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The thermal history of the south-westernmost Black Forest (Germany) and the adjacent Upper Rhine Graben were constrained by a combination of apatite and zircon fission-track (FT) and microstructural analyses. After intrusion of Palaeozoic granitic plutons in the Black Forest, the thermal regime of the studied area re-equilibrated during the Late Permian and the Mesozoic, interrupted by enhanced hydrothermal activity during the Jurassic. At the eastern flank of the Upper Rhine Graben along the Main Border Fault the analysed samples show microstructural characteristics related to repeated tectonic and hydrothermal activities. The integration of microstructural observations of the cataclastic fault gouge with the FT data identifies the existence of repeated tectonic-related fluid flow events characterised by different thermal conditions. The older took place during the Variscan and/or Mesozoic time at temperatures lower than 280°C, whereas the younger was probably contemporary with the Cenozoic rifting of the Upper Rhine Graben at temperatures not higher than 150°C.

The effects of deformation on the kinetics of the net-transfer reaction anorthite + forsterite → cpx + opx + spinel ± gt were studied using static and shear deformation experiments. Experiments were performed on dry anorthite-olivine (An92–Fo93) samples at 900°C and pressures between 1,000 and 1,600 MPa in a Griggs apparatus. Deformed (‘non-hydrostatic’) and undeformed (‘static’) samples are compared in terms of phase petrology, reaction rate and reaction mechanisms. Anorthite + olivine reactions are diffusion-controlled as seen from reaction rim structures. In undeformed samples, delayed reaction onset and low reaction rates demonstrate sluggish nucleation of reaction products and slow rates of diffusion at dry conditions, even at 700–900 MPa confining pressure overstepping. The reaction rate is enhanced in deformed An–Fo samples. The higher rate is mainly attributed to a combination of high stresses and viscous deformation processes of the reactants and products, which cause an increase in the nucleation rate of products. The results imply that viscous deformation processes alone can be responsible for the initiation and localisation of metamorphic reactions in dry rocks in the absence of fluid infiltration.

Dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change in chemical composition, driven by strain energy in the form of dislocations. Driving potentials additional to internal strain energy may contribute to the recrystallization of naturally deformed minerals, which form solid solutions such as feldspar, amphiboles and pyroxenes, if they change their composition during recrystallization. To estimate the relative importance of these driving potentials, the chemical composition of porphyroclasts and recrystallized grains of plagioclase, clinopyroxene and hornblende have been investigated in samples from a high grade shear zone of the Ivrea Zone, Italy. The plagioclases show two different recrystallization microstructures: bulging recrystallization at grain boundaries and discrete zones of recrystallized grains across porphyroclasts probably involving fracturing. Deformation took place under amphibolite facies conditions on a retrograde P,T-path. Porphyroclast and recrystallized compositions from bulging recrystallization microstructures differ only in their Or-content and yield a ΔG between mean host grain and mean recrystallized grain composition at fixed P,T-conditions of approximately 5 Joules/10−4 m3. Extreme compositional variations yield approximately 60 J/10−4 m3. The increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100 Joules/10−4 m3 for high values of dislocation densities of 1014 m−2. Thus, the effect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but may be as great as that of deformational energies for larger chemical differences. In the other type of microstructure, porphyroclasts and recrystallized grains in discrete zones differ in their anorthite content. The maximum ΔG induced by the compositional disequilibrium is on the order of 100 J/10−4 m3. This maximum value is of the same magnitude as the ΔG derived from high dislocation densities of 1014 m−2. The resulting combined ΔG is approximately twice as high as for deformational ΔG alone, and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures. The recrystallization mechanism depends on the nature of the driving potential. Grain boundary migration (GBM) and heterogeneous nucleation can release Gibbs free energy induced by compositional disequilibrium, whereas this is not likely for subgrain rotation. Therefore, only GBM and heterogeneous nucleation may link metamorphism and deformation, so that syndeformational recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction.