Corona networks as three-dimensional records of transport scale and pathways during metamorphism

Abstract

Coronal textures in metamorphic rocks are physical evidence that diffusion was the rate-limiting process in the local reactions that produced these layered structures. Mass balance, however, requires that chemical communication must also take place at length scales significantly greater than those of the corona. Evidence for the anatomy and scale of pathways along which chemical communication occurred is rarely recorded in metamorphic rocks, but orthoamphibole-cordierite gneiss from the Thor-Odin dome, British Columbia (Canada), preserves multi-layered coronal reaction textures around garnet and Al2SiO5 connected by anastomosing networks of cordierite and biotite that represent relict pathways along which mass transport occurred during reaction texture development. Calculations of component fluxes between reaction textures and surrounding mineral assemblages indicate that relatively long-distance (centimeter-scale) diffusional exchange was required for development of ∼150–500-μm-thick corona. Results of electron backscattered diffraction analysis and high-resolution X-ray computed tomography give new information about the scale and pathways of chemical communication during high-grade metamorphism, and show that length-scales of material transport were two orders of magnitude greater than those of the reaction textures.