Microstructures and crystallographic preferred orientations of omphacite in Alpine eclogites: implications for the exhumation of (ultra-) high-pressure units

Microstructures and crystallographic preferred orientations (CPOs, textures) of omphacite within eclogites from distinct (ultra-) high-pressure units of the Alps were analysed in order to constrain the deformation conditions during and after high-pressure metamorphism. Especially the omphacite CPOs bear significant details for the reconstruction of the deformational evolution of several units. CPOs from the Austroalpine Koralm-Saualm Complex correspond to S-type fabrics, related to a deformation geometry within the flattening field. This geometry is documented by the shape fabrics, too. Textures described from the Dora Maira Massif show L > S- and L-type CPO fabrics, indicating a plane strain to constrictional deformation geometry. The CPOs from the Adula Nappe may all be described in terms of very well developed L-type fabrics formed within the constrictional field. Constrictional strain (axial elongation) is interpreted to have started at the peak of high-pressure metamorphic conditions and was continued,along the retrograde path. Omphacite CPOs from the Eclogite Zone of the Tauern Window show a continuous transition from S- to L-type fabrics, corresponding to the transition from,coarse-grained eclogites to fine-grained eclogite mylonites. This evolution is related to the final phases of the prograde evolution and coincides with a transition from a flattening to a constrictional deformation geometry. This is indicated by the shape fabrics, too. The constrictional geometry prevailed along the retrograde section of the PT path. The evolution of microstructures and CPOs is assumed to correlate with the deformation geometry, which is directly linked with the mechanisms of exhumation. S-type fabrics predominantely occur within eclogites exhumed by crustal extension, in particular the Koralm-Saualm Complex. L > S- and L-type fabrics predominantely occur within eclogites exhumed by extrusion within a low-angle corner (subduction channel), in particular the Dora Maira Massif, the Adula Nappe, and the Eclogite Zone. S-type CPOs from the Eclogite Zone were formed along the prograde path and are assumed to indicate compressional deformation related to the burial of this unit. (C) 2003 Elsevier Ltd. All rights reserved.