Metamorphic and structural studies show that the Bosost dome (Central Pyrenees) is a metamorphic as well as a structural dome developed in Cambro-Ordovician to Devonian metasediments around a migmatitic core during the Hercynian orogeny. The structural-metamorphic dome occurs in epizonal terranes, and is twice as large as originally described. The structural dome shows a concentric zoning of D2 and D3 local deformations developed around the migmatites and superimposed on the D1 regional Hercynian deformation. The structural data suggest that migmatites were emplaced diapirically into Palaeozoic metasediments during D1, and are responsible for the local D2 and D3 deformations. An early medium pressure–high temperature metamorphism occurred during D1, leading to anatexis under water-rich conditions, then evolved progressively to a high temperature-low pressure metamorphism during the D2 and D3 deformations. The corresponding P–T-time path is a clockwise loop representing the evolution of the metamorphism, from migmatiza-tion to the diapiric emplacement of migmatites, in a compressional setting. Pressure values obtained from geobarometry are three times as high as expected from sedimentological data. This suggests that the Hercynian metamorphism developed in a thickened crust during the N-S shortening responsible for the large-scale E–W folding observed throughout the Pyrenean Axial Zone.

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