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Bulletin de la Societe Geologique de France; May-June 2009; v. 180; no. 3; p. 209-216; DOI: 10.2113/gssgfbull.180.3.209
© 2009 Societe Geologique de France
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Middle Ordovician U-Pb age of the Aston and Hospitalet orthogneissic laccoliths: their role in the Variscan evolution of the Pyrenees

Yoann Denele1, Pierre Barbey2, Etienne Deloule2, Ewan Pelleter2, Philippe Olivier1 and Gérard Gleizes1

1 LMTG – UMR 5563, Université de Toulouse, CNRS, IRD, OMP, 14 avenue E. Belin, F-31400 Toulouse. denele{at}lmtg.obs-mip.fr
2 CRPG, Nancy-Université, CNRS, BP 20, F-54501 Vandœuvre-lès-Nancy

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.

Key Words: Pyrenees • Aston • Hospitalet • Gneiss dome • Laccolith • Ordovician • U-Pb dating






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