Garnet peridotite and eclogite Sm-Nd mineral ages from the Lepontine dome (Swiss Alps): New evidence for Eocene high-pressure metamorphism in the central Alps

In the central Alps the upper Pennine Cima Lunga nappe (Lepontine dome, Swiss Alps) underwent eclogite facies conditions up to 850-900 °C and 3.5-4.2 GPa. Sm-Nd dating of garnet peridotites and an eclogite from the Cima Lunga nappe yields consistent mineral ages of ca. 40 Ma. The mineral ages are probably cooling ages, but petrological arguments indicate that they approximate the time of eclogite facies metamorphism, implying that the latter is related to the collision of the Adriatic plate with Europe. Emplacement of the upper Pennine nappe stack with the hot Cima Lunga nappe at its base onto European basement (lower Pennine units) is suggested to be the primary cause for the middle Tertiary Barrovian-type metamorphism in the central Alps. This process accounts for the different metamorphic evolution of the Cima Lunga nappe compared to underlying lower Pennine units and explains the time difference between Sm-Nd mineral ages from the Cima Lunga nappe and U-Pb monazite ages from the lower Pennine units.