A multidisciplinary study of the Barrovian metamorphism in the Lepontine dome gives new insights into the heating history of the Central Alps

This contribution aims to present to the Alpine scientific community some of the latest studies we performed in the Central Alps, with a focus on the origin of the Barrovian metamorphism in the Lepontine dome.

The Lepontine dome is a metamorphic and structural dome formed by crystalline basement nappes of the Penninic domain. The area is characterized by a Barrovian metamorphic imprint, which is testified by peak-temperature mineral isograds and by a pervasive mineral and stretching lineation in amphibolite-facies. Isograds locally intersect the tectonic nappe boundaries, an observation that was frequently considered as evidence of post-nappe emplacement heating. Nonetheless, the NW-SE directed lineation suggests that peak metamorphic conditions developed coeval to the emplacement of Lepontine nappes.

We addressed this inconsistency through a multidisciplinary approach. We combined extensive geological and structural mapping with U-Pb zircon dating, which permitted to identify a belt of syn-kinematic migmatites dated at 31-33 Ma. The discovery of these Alpine migmatites defines a major crustal-scale shear zone which we named “Maggia-Adula shear zone”. This thrust divides the Adula and Maggia nappes on top from the Simano below, with the Cima Lunga unit pinched and sheared between them.

We modelled nappe emplacement along this main shear zone with a simple thermo-kinematic numerical model, which revealed that heat was mainly advected from depth during overthrusting. Heat conduction also contributed to the final configuration of the peak-temperature isotherms and shear heating played a role in shaping the inverted metamorphism observed around the thrust.

Finally we investigated the cooling history of Lepontine garnet-paragneisses sampled at different tectonic levels within the nappe pile. We applied multicomponent diffusion modelling to Alpine garnet rims which experienced re-equilibration at close-to-peak conditions. The rocks within the main shear zone experienced post-peak conditions of ca. 635 °C and 0.8 GPa, with a subsequent very fast cooling of 100-400 °C/Myr. These high cooling rates can’t be explained with regional exhumation, and confirm the hypothesis of shear heating acting within the shear zone.

In conclusion, our results indicate that the origin of Barrovian metamorphism in the Lepontine dome can be ascribed to the emplacement of a hot Alpine nappe, which we refer to as the “Maggia-Adula nappe”. This event produced Barrovian isograds, amphibolite-facies lineation and migmatites at ca. 31 Ma. In specific locations within the Lepontine dome, our results also suggest that we should re-consider the distribution of peak-temperature isotherms. The subsequent cooling of the Lepontine area was spatially and temporally heterogeneous and could have determined a later re-heating in the northern units.

Assessing the extent of Barrovian metamorphism and its finite imprint in the field at different tectonic levels is challenging and requires an interdisciplinary study. Geometrical interpretation alone is insufficient to understand the origin of Barrovian metamorphism without considering the physical forces leading to deformation and metamorphism in mountain-building processes.