Deformation and recrystallization of plagioclase along a temperature gradient: an example from the Bergell tonalite

Abstract

Syn- to post-mylonitic tilting of the Bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies. At the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains. In contrast, at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration. The temperature increase is inferred to induce this transition in two ways. First, at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower. Hence, the chemical driving potential is small compared with recrystallization at lower temperatures. Second, higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility. The temperature increase is also associated with a transition in the dominant deformation mechanism. Intracrystalline plasticity dominates deformation at lower temperatures, whereas at higher temperatures deformation occurs primarily by diffusion-accommodated grain boundary sliding, as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase–biotite matrix with increasing deformation. Therefore, the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase. Lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding.