MICROSTRUCTURES AND METAMORPHIC CRYSTALLIZATION

Environmental factors such as pressure-temperature-time-deformation conditions and paths influence chemical and physical textures in metamorphic rocks. Local factors such as crystallographic orientation of reactant and product phases in metamorphic reactions may also affect textures. Crystallographic orientation data integrated with mineral composition information document grain-scale metamorphic processes in the overall context of P-T-t-d paths. Relative crystal orientations may influence the patterns of reaction textures (symplectites, coronas, pseudomorphs). For example, electron back-scattered diffraction (EBSD) data from kyanite partially replaced by cordierite in a granulite facies rock show a correlation between the orientation of c-axes of kyanite and the surrounding cordierite, independent of the overall fabric of the rock.

Another example of how crystallographic data can be used to understand metamorphic (re)crystallization is the relationship of mineral inclusion shape and orientation to their host porphyroblast. Aligned or elongate mineral inclusions are commonly used to interpret deformation fabrics that formed pre- or syn-porphyroblast growth, but at high grades (> 650 C) the host mineral may influence the shape of inclusions during recrystallization along inclusion-host interfaces. To document the P-T conditions and mechanisms of textural and chemical interaction of garnets and inclusions, we obtained EBSD data from minerals in pelitic schists from a variety of metamorphic terrains that record conditions ranging from the garnet to the sillimanite zone. Quartz, plagioclase, and other inclusions in garnet exhibit incipient facets in upper staurolite and kyanite zone rocks and are well faceted in sillimanite zone and higher-grade rocks. The orientation of facets corresponds to the crystallographic orientation of host garnet.

The influence of garnet on the shape of inclusions at lower grades is more ambiguous: in the garnet zone, the shape-preferred orientation (elongation) of qz inclusions does not correspond to crystallographic orientation of qz or garnet. The shape may be inherited from a matrix geometry or may represent syn-entrapment dynamics between garnet and qz. Ongoing work investigates the influence of P-T-t-d paths on inclusion shape and orientation.