Geothermal gradients in the Earth crust are often represented as purely conductive with heat source from the mantle and radioactive decay. Nevertheless, active collisions usually involve magma transfer and emplacement in the crust, resulting in non-stady-state, sometimes high, geothermal gradients. The Agly massif (Eastern Pyrenees) is an exceptional laboratory to study ancient high geothermal gradient since we have access to a unique cross-section from green-schist facies (upper crust) to granulite facies (lower crust). The metamorphic gradient has been constrained using thermodynamic modeling on more than 20 samples from different structural position and all over the massif. The result confirm a high geothermal gradient of 55°C/km in the upper crust and a nearly isothermal gradient in the deeper part of the massif (7°C/km), at very high temperature, i.e. 750 °C. The age of this metamorphic event has been constrained on samples from the gneissic core using in-situ U-Th-Pb at around 305 Ma. We interpret the discontinuity of the gradient as well as the high temperature in the gneissic core as the cumulated result of several intrusions between 308-304 Ma and the strong buffering effect of the biotite dehydration reaction. This gradient has been altered later by at least one big detachment fault localized at the anatexis front, resulting in the thinning of the massif from a thickness of 15 km to 5-6 km.