Identifying viscous heating in metamorphic sequences using paired Sr-diffusion chronometry in albite-apatite

Understanding orogen dynamics requires quantifying the relative contributions of heat sources that drive metamorphism in the continental crust. The importance of viscous heating in the orogen-scale heat budget remains debated, largely because it is difficult to identify in the field. Short metamorphic durations at near-peak conditions have been proposed as a potential diagnostic of viscous heating, but other processes such as rapid thrust advection or magmatic intrusions can also produce brief thermal events. To better constrain metamorphic time scales and discriminate between heating mechanisms, we develop a paired albite-apatite Sr-diffusion chronometry approach and apply it to a gneiss (peak temperature [T] ∼530 °C) from the Carboniferous Variscan nappe stack of the French Massif Central. We characterize the mineral textures, constrain their crystallization along the prograde path, and document sharp Sr growth zoning. Forward diffusion modeling shows that the observed profiles, especially the preservation of Sr-poor moats in albite, are reproduced if the rock resided for ∼0.1−0.2 m.y. at ΔT ≤40 °C relative to peak, with near-peak heating and cooling rates of ∼500 °C/m.y. These rates exceed those predicted by thermo-kinematic thrust advection models and, given the lack of Variscan magmatism in the study area, are best explained by a viscous heating pulse. Paired albite-apatite Sr-diffusion chronometry can therefore resolve rapid thermal histories in garnet-free rocks and provide a mineral-scale criterion to identify viscous heating in metamorphic belts.