Deformation of mantle pyroxenites provides clues to geodynamic processes in subduction zones: Case study of the Cabo Ortegal Complex, Spain
Résumé
In the Herbeira massif, Cabo Ortegal Complex, Spain, a well exposed assemblage of deformed dunites and pyroxenites offers a unique opportunity to investigate key upper mantle tectonic processes. Four types of pyroxenites are recognized: clinopyroxenites with enclosed dunitic lenses (type-1), massive websterites (type-2), foliated and commonly highly amphibolitized clinopyroxenites (type-3) and orthopyroxenites (type-4). Field and petrological observations together with EBSD analysis provide new insights on the physical behavior of the pyroxenes and their conditions of deformation and reveal the unexpected journey of the Cabo Ortegal pyroxenites. We show that, during deformation, type-1 pyroxenites, due to their enclosed dunitic lenses, are more likely to localize the deformation than types-2 and -4 pyroxenites and may latter act as preferred pathway for fluid/melt percolation, eventually resulting in type-3 pyroxenites. All pyroxenite types display a similar response to deformation. Orthopyroxene deformed mostly by dislocation creep; it shows kink bands and undulose extinction and its fabric is dominated by [001](100). Clinopyroxene displays subgrain rotation, dynamic recrystallization and fabric with [010] axes clustering next to the foliation pole and [001] axes clustering next to the lineation suggesting activation of [001]{110} and [001](100) in some samples. These observations are in good agreement with deformation at temperatures greater than 1000 °C. Olivine in type-1 and type-4 pyroxenites shows [100](010) or [001](010) fabrics that are consistent with deformation at temperatures >1000 °C and may indicate deformation in a hydrous environment. The amphibole [001](100) fabric gives insights on a lower-temperature deformation episode (∼800 to 500 °C). Our results, interpreted in the light of published experimental data, together with the regional geological and geochemical studies are consistent with the following tectonic evolution of the Cabo Ortegal pyroxenites: (1) delamination from an arc root in a mantle-wedge setting at temperatures above 1000 °C and (2) introduction into a relatively softer subduction channel where deformation was accommodated by localized shear zones, thus preserving the high-temperature fabrics of pyroxenites. The Cabo Ortegal pyroxenites may therefore be seen as a rare exposure of deformed mantle-wedge material.