Petrologic and thermobarometric study of the Riás schists (NW Iberian Massif)

Abstract

The Riás Schists crop out in the so-called Iberian Variscan parautochthon, surrounding the Malpica-Tui Complex (NW Iberian Massif), as part of the westernmost internal areas of the European Variscan belt. Three Variscan metamorphic events have been identified in the Riás Schists (M1, M2 and post-M2). M1 comprise unoriented microinclusions that have only been identified in garnet porphyroblast cores and inside staurolite crystals. M2, comprises the matrix foliation (S2) and the assemblage garnetRIM + staurolite + muscovite + biotite + chlorite + rutile/ilmenite + magnetite + quartz and the development of syntectonic andalusite. Finally, post-M2 (post-S2) includes andalusite and plagioclase together with secondary muscovite, biotite and chlorite and accessory tourmaline, Fe/Ti oxides, apatite and carbonate. Results of multi-equilibrium thermobarometry (pressure-temperature pseudosections), for M2 indicates a medium-pressure Barrovian metamorphic event (0.5-0.6 GPa; 580-570 ºC; ca. 20 km deep), followed by moderate cooling during decompression, together with the extensional collapse of the Variscan orogenic pile (post-M2;0.3 GPa; 540 ºC; ca. 10 km deep). Inferred P T conditions in the Riás Schists, and their spatial relationship with the overlying allochthonous high-pressure pelitic schists (i.e. Ceán Schists), suggest that both lithologies were part of the same continental margin during the beginning of the Variscan orogeny. However, they experienced very different tectonothermal evolutions due to their putative locations in the orogenic wedge.