Deformation and emplacement of late hercynian leucogranites in the french Massif Central from AMS, Gravity and Geochronology data.

Hercynian leucogranitic plutons crop out over large areas in the French Massif Central hiding the earlier crustal structures. In the Limousin area, these granitic intrusions are spatially associated with normal faults and major strike-slip shear zones that continue the South Armorican shear zone. New 40Ar/39Ar dating of wrench fault mylonites in Limousin yield ages bracketed between 310 Ma and 320 Ma. These faults which constitute the boundaries of hercynian leucogranites played a significant role in their emplacement and subsequent exhumation. From the example of the Millevaches granitic complex located in the middle of the French Massif Central, our study intends to highlight the massif structure at depth and to discuss the mode of emplacement of granites in intracontinental extensional context and the relationships between thermal and tectonic events experienced by the Hercynian orogen in Limousin between 360 and 300 Ma. Analysis and inversion of the residual Bouguer anomaly show that the N-S trending Millevaches massif is a 2 to 4 km thick laccolith. On the basis of AMS measurements, the micas subfabric suggests that the magnetic foliation and lineation display a general NW-SE sub-horizontal pattern on both sides of the N-S Pradines dextral wrench fault that deforms the core of the massif on 4 to 5 km width. The new U/Pb and 40Ar/39Ar dates around 315 Ma comply with the microstructural observations made on leucogranite mylonites show that the activity of the Pradines fault is coeval with granite emplacement. We propose that once the magma, reached the upper crust through the Pradines dextral wrench fault, it spread-out laterally into the sub horizontal micaschist foliation. The weak gravity anomaly below the Pradines fault, suggests that narrow and unstable feeding zones disappeared after the passage of magma.