MICROSTRUCTURES AND FABRIC OF AN E-TYPE SHEAR ZONE WITHIN THE RONDA PERIDOTITE MASSIF, SOUTHERN SPAIN

The Ronda peridotite massif is texturally and mineralogically zoned, containing in structural sequence from bottom to top: plagioclase tectonites, granular spinel peridotites, spinel tectonites, and garnet-spinel bearing mylonites, which represent equilibration within a thermal gradient during exhumation of the body from >2 GPa to <0.8 GPa. The upper tectonites and mylonites together represent the shear zone by which the peridotite was exhumed to crustal levels. The lower plagioclase-bearing tectonites likely represent lower-pressure deformation associated with the emplacement into the crust. The coarse-grained granular spinel zone is cross-cut by a narrow shear zone with distinct microstructures and fabric. This shear zone comprises a banded mylonite consisting of slightly coarser (227 μm) pure olivine bands; finer (90 μm) bands of mixed olivine, pyroxene, and spinel; and porphyroclasts of pyroxene that are either round or stretched, some to aspect ratios of 1:10 or more. Asymmetrical recrystallized tails on orthopyroxene and oblique new grain shape fabrics indicate top to the northwest sense of shear. Olivine in this shear zone has an E-type crystallographic lattice-preferred orientation produced by slip on (001) in the [100] direction. E-type fabrics have been proposed to occur at high water content (200-1000 ppm H/Si) and low stress (Jung et al., 2006). Samples taken meters away from the shear zone contain olivine porphyroclasts with sharp subgrain boundaries, and neoblasts comparable in size to subgrain width and show new grain orientations compatible with subgrain rotation.