Folding and granite emplacement inferred from structural, strain, TEM and gravimetric analyses: the case study of the Tulle antiform, SW French Massif Central

In the South Limousin (French Massif Central), the Tulle anticline is a kilometre-scale, upright, open fold which re-folds an already foliated Variscan stack of nappes. Three main granodioritic plutons associated with numerous folded dykes and boudinaged sills crop out in the core of the Tulle anticline. Microstructural study, including quartz 〈c〉-axis petrofabrics and transmission electron microscope (TEM) analysis allows us to recognize the timing of deformation during pluton emplacement and cooling. The earliest fabric is a pre-full crystallization deformation. The second stage corresponds to a high-temperature (ca 650°C) solid-state deformation. The last deformation is responsible for the 〈c〉-axis preferred orientation at low temperature. Several methods of strain analysis applied in various mineral phases show that pluton emplacement is coeval to a constrictional strain. This strain is characterized by a NW–SE maximum stretching axis (X) parallel to the Tulle anticline axis and an intermediate, NE–SW-trending, shortening axis (Y) also responsible for large open folding of the host rocks. Gravimetric modelling allows us to determine a laccolithic shape for the plutons. Hot Stokes diapirism is ruled out and the hypothesis of granitic magma channelling through dykes is preferred. A laccolith emplacement model based on a syn-folding décollement level is proposed.