(U-Th)/He dating of low T metamorphism in the Penninic sector (Ligurian Alps, Northern Italy): Inferences on deformation ages of Alpine tectonic evolution

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Maino, Matteo۱; Dallagiovanna, Giorgio۱; Gaggero, Laura۲; Persano, Cristina۳; Seno, Silvio۱; Stuart, Finlay M.۴
Holding Date 20 September 2008

The present structure of the Ligurian Alps chain is represented by several tectonic units belonging to the Briançonnais, Prepiedmontese and Piedmont-Ligurian domains of the Penninic realm. The Briançonnais units are the only ones that show a continuous stratigraphy, composed of a pre-Variscan basement, a Permian volcanic succession and a Mesozoic cover, whereas the Pre-piedmontese and Piedmont-Ligurian units are represented by detached cover or basement only.
Units were displaced from their original positions and transported outwards during the principal (Eocene) Alpine tectogenetic event. Presently, the Briançonnais units are the base of the Penninic nappe pile, in turn thrust onto the Dauphinois domain. After the SSW-wards directed stacking (D1 phase), the whole pile of units underwent a backfolding event (D2 phase), whose intensity decreased towards the more external units. The early metamorphism linked was characterised by high conditions in the internal units (up to 1.3 GPa, 400 °C) and it decreased towards the outer sectors (0.3 GPa, 200 °C). The intensity of the D2 backfolding event increases towards the inner parts of the chain, and the accompanying metamorphic paragenesis indicate decompression (0,5 GPa) possibly accompanied by a slight temperature increase. The timing of HT (>300°C) metamorphism is relatively well known in the Penninic sector of the Western and Ligurian Alps whereas the low T events are poorly constrained. In order to detect the timing and the boundaries of the low T metamorphism as well as to better constraint the deformation phases we carried out 11 zircon (U-Th)/He analysis from samples of volcanic formations that are preserved in all of the outer, intermediate and inner Alpine tectonic units belonging to the Briançonnais domain. The geochemical characters, paragenesis and U-Pb emplacement ages of the Permian volcanites have been also determined. The lithologies chosen for the zircon (U-Th)/He analyses are calc-alkaline granodioritic bodies (SHRIMP U-Pb age: 300-294 Ma), rhyolitic pyroclastites (U-Pb age by LA ICP MS: 285.6~aline rhyolites (U-Pb age by LA ICP MS: 258.5~1.4 Ma).
The zircon (U-Th)/He thermochronometer has a closure temperature of c. 200 °C that is optimal to detect the lower boundary of the metamorphism. Zircons crystals are selected on the basis of size, morphology, abundance of inclusions and colour. The analyses were performed at the (U-Th)/He facility at the Scottish Universities Environmental Research Centre (SUERC, East Kilbride, UK).