Metastable staurolite-cordierite assemblage of the Bossòst dome : Late Variscan decompression and polyphase metamorphism in the Axial Zone of the central Pyrenees

A kilometre-scale shear zone is recognized in the Cambro-Ordovician schist of the Bossòst dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and staurolite porphyroblasts following regional metamorphism M1, while coaxial conditions prevailed during later contact metamorphic M2 growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet-staurolite-andalusite-cordierite assemblages are significantly enriched in Mg and Mn over the garnet-staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet-staurolite assemblage preserves conditions during M1, estimated by AFM diagrams and P-T pseudosections to be 5.5 kbar and 580 °C, respectively. Pseudosections also indicate that staurolite is not a stable phase in cordierite-andalusite assemblages of M2, suggesting polyphase metamorphism and decompression along a clockwise P-T path for the staurolite-cordierite-andalusite assemblages. This concurs with proposed extensional tectonics along the regional shear zone. To cite this article: J.E. Mezger et al., C. R. Geoscience 336 (2004)