MODELING SYMPLECTITES

Symplectites are fine-grained intergrowths of two or more phases, and are common in high-grade metamorphic rocks that experienced a rapid decrease in pressure or temperature. Common models for symplectite formation invoke oxidation or exsolution, but many symplectites (e.g., those associated with aluminous porphyroblasts in granulites and eclogites) are best explained by net-transfer reactions. We are investigating the growth and textural evolution of symplectites associated with Al2SiO5 and garnet in gedrite amphibolite from the Thor-Odin dome, BC, Canada. The symplectites consist of vermicular spinel +/- sapphirine +/- corundum in coronas of anorthite (inner shell) and cordierite (outer) surrounding resorbed kyanite/sillimanite. BSE images, X-ray maps, and electron backscatter diffraction (EBSD) analyses from serially polished slices allow detailed imaging of physical and chemical patterns in 3 dimensions. Spinel crystals that are vermicular in thin section (2-D) are seen in 3-D to form an interconnected, branching network. EBSD data document a relationship between spinel orientation and the orientation of the host phase (An, Crd). Cordierite crystals are strongly aligned, with the c-axes parallel to the c-axis of the central Al2SiO5 crystal; spinel hosted by Crd shows a weak crystallographic preferred orientation (CPO). In contrast, spinel hosted by anorthite shows a very strong CPO within each anorthite grain, although anorthite crystals show no overall CPO around the central Al2SiO5. We are using modeling, in part guided by EBSD data for crystal growth parameters to i) test a new growth model for net-transfer type symplectites, and ii) test whether there is primary crystallographic control of symplectite textures by the resorbing or neocrystallizing phases. Preliminary results for random crystal growth using 3-D diffusion limited aggregation models suggest that first-order, symplectite-like patterns can form without crystallographic control by any phase. With the ultimate goal of using symplectite textures to determine quantitative metamorphic variables (rates, paths), our ongoing work seeks to understand the relationship between 3-D symplectite patterns and crystallographic orientation of phases in the context of geometric, thermodynamic, and kinetic factors.