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. I n the case of:ow-to medium-grade metamorphic rocks, a satisfactory exp lanation has not been proposed yet for elongated garners. We have stud ied elongated garnets grown under low-grade metamorphic conditions in a shear zone with a composite planar-linear fabric. Garnet shapes in t hree dimensions define oblate ellipsoids. Drawing on evidence from com positional X-ray maps, it can be deduced that growth zoning is truncat ed 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 deformati on of the garnets under examination here. More specifically, dislocati on-enhanced dissolution, which occurs at low temperatures and low disl ocation mobility, is arguably the mechanism responsible for partially dissolving the garnet grains. Selective dissolution is expected to yie ld plane-strain oblate ellipsoids (i.e. with a volume loss) as measure d in the elongated garnets. However, the rock as a whole has been defo rmed by plastic flow in a simple-shear regime, as shown by the existen ce of numerous shear criteria and the strain calculations performed. ( C) 1997 Elsevier Science Ltd.