SYNDEFORMATIONAL RECRYSTALLIZATION - DYNAMIC OR COMPOSITIONALLY INDUCED

Dynamic recrystallization in the strict sense of the term is the recon stitution of crystalline material without a change in chemical composi tion, driven by strain energy in the form of dislocations. Driving pot entials additional to internal strain energy may contribute to the rec rystallization of naturally deformed minerals, which form solid soluti ons such as feldspar, amphiboles and pyroxenes, if they change their c omposition during recrystallization. To estimate the relative importan ce of these driving potentials, the chemical composition of porphyrocl asts and recrystallized grains of plagioclase, clinopyroxene and hornb lende have been investigated in samples from a high grade shear zone o f the Ivrea Zone, Italy. The plagioclases show two different recrystal lization microstructures: bulging recrystallization at grain boundarie s and discrete zones of recrystallized grains across porphyroclasts pr obably involving fracturing. Deformation took place under amphibolite facies conditions on a retrograde P,T-path. Porphyroclast and recrysta llized compositions from bulging recrystallization microstructures dif fer only in their Or-content and yield a Delta G between mean host gra in and mean recrystallized grain composition at fixed P,T-conditions o f approximately 5 Joules/10(-4) m(3). Extreme compositional variations yield approximately 60 J/10(-4) m(3). The increase of free energy due to dislocations calculated for common glide systems in plagioclase ar e on the order of 100 Joules/10(-4) m(3) for high values of dislocatio n densities of 10(14) m(-2). Thus, the effect of chemically induced dr iving energies on grain boundary velocity appears small for mean compo sitions but may be as great as that of deformational energies for larg er chemical differences. In the other type of microstructure, porphyro clasts and recrystallized grains in discrete zones differ in their ano rthite content. The maximum Delta G induced by the compositional diseq uilibrium is on the order of 100 J/10(-4) m(3). This maximum value is of the same magnitude as the Delta G derived from high dislocation den sities of 10(14) m(-2). The resulting combined Delta G is approximatel y twice as high as for deformational Delta G alone, and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures. The recrystallization mechanism depe nds on the nature of the driving potential. Grain boundary migration ( GBM) and heterogeneous nucleation can release Gibbs free energy induce d by compositional disequilibrium, whereas this is not likely for subg rain rotation. Therefore, only GEM and heterogeneous nucleation may li nk metamorphism and deformation, so that syndeformational recrystalliz ation may represent a transitional process ranging from dynamic recrys tallization to metamorphic reaction.