Francisco González Lodeiro

Deformation of garnets in a low-grade shear zone

Elongated shapes of garnets in high-grade metamorphic rocks have been explained as a result of plastic crystal flow or anisotropic growth. In the case of low- to medium-grade metamorphic rocks, a satisfactory explanation has not been proposed yet for elongated garnets. We have studied elongated garnets grown under low- grade metamorphic conditions in a shear zone with a composite planar-linear fabric. Garnet shapes in three dimensions define oblate ellipsoids. Drawing on evidence from compositional X-ray maps, it can be deduced that growth zoning is truncated along the long borders of grains, whereas subcircular garnets show non-truncated concentric growth zoning. This fact shows that selective dissolution along planes parallel to the foliation and the C surfaces can be claimed to be the main mechanism responsible for the deformation of the garnets under examination here. More specifically, dislocation-enhanced dissolution, which occurs at low temperatures and low dislocation mobility, is arguably the mechanism responsible for partially dissolving the garnet grains. Selective dissolution is expected to yield plane-strain oblate ellipsoids (i.e. with a volume loss) as measured in the elongated garnets. However, the rock as a whole has been deformed by plastic flow in a simple-shear regime, as shown by the existence of numerous shear criteria and the strain calculations performed. 0 1997 Elsevier Science Ltd.