Investigating the time of deformation and heat transfer in the Lepontine Dome (Central European Alps)

Tagliaferri, Alessia and Schenker, Filippo and Schmalholz, Stefan and Ulianov, Alexey and Seno, Silvio (2022) Investigating the time of deformation and heat transfer in the Lepontine Dome (Central European Alps). In: Abstract Book, Congresso SGI 2022, 19-21.9.2022, Torino.

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Deformation is a main topic when it comes to the study of (poly-)metamorphic crystalline basements: in which stage of the metamorphic history did it occur and how is it correlated to far-field thermo-tectonic processes? We can answer this question adopting a combined approach comprising geological mapping (performed over 30kmq covering the Valleys of Ticino, Switzerland), meso- and micro-structural analysis, petrology and finally U-Pb geochronology on zircons. We applied this method to the high-grade rocks of the Lepontine nappes constituting the Penninic domain of the Central European Alps. The Lepontine Dome is a metamorphic and structural dome formed by the nappe-stack of Europe-derived crystalline basement units. It is characterized by a widespread Barrovian metamorphism generally considered to peak end of Oligocene, after nappe formation. The relative mineral-zone and isograde boundaries have an asymmetric-concentric shape, that doesn’t coincide with the dome shape defined by regional attitudes of foliation and thrust sheets. This metamorphic pattern suggests a post-thrusting thermal event. However, the pervasive NW-SE-directed amphibolite-facies mineral lineation and the local migmatites, both associated with top-to-the-foreland shearing, indicate that peak temperature conditions reached during nappe emplacement. Hence, a key element to solve this apparent paradox is to unravel the timing of the pervasive high-grade deformation. Geological survey, focused on a N-S transect along the main tectonic contacts, permitted to sample leucosomes (in shear bands, boudinated and in complex deformed networks) of contact-parallel syn-tectonic migmatites and post-foliation aplitic to pegmatitic dikes to date their genesis through U-Pb zircon dating technique with LA-ICP-MS (Laser Ablation Inductively-Coupled Plasma Mass Spectrometry, 20 μm laser spot) and SIMS (SwissSIMS ion probe, 8x12 μm ions spot). The ages of metamorphic zircon rims pinpoint at 31-33Ma the deformation common to all the lithotypes, in different structural levels in the nappe pile, from north to south, coeval with migmatization and leucosomes genesis, hence considered to date the timing of deformation close to peak conditions. While the 22-24Ma ages of magmatic zircons belonging to post-foliation granitic dikes represent the minimum age of ductile deformation cease, indicating a post-tectonic event affecting only the southernmost units, close to the SSB (Southern Steep Belt). These events suggest the presence of two main heat pulses: the first related to thrusting, hence pervasive along the nappe boundaries and strictly related to deformation, and the second to magma/fluid advection and/or heat diffusion from the SSB. Which one of these is responsible for the overall Barrovian metamorphism is still unclear and quantitative thermo-kinematic numerical models will help us to discriminate the different scenarios in future studies.

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