DRIVING FORCES FOR SYMPLECTITE DEVELOPMENT ON KYANITE AND SILLIMANITE, THOR-ODIN GNEISS DOME, BRITISH COLUMBIA, CANADA

Granulite facies Mg-Al-rich rocks from the Thor-Odin gneiss dome, BC, Canada, contain complex replacement assemblages after kyanite/sillimanite. Microstructures in samples with similar bulk chemistry vary systematically as a function of structural position in the dome, and therefore possibly as a function of decompression magnitude and/or rate. We use integrated results from field observations, mineral and bulk rock composition, crystallographic orientation, pseudosection analysis, and X-ray computed tomography to evaluate symplectite textures in the context of bulk composition and P-T paths. Samples from the dome margin contain kyanite with little to no replacement by sillimanite. Kyanite is rimmed by cordierite or cordierite + dendritic corundum. Samples from the dome core contain complete or near complete transformation of kyanite to sillimanite, with replacement by an inner rim of anorthite + spinel and an outer rim of cordierite + spinel, +/- corundum and sapphirine in both layers. Samples from intermediate structural positions contain spinel, corundum, and partial replacement of kyanite by sillimanite. 3D pattern analysis demonstrates that spinel and, more rarely, corundum, occur as 3D and 2D dendritic arrays, respectively. The degree of symplectite/corona development correlates with the degree of transformation of kyanite to sillimanite, possibly as a result of the difference in surface energy of sillimanite vs. kyanite and increased nucleation sites and/or to the isothermal decompression path. These differences cannot be accounted for by variation in bulk chemistry, hence our interpretation that the P-T-t path and crystallographic controls of kyanite vs. sillimanite were major controlling factors in symplectite development and textural variations. Future work will utilize P-T-t data and diffusion controlled, disequilibrium pattern formation models to quantify the relationship between rates/path and development and evolution of these textures.