Diffusional creep as a possible deformation mechanism for
s-shaped quartz porphyroclasts within a calcite marble shear zoneMichel Bestmann, David J. Prior, Kees T.A. Veldkamp
Department of Earth Sciences, Liverpool University, L69 3GP, UK
Many deformed rocks, contain porphyroclasts embedded as relative rigid inclusions in a fine-grained, ductile matrix. In contrast to most natural porphyroclasts with recrystallized wings, the
s-shaped quartz grains, we present in this study, are single crystals. Optical microscopy, electron backscatter diffraction (EBSD) technique and cathodoluminescence analysis in a scanning electron microscope were applied in order to gain microstructural data that can constrain the quartz deformation mechanisms of the quartz grains during strain localization within a calcite shear zone.Within a greenschist facies marble shear zone, quartz grains that are dispersed within the calcite matrix change shape across the shear zone profile (Bestmann et al., 2000). Whereas these quartz grains are isometric in the coarse-grained marble outside the shear zone, they are asymmetric with wedge shaped appendages within the fine-grained calcite ultramylonite (Fig. 1). The shape controlling deformation processes of these single crystals are grain size sensitive. Only quartz grains with a larger grain size than the average grain size of the surrounding dynamically recrystallized calcite matrix show the characteristic asymmetrical shape. A type of
diffusional creep can best explain the evolution of the asymmetrical quartz grains. Fluids, released from fluid inclusions during dynamic recrystallization of the calcite matrix, may assisted the diffusive mass transfer along phase boundaries between areas of relatively high compressive stress to areas of relatively low compressive stress induced by the flow pattern of the surrounding calcite matrix. Cathodoluminescence images suggest a combination of internal mass transfer and mass removal for the development of the s-shaped quartz grains. However, a variety of lattice distortion patterns suggest that dislocation creep has occurred within individual quartz grains during some stage of deformation. A strong but uncommon crystallographic preferred orientation (CPO) for quartz under greenschist facies condition with c-axes parallel to the shear direction can be explained by a strengthening of an existing post-sedimentary weak CPO.
Fig.1. Three-dimensional shape of an asymmetrical (s-shaped) quartz grain situated in-situ on the shear plane of an ultramylonite marble. The two-dimensional section reveals that the grains are single crystals.
Bestmann, M., Kunze, K., Matthews, A., 2000. Evolution of a calcite marble shear zone complex on Thassos Island, Greece: microstructural & textural fabrics and their kinematic significance. Journal of Structural Geology 22, 1789-1807.