viscous rheology, even if microstructures would indicate a diffusion creep
mechanism.
Pennacchioni G., Menegon L., Leiss B., Nestola F., Bromiley G., 2010: Development of
crystallographic preferred orientation and microstructure during plastic deformation of
natural coarse?grained quartz veins. Journal of Geophysical Research, Vol. 115, B12405 |
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