LA-ICP-MS U-Pb dating on zircons from the Lepontine Dome (Central European Alps)

The Lepontine Dome is a structural and metamorphic dome formed by crystalline basement nappes belonging to the Penninic domain of the European Alps (Switzerland). The mineral-zone boundaries of the Barrovian Tertiary metamorphism show an asymmetric concentric zonation not coinciding with the dome shape defined by the regional attitudes of foliation and thrust sheets. The related Barrovian isogrades locally dissect the tectonic nappe contacts suggesting a post-thrusting thermal event. However, the extremely pervasive and NW-SE directed mineral and stretching lineation, also developed during the upper amphibolite facies metamorphism, suggests non-coaxial deformation during thrusting at peak metamorphic conditions. This apparent paradox may be explained with several geodynamic scenarios that are still debated by the scientific community. One crucial element helping to evaluate the different scenarios is the timing of the upper amphibolitic, non-coaxial deformation along the tectonic contacts, which is still poorly constrained. Hence, the goal of our work is to date this deformation with a multidisciplinary approach that aims to solve the relation between the geologic structures and the distribution of heat in the nappe pile.

In the studied domain, the lower unit (the Simano nappe) is formed by metagranitoids and by minor paragneiss. The upper thrusted unit (the Cima Lunga/Adula nappe) is made of metasediments, mainly quartz-rich gneiss intercalated with amphibole-gneiss, peridotitic lenses and, locally, calcschist and/or marble. The alternation of lithotypes is mostly parallel to the nappe boundary, and constant over its kilometer-scale length. Below the Cima Lunga/Adula, the transition to the Simano nappe is marked by a progressive change in gneiss texture: more stretched towards the top of the sequence, indicating a strain increase. Migmatitic leucogneisses have been found parallel to the tectonic contacts. Field observations indicate that their deformation is syn-tectonic, hence suggesting partial melting conditions during nappe emplacement. Their foliation is locally crosscut by granitic dikes of aplitic and pegmatitic texture.

To define the temporal duration of melting, U-Pb zircon dating with LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) has been performed on migmatites, paragneiss, gneiss, and granitic dikes. The results show two main groups of (metamorphic) ages centring at ca. 31 and 22 Ma. The younger ages date the intrusion of the post-tectonic dikes found exclusively in the southernmost area, proximal to the roots of the Lepontine nappes, likely related to the melt production along the Southern Steep Belt which lasted until ca. 22 Ma (according to U-Pb zircon dating by other authors). Ages indicating ca. 31 Ma are widespread from north to south, representing the nappe emplacement stage, coeval with migmatization.

Our results suggest the existence of two main heat sources: one related to thrusting and the other to fluid advection and/or diffusion of heat from the bottom along the Southern Steep Belt. Which heat source is responsible for the regional Barrovian metamorphism remains unclear. Our future studies will focus on the comprehension of the mechanisms of heat transfer and the relative roles of diffusion, advection and production to understand how these events are responsible for the net Barrovian heat budget of the Lepontine Dome.