Description of metamorphic and tectonic events in the Cambro-Ordovician metasedimentary rocks of the western Aston and Hospitalet domes is best accomplished by comparison with the evolution of the Bossòst dome (Table 3; Mezger & Passchier 2003; Mezger et al. 2004).
In the Bossòst dome nucleation of the earliest spessartine garnets occurred during early regional metamorphism M1. Crenulation folds are preserved as garnet inclusion trails. Next, staurolite and almandine garnet grew during periods of deformational quiescence. A phase of intense non-coaxial shearing, evident in rotated staurolite porphyroblasts, separates earlier staurolite growth from nucleation of andalusite. There is clear textural evidence that andalusite consumes staurolite (reaction 4) and andalusite grew during coaxial deformation. In the Aston and Hospitalet domes crenulation folding is preserved in garnet, staurolite and andalusite. In other localities coexisting garnet, staurolite and andalusite overgrew an already existing foliation. The main difference to the Bossòst dome is that staurolite and andalusite belong to the same paragenesis, since there is no evidence in these rocks of growth of one phase on the expense of the other. Also, there is no distinct deformational event that would separate metamorphic phases. Preservation of crenulation folding in one case and a well developed foliation in another could be interpreted as development of the assemblages during different times, or a different structural environment during metamorphism. At the moment there is no definite answer to that problem.
In the Bossòst dome simultaneous breakdown of staurolite to andalusite and cordierite, and contemporaneous consumption of andalusite by cordierite is evident. This can be linked to the intrusion of the late Variscan (?) Bossòst granodiorite. In the Aston and Hospitalet domes, consumption of staurolite by andalusite, staurolite and andalusite by cordierite can also be inferred from textural relationships. Peak metamorphic temperatures are reached in the sillimanite-biotite schist, which are found within the migmatite zone, close to the contact with Aston orthogneiss. The important question is, which thermal event is responsible for peak metamorphism? The intrusion of the protolith of the Aston and Hospitalet orthogneisses would create a contact aureole of considerable width, if intrusion occurred in mid-crustal depth. However, the foliation that was imparted on the orthogneiss and the mantling metasedimentary rocks predates development of andalusite and cordierite, with the exception of some staurolite assemblages. The next possible event, the migmatization of the mantling metapelitic rocks obviously effects already foliated rocks, and contains the rocks of the highest metamorphic grade. Lacking any geochronological data, timing of migmatization is still unclear. It may well be related to an earlier phase of late Variscan plutonism, that resulted in the intrusion of the Bassiès pluton to the northwest of the Aston dome, and emplacement of the Soulcem plutons. Alternatively, cordierite and sillimanite growth of the Soulcem thermal dome is of younger age than metamorphism along the western margin of the Aston migmatite zone (Verhoef et al. 1984).
In the Bossòst dome no significant non-coaxial deformation occurred after peak metamorphism. In the Aston dome top-to-the-ESE shear is recorded in c'-type shear bands indicating hanging-wall up motion that clearly post-date peak-metamorphism (Figure 13). At the top of the Hospitalet gneiss further east van den Eeckhout (1986) postulated an extensional shear zone, the Contact High Strain Zone, was developed following peak-metamorphism, agreeing with observations in the western Aston dome.
Figure 13. Well developed c'-type shear bands
Well developed c'-type shear bands in an andalusite-sillimanite schist from south of Etang d'Izourt, close to the western migmatites. Note that the shear bands deflect the fibrolite, indicating post-peak metamorphic non-coaxial deformation, with a top-to-the-SE sense of shear.
Both the Bossòst and the Aston-Hospitalet domes are divided into two parts by a late fault or mylonite zone. While in the Bossòst dome the faulted zone is narrow, the mylonite zone separating the Aston and Hospitalet gneiss in the central part can be up to one kilometre wide (Besson 1991). In both regions, the northern block records higher metamorphism. This suggests that the Bossòst and the Merens faults accompanied significant vertical motion, prior to their development into latest Variscan or Alpine mylonite zones (Mezger & Passchier 2003).