Eocene ultra high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and monazite dating in petrographic context.

The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic UHT metamorphism in the world but its age is still a matter of debate. Here we present LA-ICPMS dating in petrographic context of zircon and monazite from an UHT restitic granulite. Zircons and monazites are both included in large crystals and in retrograde symplectites. In such restitic rocks, partial melting or fluid interactions are unlikely precluding resetting of the monazite chronometers. Zircon cores yield Permian ages interpreted as age of charnockitisation. They are sometimes surrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma 15 age interpreted as the time of complete (re-)crystallisation during the UHT paragenesis. We propose that the zircons dated a post Hercynian metamorphism which is responsible of the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluid-absent melting event produced refractory lithologies such as restites in charnockites. We suggest that Gruf UHT paragenesis is alpine in age and cristallised from these refractory lithologies. We conclude that the lower restitic crust produced at the Permian time had the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time