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

In the Agly Massif (Pyrenees), two Variscan plutons, the Saint-Amac pluton and the Ansignan chamockite, intrude different levels of a c. 10 km thick crustal section. The Saint-Amac 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 D-2 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 chamockite 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-Amac pluton was injected in the upper crust at the beginning of formation of a gneissic dome.