Cooling-rate constraints from metapelites across two inverted metamorphic sequences of the Alpine-Himalayan belt; evidence for viscous heating

Despite the occurrence of high-grade metamorphic rocks next to and along crustal-scale shear zones, the temporal character of their formation and evolution is difficult to extract. We utilize the major-element diffusion in the compositional re-adjustment of garnet from metapelites in two crustal-scale shear zones as a complementary method to extract cooling rates from deforming/reacting rocks. The two thrust zones, the Nestos Thrust Zone (NTZ) in Rhodope, Greece, and the Main Central Thrust (MCT) in Sikkim, Himalaya, exhibit inverted metamorphic zonation. We applied phase equilibria modelling and geothermometry to constrain the peak- and the post-peak-temperature conditions relevant for the cooling-rate estimates. Results are 50–80 °C/Myr in the footwalls of both thrust zones, in consistency with published estimates using geochronology methods for MCT. However, results are much less (∼0.5–5°C/Myr) for the base of the MCT hanging wall. The estimated cooling rates are between 300 and 2500 °C/Myr for the NTZ hanging wall. The exceedingly fast cooling rates indicate the operation of transient and proximal thermo-mechanical processes consistent with the contribution of thrust-related viscous heating during metamorphism. The very slow cooling rate of the MCT hanging wall may reflect a complex thermal history or other overlooked processes.