Intragranular dynamic recrystallization in naturally deformed calcite marble: diffusion accommodated grain boundary sliding as a result of subgrain rotation recrystallization

Michel Bestmann, David J. Prior

Department of Earth Sciences, Liverpool University, L69 3GP, UK

michelb@liv.ca.uk

Intragranular microshear zones within a greenschist facies calcite marble (Thassos Island, Greece) were studied to try to constrain better the processes of dynamic recrystallization as well as the deformation processes that occur within newly recrystallized grains. Intragranular recrystallized grains within large, twinned calcite porphyroclasts can be related to the host from which they have recrystallized and are the focus of an electron backscatter diffraction study.

Analysis of misorientation distributions is a powerful tool to quantify microstructural features and to constrain deformation processes and history. Although the orientation mapping does not provide the orientation of the boundary/interface plane to describe fully the geometry, the misorientation analysis alone helps to discriminate the role of various processes in forming microstructures.

Lattice distortions, low angle boundaries and some high angle boundaries (>15° ) in the microshears within a porphyroclast have the same misorientation axes suggesting that deformation occurred by climb-accommodated dislocation creep involving subgrain rotation recrystallization. Changes in the ratio of host and twin domain, as the deformation zone is entered, show that twin boundary migration also occurred. Recrystallized grains have similar sizes (10-60 mm) to subgrains, suggesting that they formed by subgrain rotation. However, within the intragranular microshear zones the misorientations between recrystallized grains and porphyroclasts are considerably larger than 15° and misorientation axes are randomly oriented. Moreover recrystallized grain orientations average around the porphyroclast orientation. We suggest that the recrystallized grains, once formed are able to deform partly by diffusion accommodated grain boundary sliding, which is consistent with predictions made from lab flow laws.

Fig. 1. EBSD orientation map of the analysed intragranular shear zone within a calcite porphyroclast. For the host/twin domains and the new recrystallized grains the crystallographic orientation is presented as pole figures. The misorientation axes and their sense of rotation are constructed from the dispersion patterns. Bulk sense of shear is given.