Deformation, high-pressure metamorphism and exhumation of ultramafic rocks in a deep subduction/collision setting (Cabo Ortegal, NW Spain)

New petrological and microstructural data from various metaperidotite 'boudins' within large ductile shear zones in the Cabo Ortegal allochthonous complex in NW Spain have important implications for the tectonic models of the area. The peridotites (mylonitic garnet harzburgite, Ti-clinohumite and magnesite–olivine orthopyroxenite) contain mineral assemblages that equilibrated at high- to ultra-high-pressure metamorphic conditions as well as microstructures of tectonic origin formed at temperatures well above 800  °C. Olivine and orthopyroxene fabrics resulted from flow at high temperature (>1000  °C) and solid-state non-coaxial plastic flow at intermediate temperature (800–900  °C). Flow caused dynamic recrystallization and formation of moderate to strong lattice preferred orientations under low to moderate differential stresses and strain rates characteristic of upper mantle and deep crustal deformation. The microstructures and textural relationships suggest that the mylonitic garnet harzburgite represents mantle fragments with lithospheric and asthenospheric imprints, whereas the olivine orthopyroxenite resulted from serpentinite burial to depths where it acquired a characteristic high/ultra-high-pressure metamorphic signature. Both types of ultramafites converged to a common site in a subduction zone that was later incorporated during continental collision to the NW Iberian Massif as exotic, allochthonous complexes that record structural and metamorphic evidence of the earliest phases of the Hercynian orogeny.