Grain and sub-grain size variations across a mylonite zone |
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| Authors: | S White |
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| Institution: | (1) Department of Geology, Royal School of Mines, Imperial College, SW7 2BP London, Great Britain |
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| Abstract: | The results are reported of a combined optical and electron microscopy study of microstructural variations across a quartz
mylonite zone with increasing shear strain. The mylonite developed by recrystallization of the deformed quartz grains with
increasing shear strain. It was found in a given specimen that the size of recrystallized grains and of sub-grains were always
smaller in electron micrographs. The possible reasons for this are discussed. The size of both features decreased with increasing
shear strain irrespective of the microscope used. However the density of unbound dislocations remained constant. A marked
grain size reduction occurred in phyllosilicate rich layers. Variations in sub-grain size were observed within the relict
old grains which remained at low shear strains. These are thought to reflect stress intensification adjacent to grain boundaries
during deformation. The relict grains recrystallized at higher strains. Stresses were estimated from grain and sub-grain sizes
and from the dislocation density. The results indicate that estimates based on grain size are unreliable if phyllosilicates
inhibit the growth of grains during recrystallization, and that the dislocation densities are altered during uplift and are
unlikely to give meaningful estimates. It is also concluded that the microstructures reflect stress gradients present during
the formation of the mylonite, that is the initiation and propagation of the shear zone and that these were subsequently replaced
by strain rate gradients. |
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