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

Tagliaferri, Alessia and Schenker, Filippo and Schmalholz, Stefan and Ulianov, Alexey (2021) LA-ICPMS U-Pb dating on zircons from the Lepontine Dome (Central European Alps). In: Abstract Volume19th Swiss Geoscience Meeting 2021 19th Swiss Geoscience Meeting 2021, Geneva.

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The Lepontine Dome (Central Alps, Ticino, Switzerland) is a structural dome belonging to the Penninic domain of the European Alps. It is formed by crystalline basement nappes characterized by a strong amphibolite facies imprint. The related barrovian isogrades locally dissect the tectonic contacts. These nappes show an extremely pervasive mineral and stretching lineation (NW-SE directed) suggesting non-coaxial deformation during shearing at peak temperature metamorphic conditions. In the studied area, the Simano nappe is formed by metagranitoids and by minor paragneisses. The Cima Lunga/Adula nappe above is made of metasediments, mainly quartz-rich gneisses intercalated with amphibolite-gneisses, peridotitic lenses and local calcschists and/or marbles. The alternation of lithotypes is mostly parallel to the nappe boundaries and usually constant over their whole, kilometer-scale, length. The progressive change in gneisses’ texture marks the transition between the Simano and Cima Lunga/Adula nappes: the more stretched texture is the evidence of the strain increase from the bottom to the top of the sequence, accompanied by a change in the constituent lithotypes. Finally, migmatitic leucogneisses have been found at the tectonic contacts between these nappes, indicating syn-tectonic top-to-the foreland deformation accompanied by partial melting. Locally the foliation of these rocks is crosscut by aplitic and pegmatitic dikes. In order to define the temporal duration of melting, U-Pb zircon dating with LA-ICPMS (Laser Ablation Inductively-Coupled Plasma Mass Spectrometry) has been performed on leucosomes of migmatites, paragneisses, meta-arenites and post-tectonic dikes. A 20µm laser spot permitted to date the thin metamophic rims associated with migmatization. The results show two main groups of ages pointing at ca.31Ma and 22Ma. The latter group dates the intrusion of the post-tectonic dikes. The distribution of ages in samples collected at different structural levels permitted to distinguish the metamorphic events that affected these rock units, and their relationships with the more southern migmatites belonging to the Southern Steep Belt.

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