Late-Orogenic Thermo-Tectonic Evolution of an Orogen External Zone: Coupled Thermometry and 40Ar/39Ar Dating of the Para-Autochthonous Unit of the Variscan Belt (Cévennes Area, French Massif Central)

This study combines Raman Spectroscopy on Carbonaceous Matter (RSCM) thermometry and 40Ar/39Ar dating to constrain the thermal structure of the Variscan Para-autochthonous Unit (PAU) in the southern French Massif Central. The RSCM data define a regionally uniform peak-thermal pattern at TRSCM = 535 ± 50 °C throughout much of the PAU that coincides with P-T thermodynamic modeling of Grt-bearing metapelite indicating 525 ± 20 °C and 0.45 ± 0.05 GPa. This pattern post-dates thickening. No RSCM temperature increase can be seen around late granitic plutons, presumably as a consequence of unfavorable transformation kinetics. In contrast, 40Ar/39Ar step-heating dating of syn- and post-folial muscovite and biotite populations record regional N-S variations from 330 Ma to 325 Ma, down to 305 Ma around plutons. In pluton contact aureoles, 40Ar/39Ar systematics appear controlled by synkinematic recrystallization at 315–300 Ma, indicating a continuum of P-T variations during pluton emplacement. Older biotite and muscovite ages at 325 Ma record an early deformation stage predating regional muscovite ages between 320 and 315 Ma. In the eastern PAU, 332–322 Ma ages argue for an early thickening event. The decoupling between regional 40Ar/39Ar variations and the RSCM temperature record indicates that the thermal regime was associated with a late-thickening thermal event.