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Journal of the Geological Society

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Structure and U–Pb dating of the Saint-Arnac pluton and the Ansignan charnockite (Agly Massif): a cross-section from the upper to the middle crust of the Variscan Eastern Pyrenees

¹ LMTG–UMR 5563, Université de Toulouse, CNRS, IRD, OMP, 14 Avenue Edouard Belin, 31400, Toulouse, France (e-mail: olivier{at}lmtg.obs-mip.fr)
² CNRS–UMR 6524 Magmas et Volcans, Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France
³ Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile

In the Agly Massif (Pyrenees), two Variscan plutons, the Saint-Arnac pluton and the Ansignan charnockite, intrude different levels of a c. 10 km thick crustal section. The Saint-Arnac pluton intrudes through upper crustal rocks and the Ansignan charnockite cuts mid-crustal country-rocks. A structural study of these plutons based on the anisotropy of magnetic susceptibility technique, combined with a kinematic study of the country-rocks, shows that the structures and emplacement modes of the plutons are compatible with those of the other plutons of the Pyrenees emplaced during the D₂ transpressive phase. U–Pb dating on zircons from the Saint-Arnac pluton yields a 308.3 ± 1.2 Ma age for a diorite and a 303.6 ± 4.7 Ma age for a granodiorite. The charnockite was previously dated at 315 Ma. The emplacement ages of these two intrusions are thus separated by at least 5 Ma. First, numerous sills and laccoliths, such as the Ansignan laccolith, were injected in the middle crust, and this induced heating and thickening. Subsequently, the large Saint-Arnac pluton was injected in the upper crust at the beginning of formation of a gneissic dome.