Dynamic recrystallization near the brittle-plastic transition in naturally and experimentally deformed quartz aggregates

The electron backscattering diffraction technique (EBSD) was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates, both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains. For the Tonale mylonites we investigated, a temperature range of 300-380 degrees C, 0.25 GPa confining pressure, a flow stress range of similar to 0.1-0.2 GPa, and a strain rate of similar to 10-(13) S-1 were estimated. Experimental samples of Black Hills quartzite were analyzed, which had been deformed in axial compression at 700 degrees C, 1.2-1.5 GPa confining pressure, a flow stress of similar to 0.30-0.4 GPa, a strain rate of similar to 10(-6) s(-1), and to 44% to 73% axial shortening. Using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development. Our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains. Grain boundary bulges are not only formed by local grain boundary migration, but they also display a lattice misorientation indicative of subgrain rotation. Dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation, i.e. with an orientation unsuitable for easy basal slip. In addition, Dauphine twins within porpbyroclasts are preferred sites for recrystallization. We interpret large misorientation angles in the experimental samples, which increase with increasing strain, as formed by the activity of fluid-assisted grain boundary sliding. (c) 2007 Elsevier B.V. All rights reserved.