Constraining the thermal gradient in shear zones bounding the Chimparra high-pressure gneisses of the Cabo Ortegal Complex (NW Spain): an EBSD approach

The petrographic study and structural analysis of high-pressure rocks can bring to light some of the mechanisms involved in the subduction and subsequent exhumation of the crustal materials affected by these processes. In the Cabo Ortegal Complex (NW Spain) the emplacement, amalgamation and progressive deformation of still hot peridotites between the Chimparra gneisses and the Bacariza high-pressure granulites imposed a thermal gradient in the shear zones developed at the contact with the high-pressure gneissic formation. This gradient lead to a change of deformation mechanisms and operative intracrystalline slip systems in all the constituent minerals during initial stages of the exhumation of the complex in the subduction channel.

In external areas of the shear zone affecting the gneissic materials, large garnets accommodated strain by rigid rotation. However, close to the shear zones elongated garnets behaved more plastically assisted by 1/2<111>{110} and <100>{010} intracrystalline slip systems. Kyanite, in turn, shows kink bands, sigmoidal geometries and subgrain boundaries disposed subperpendicular to the X structural direction. This feature suggests the activation of [001](100) and [001](010) intracrystalline slip systems in outer sectors and glide on (100) planes along the [001] direction in areas closer to the contact. Although oligoclase does not show systematic orientation distribution patterns in distant sectors the lattice preferred orientation (LPO) obtained at the contact indicates clearly the activation of [100](001) intracrystalline slip systems which have been recognized in rocks deformed under medium- to high-grade conditions. This is in accord with the operation of prism-<a> slip systems in quartz for those samples closer to the ultramafic massif, indicative of temperature conditions up to 700º C. Away from the contact, the temperature decreases, leading to activation of basal- and rhomb-<a> systems in this phase suggesting lower deformation temperature or, more likely, higher strain rates along narrower sectors of the subduction channel accommodating deformation.