OMPHACITE FABRIC VARIATION IN THE CABO-ORTEGAL ECLOGITE (NW SPAIN) - RELATIONSHIPS WITH STRAIN SYMMETRY DURING HIGH-PRESSURE DEFORMATION

Petrostructural study of Cabo Ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice-preferred ori entation and the fundamental deformation parameters, in particular str ain symmetry. Microstructural evidence is consistent with a tectonic o rigin of eclogite fabrics by coeval pyroxene plastic deformation and r ecrystallization under eclogite-facies metamorphic conditions. Omphaci te shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constric tion. 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. Om phacite crystallographic fabrics are consistently asymmetric with resp ect to the structural framework defined by foliation and lineation. Th is suggests that nan-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 de termination 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 de grees C, low differential stress (< 15-20 MPa) and slow strain rate (1 0(-12) - 10(-15) s(-1)). (C) 1997 Elsevier Science Ltd.