Testing Models of Collisional Magmatism: A Source-to-Sink Approach Applied to the Central Alps

Competing models have been proposed to explain syn-orogenic magmatism in the Alps during the Eocene-Oligocene (~45-30 Ma). During this timeframe, magmatism occurred near the Periadriatic Fault System due to slab break-off or slab steepening of the European plate beneath the Adriatic-African plate, leading to emplacement of intrusions, including the Bergell Pluton. Understanding the emplacement, cooling, exhumation, and erosional history of the Bergell is key to test models of syn-orogenic magmatism. Short-lived magmatism and rapid exhumation is expected in the case of slab break-off, whereas a longer history of magmatism and slower exhumation is anticipated in the case of slab steepening leading to enhanced corner flow. Our approach involves applying multiple geochronometers to determine a complete time-temperature history of pluton emplacement, cooling, and exhumation. Preliminary zircon U-Pb, (U-Th)/He, and Lu-Hf data have been collected from the Bergell Pluton, Novate Pluton, country rocks (Gruf complex and European crust), and individual cobbles from the Como and Villa Olmo conglomerates in the proximal foreland basin. These data constrain crystallization ages of the Bergell (32.15 ± 0.16 Ma) and Novate (25.73 ± 0.18 Ma), as well as showing the prominent Permian signature of the Gruf. Zircon (U-Th)/He ages of 23-24 Ma for Bergell-derived clasts within the syn-orogenic conglomerates with depositional age of approximately 23 Ma indicate lag times of less than 1 Ma, implying rapid exhumation. A granite cobble with a zircon U-Pb age typical of European basement (286.2 ± 1.3 Ma), yielded Oligocene zircon (U-Th)/He ages (31-35 Ma). Cobbles of granodiorite derived from the Bergell (with ~32 Ma zircon U-Pb ages and matching initial εHf) show younger, faster exhumation than cobbles derived from country rocks, indicating the ability of geo-thermochronology to discriminate different sources. Zircon (U-Th)/He thermochronological data from in situ Bergell and Gruf exposures yield Miocene cooling ages (19-15 Ma). These data, when combined with existing thermochronological data from the Bergell, indicate rapid early exhumation of the Bergell at 25-20 Ma and 17-10 Ma, with slower exhumation after that. The short lag times between cooling and deposition, and the presence of Bergell-derived clasts in the Como and Villa Olmo conglomerates, require episodic and rapid early exhumation, supporting the slab break-off model for syn-orogenic magmatism in the Alps.