The role of magmatic reaction, diffusion, and annealing in the evolution of coronitic microstructure in troctolitic gabbro from Risör, Norway: a discussion

Abstract

AbstractThe data of Joesten (1986) are re-interpreted. The petrography of the coronas is not consistent with magrnatic origin. Both microstructural types described by Joesten (1986), here re-named ‘columnar’ and ‘tabular’, formed by solid-state replacement of plagioclase and of adjacent olivine or ilmenite. Tabular microstructures are not annealed, but result from overgrowth or epitaxy of amphibole and pyroxene on pre-existing grains. Since the diffusion-controlled models of Joesten (1986) can account for major aspects of the coronas, it seems possible that a slightly modified, less simplified theory might explain them fully. Open-system behaviour must be admitted, with some constraints provided by symplectites. It may also be necessary to develop the theory in more than one dimension, and to allow for departures from local equilibrium at layer boundaries.