Omphacite fabric variation in the Cabo Ortegal eclogite (NW Spain): relationships with strain symmetry during high-pressure deformation

Abstract

Petrostructural study of Cabo Ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice-preferred orientation and the fundamental deformation parameters, in particular strain symmetry. Microstructural evidence is consistent with a tectonic origin of eclogite fabrics by coeval pyroxene plastic deformation and recrystallization under eclogite-facies metamorphic conditions. Omphacite shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constriction. Omphacite lattice-preferred orientations are dominated by [010]-axis maxima normal to the foliation ( S-type or flattening fabrics), by [001]-axis maxima parallel to the lineation ( L-type or constriction) or correspond to other fabric patterns described in the literature. Omphacite crystallographic fabrics are consistently asymmetric with respect to the structural framework defined by foliation and lineation. This suggests that non-coaxial deformation components (either simple or general shear) accompanied eclogite fabric development in addition to any flattening or constriction strain paths, and permit shear-sense determination. These petrostructural features were acquired in the course of eo-Hercynian ductile deformation (probably during deep subduction) under 1.5 − 1.7 GPa confining pressure, temperatures above 600–700 °C, low differential stress (< 15–20 MPa) and slow strain rate (10 −12−10 −15s −1).