Crystal heterogeneity controlling the grain size of dynamically recrystallized plagioclase

Recrystallization of plagioclase is induced by deformation and/or chemical disequilibrium. It can be accomplished by several deformation mechanisms or their combination and it is typically accompanied by a change in plagioclase composition. The known agents facilitating the recrystallization are dislocations, mechanical twinning and cracking. In this contribution, we present incipient stages of dynamic recrystallization imposed on the magmatic plagioclase crystals in metagabbro from the Teplá-Barrandian Unit in the Bohemian Massif. The plagioclase crystals show chemically and mechanically heterogeneous internal structure related to its metamorphic-deformation transformation.

The chemical heterogeneity is manifested by decomposition of magmatic porphyroblasts of labradorite composition to the mixture of randomly oriented laths of bytownite and surrounding andesine. Crystallographically the laths are perfectly coherent with the rest of the crystal. The mechanical heterogenity is due to subsequent deformation that led to mechanical twinning followed by recrystallization. The initial low angle boundaries of the newly developing grains follow the network of bytownite laths while the later high angle boundaries are based on the original laths together with segmented twin boundaries. The resulting recrystallized microstructure shows small individual grains with andesine cores and bytownite rims. The misorientation analysis of the low angle boundaries indicate the geometry of till and twist boundaries resulting from dislocation glide and operation of (010)[001] slip system. Once the high angle boundaries are established they start to migrate and equillibrate, driven by chemical disequilibrium at the bytownite-andesine interfaces. The resulting fine-grained plagioclase shows evidence for grain size sensitive creep during subsequent deformation. Our findings indicate that crystal heterogeneity in feldspars may be an important parameter in the grain refinement process thus influencing the switch from dislocation creep to viscous flow.