NEW INSIGHTS INTO PROCESSES OF SYMPLECTITE FORMATION FROM HIGH-RESOLUTION X-RAY COMPUTED TOMOGRAPHY AND EBSD

Metamorphic reaction textures are chemical and physical records of metamorphic P-T paths and reaction rates. High-resolution X-ray computed tomography (HRXCT) and electron backscatter diffraction analysis (EBSD) allow investigation of textures in 3D and at multiple scales, allowing investigation of reaction domain size, diffusion length scales and pathways, crystallographic relationships of fine-scale intergrowths, and other information relevant to application of phase equilibria modeling of reactions and P-T histories.

Crd-Ged/Bt gneiss from the Thor-Odin dome, BC, Canada, contain coronal reaction textures after Al2SiO5 and garnet. The gneiss experienced isothermal decompression from 10 kbar, >700C, is crosscut by Crd-Qz veins, and is surrounded by migmatite. Corona formation may have been driven by rapid decompression and/or by infiltration of H2O associated with crystallizing migmatites/veins, processes that may be recorded in chemical and physical features of the reaction textures.

Coronas on Ky that has been partially to completely pseudomorphed by Sil are comprised of successive two-phase, symplectitic assemblages: An + Spl (innermost), Crd + Crn, Crd + Spl, and these are surrounded by a moat of polygonal Crd. HRXCT data reveal a high-degree of interconnectivity between Ky/Sil-hosted symplectitic corona and corona centered on garnet. This interconnectivity defines the size of the chemical system, and may account for the presence of anorthite rims on Ky/Sil. EBSD analysis coupled with serial sectioning reveals that Spl vermicules form narrow domains of interconnected networks emanating from the central Ky/Sil. Spl vermicules are surrounded by Crd or An crystals aligned parallel to the shape orientation of Spl. These relationships are critical to identifying the growth mechanism(s) active during symplectite formation. HRXCT and EBSD analyses together provide an integrated physical and chemical view of reaction texture formation, and may result in a clearer understanding of the relationships between textural features and the driving forces of metamorphic reactions.