Low- to medium-pressure Variscan metamorphism in Galicia (NW Spain): evolution of a kyanite-bearing synform and associated bounding antiformal domains

This paper is included in the Special Publication entitled 'What drives metamorphism and metamorphic reactions?' edited by P.J. Treloar and P.J. O'Brien. Evidence for different substitution relationships between aluminosilicates in two domains east of the Ollo de Sapo gneiss antiform in Galicia (NW Spain) suggests a high dependence of metamorphic evolution on the local tectonic and granitoid intrusion history. The Vivero pelite belt constitutes a pinched synformal domain located between the Ollo de Sapo antiform to the west and the Lugo dome to the east. Within this synform, black, Al-rich Silurian pelites contain kyanite that, along with muscovite, formed pseudomorphically after early pre-D 2 andalusite. Syn-D 2 inversion of andalusite to kyanite indicates an evolution from early, low-P to medium-P regimes. Locally, near the Vivero transtensional fault, which constitutes the E boundary of the synform new, late post-D 2 pleochroic andalusite grew from staurolite and biotite indicating reinstallment of a new low-P regime during and after the D 3 deformation phase, which is related to movement along the fault zone. The Ollo de Sapo domain contains only a high T-low P series of metamorphic assemblages. The peak assemblage within this domain was cordierite-biotite-K-feldspar-andalusite/fibrolite related to emplacement of a pre- to syn-D 2 swarm of sheet-like peraluminous granitoids located mainly in the core of the antiform. Early pre- to syn-D 2 staurolite-biotite±garnet, and late sillimanite-staurolite- biotite assemblages present on the west side of the Lugo dome testify to an early stage of nearly isothermal decompression followed by decompression with local heating. Stabilization of late andalusite in the Vivero pelite belt and of sillimanite west of the Lugo dome appear almost synchronous and clearly related to a syn-D 3 thermal peak induced by Upper Carboniferous granitoid intrusion along the Vivero transtensional fault separating both domains.