Abstract

Two identical zircon U-Pb ages have been obtained from the Riète orthogneisses at 470 ± 6 and 472 ± 2 Ma in the Aston and Hospitalet domes (Ariège, Pyrenees), respectively. New mapping data show that the protolith of these orthogneisses corresponds to Ordovician granitic laccoliths. Combined study of thin-sections and magnetic susceptibility on these rocks show that the laccoliths correspond to a suite consisting of granodiorites to leucogranites.

U-Pb ages of the Aston and Hospitalet orthogneisses, very similar to the ages recently obtained from the Canigou (473 ± 4 Ma) and the Montagne Noire (southern French Massif Central) 469 ± 4 Ma orthogneisses, point to a major Early to Middle Ordovician event of granitic laccolith emplacement in the southwestern part of France, and more generally in western Europe.

We underline that these laccoliths influenced the mechanical and thermal behaviour of the Variscan crust of the Pyrenees. Indeed, they have induced a rheological heterogeneity in the Variscan middle crust, which is at the origin of a structural contrast between the middle and upper crust. Moreover, these laccoliths played the role of screens, which have controlled transfer of magmas from the lower to the upper crust.

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