Quantitative P–T–t–D Paths: A Key to decipher orogenic dynamics in the Variscan Aiguilles Rouges Massif (External Crystalline Massifs, Western Alps)

This study investigates the P–T–t–D evolution of two metapelitic samples from the middle crust exposed in the Aiguilles Rouges Massif. Garnet compositional mapping, phase equilibrium modelling, zirconium-in-rutile thermometry, trace element geochemistry of garnet and monazite, and U-Pb LA-ICP-MS dating on monazite were used to better understand the tectonic and thermal history of the variscan External Crystalline Massifs. In the sample representing the upper-middle crust (AR736, southwestern part of the massif), using the preserved mineral assemblage in garnet inclusion (Grt + St + Bt + Ms + Qz + Pl + Rt) and garnet compositions, the prograde P–T path was constrained from ∼0.5–0.6 GPa and 550–625 °C to ∼0.76–0.82 GPa and 600–640 °C. The P–T conditions at the onset of this prograde evolution suggest a high geothermal gradient (∼30–35 °C/km) prior to the onset of crustal thickening. In the sample representing the lower-middle crust (AR14, central part of the massif), using the preserved mineral assemblage in garnet inclusion (Grt + Bt + Ms + Qz + Pl + Rt), the occurrence of sillimanite and ilmenite in the matrix and garnet compositions, a β-shaped P–T path characterised by a late temperature increase during exhumation was identified. Both samples recorded a retrograde P–T stage at ∼0.4 GPa and 545 °C, dated at 315–305 Ma. Microstructural analysis indicates dextral transcurrent deformation from the late crustal thickening stage to the exhumation phase. Comparison with previously published P–T paths from eclogitic lenses highlights the juxtaposition of middle and lower crustal domains during dextral transcurrent deformation. We propose a tectonic model in which the formation of supra-subduction volcano-sedimentary basins (∼350 Ma) is followed by crustal thickening between 350 and 340 Ma under a thermal gradient of ∼5–15 °C/km. The exhumation of the lower and middle crust took place in a transcurrent regime between 340 and 305 Ma. This prolonged transcurrent tectonic activity suggests that the numerous transcurrent shear zones in the Variscan belt are not merely late orogenic structures but played a significant role in the geodynamic evolution, particularly in the exhumation of the orogenic crust, from the end of continental collision to the closure of the Variscan orogeny.