Comparison of the western Aston-Hospitalet and the Bossòst domes: Evidence for polymetamorphism and its implications for the Variscan tectonic evolution of the Axial Zone of the Pyrenees

J. E. Mezger

The Bossòst and the western Aston-Hospitalet structural and metamorphic domes have similar metamorphic characteristics. Textural observations show garnet and staurolite as earlier phases, andalusite, cordierite and sillimanite as later phases. Metastable staurolite–cordierite–biotite–muscovite assemblages found in both domes indicate a decompressional path from medium pressure–medium temperature regional metamorphism producing garnet and staurolite to a low pressure–higher temperature contact metamorphism resulting in the growth of andalusite, cordierite and sillimanite. In the Bossòst dome these two metamorphic events are separated by a phase of strong non-coaxial deformation which is preserved in a prominent shear zone. The shear zone is inferred to have facilitated decompression as an extensional shear zone. In the Aston-Hospitalet domes, the similarity of the metamorphic characteristics with the Bossòst dome is not matched with the mircostructural evidence. This may be due to the lack of a suitable lithology which prevented subsequent metamorphic phases to form, so that earlier fabrics had been completely obliterated by later thermal and tectonic events. The presence of large migmatite zone along the western Aston gneiss may record such an event. The tectonic conclusions derived from the Bossòst dome can not readily be applied to the Aston-Hospitalet domes.