Geological mapping and U-Pb dating of detrital and magmatic zircon crystals in the Lepontine dome (Central European Alps)

Tagliaferri, Alessia and Schenker, Filippo and Schmalholz, Stefan and Ulianov, Alexey (2022) Geological mapping and U-Pb dating of detrital and magmatic zircon crystals in the Lepontine dome (Central European Alps). In: Abstract Volume, 20th Swiss Geoscience Meeting, Lausanne 20th Swiss Geoscience Meeting, 18-20.11.2022, Lausanne.

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The Barrovian metamorphism of the Lepontine dome (Central European Alps) is manifested by isogrades that cross-cut tectonic nappe contacts, which is commonly interpreted as a metamorphism that occurred after nappe emplacement. However, the pervasive mineral and stretching lineation in amphibolite facies, associated to top-to-foreland shearing, suggests that peak Barrovian conditions are coeval to thrusting. Here, we combined extensive fieldwork and U-Pb zircon dating on gneisses and post-foliation dikes to better constrain the relation between metamorphism and nappe emplacement. Metamorphic zircon rims show two groups of ages, at 33-31 Ma and 24-22 Ma. The younger group is observed in post-tectonic dikes and related metasomatic overprint in the country rock. The older group occurs in syn-kinematic migmatites along a crustal-scale shear zone. On its footwall, magmatic and detrital zircon cores suggest that the Cima Lunga unit, previously interpreted as a mélange with Mesozoic fragments, was a pre-Variscan metasedimentary sequence intruded by Permian granitic sills, now orthogneisses. This unit was tectonically reworked within the Simano nappe during the overthrusting of a major Alpine high-pressure nappe, here defined as the Maggia-Adula nappe, imprinting the regional lineation and peak-temperature metamorphism until ca. 31 Ma. Péclet (1-10) and Brinkman (0.002-1.8) numbers, computed for estimated values on the newly-defined Maggia-Adula shear zone system, suggest an advection-dominated heat transfer with significant diffusion during and after nappe emplacement, generating Barrovian isogrades discordant to the thrust. Shear heating played a not-negligible role if effective viscosities were >1021 Pa·s. The local advection of magma/fluids cutting the nappe contact at 24-22 Ma sourced from deeper migmatites exhuming along the Alpine backstop. In this contribution, we focus on the field relationships between the Cima Lunga unit and the Simano nappe as well as on the pre-Variscan detrital and Permian magmatic U-Pb zircon ages. Our results have consequences on the definition of the nappe structure of the Lepontine dome, suggesting that the Cima Lunga unit has a pre-Variscan origin and was already part of the Simano nappe during the formation of the Alps.

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