Crystallographic orientation, chemical composition and three-dimensional geometry of sigmoidal garnet: evidence for rotation

Studies of the microstructure, crystallographic orientation, chemical composition and three-dimensional shape of sigmoidal garnet reveal a number of features that can be used to discuss its origin, and in particular to distinguish between rotational versus non-rotational models. Crystallographic orientation mapping of sigmoidal garnet shows no evidence of subgrain formation or of being polycrystalline, suggesting that neither ductile nor brittle deformation is significant. Chemical mapping shows that the garnet grew during a single metamorphic event, arguing against ideas that sigmoidal garnet forms as the result of a series of independent growth events. The chemical mapping also reveals anisotropic growth of garnet with the long axis of the ellipsoidal grain rotating in the same direction but to a lesser degree than the inclusion trails. This is best explained as the result of syn-growth rotation of the garnet with respect to the foliation and maximum growth direction. High-resolution X-ray CT scanning shows that the inclusion trails have a complex three-dimensional spiral geometry. This type of geometry is predicted by rotational models of sigmoidal garnet formation.