A pressure-temperature-time path for the NE Mont-Blanc massif; fluid-inclusion, isotopic and thermobarometric evidence

The Mont-Chemin region at the NE extreme of the Mont-Blanc massif, Canton Valais, Switzerland is predominantly comprised of the granitic rocks of the Mont-Blanc intrusive rock suite and the Mont-Blanc basement gneisses. Fluid inclusions, fluid-mineral equilibria, stable-isotope and radiogenic-isotope studies have been used to derive pressure, temperature, age (PTt) and fluid-composition constraints for a number of Alpine events. The earliest of these events is recorded in a paragonite-katophorite schist hosted within the basement gneisses. The paragonites yield a (super 40) Ar/ (super 39) Ar age of 47 Ma. Mineral thermobarometry is consistent with formation temperatures in excess of 300 degrees C, with minimum pressures of 1500 bars. A well-defined pressure-temperature uplift path is recorded in minerals hosted by veins of different generations. The overall PTt path defines a geothermal gradient of 25 degrees C/km, but the younger portions of this PTt path are consistent with geothermal gradients slightly in excess of 50 degrees C/km, similar to those observed to the East along the Rhone-Simplon line.