REVEALING THE COOLING HISTORY OF THE LEPONTINE DOME: INSIGHTS FROM GARNET MULTICOMPONENT-DIFFUSION MODELING

The Lepontine dome, located in the Central European Alps, is a metamorphic and structural dome formed by the accretion of crystalline basement nappes during continental collision. The area is characterized by regional Barrovian metamorphism, with peak amphibolite-facies conditions coinciding with the final stages of nappe emplacement at ca. 31 Ma. The determination of peak conditions was achieved through U-Pb zircon dating of syn-tectonic migmatites along the major Maggia-Adula shear zone.

Although the age of the temperature peak is well-constrained, the duration of subsequent cooling remains unclear. Here, we employed inverse multicomponent-diffusion modeling to estimate the cooling rates by examining the garnet compositional re-adjustment due to major-element diffusion at garnet crystal rims.

We selected six garnet-paragneiss at different tectonic levels within the core of the nappe pile. Garnets are pre- to syn-kinematic with respect to the amphibolite-facies metamorphic foliation and display at their rim coupled Mn increase and Mg decrease, indicative of retrograde reactions with matrix biotite. Applying geothermometry and geobarometry techniques, we estimated the post-peak temperatures of re-equilibration to range between 577 and 661 °C at pressures between 0.5 and 1.3 GPa.

The cooling rates exhibit spatial variability across the study area. The main shear zone shows significantly higher cooling rates, ranging from 100 to 400 °C/Ma, while the footwall exhibits a much slower cooling rate of approximately 2 °C/Ma. Additionally, the migmatitic belt bordering the Lepontine dome to the south displays cooling rates between 20 and 50 °C/Ma.

To better understand these different cooling behaviors, one-dimensional thermal models were constructed. Our results suggest that the high cooling rates at approximately 635°C and 0.8 GPa observed in the main thrust cannot be solely attributed to regional exhumation processes, even when considering high exhumation velocities. Instead, in the middle and lower crust, cooling rates greater than approximately 50 °C/Ma are likely caused by additional local heat sources beyond regional heating, such as the percolation of hot fluids or shear heating.