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

Co-auteur étrangerInternational audienceThe Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic ultra-high temperature (UHT)metamorphism in the world, but its age is still a matter of debate. Herewe present LA-ICPMS dating in a petrographic context ofzircon and monazite from a UHT restitic granulite. Zircons and monazites are both included in large crystals and in retrogradesymplectites. In such restitic rocks, partial melting or fluid interactions are unlikely, precluding resetting of the monazitechronometers. Zircon cores yield Permian ages, which are interpreted as the age of charnockitization. They are sometimessurrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma age interpreted as the time ofcomplete (re-)crystallization during the UHT paragenesis. We propose that the zircons dated a post-Hercynian metamorphismwhich is responsible for the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluidabsentmelting event produced refractory lithologies, such as restites in charnockites. We suggest that Gruf UHT paragenesis isalpine in age and crystallized from these refractory lithologies.We conclude that the lower restitic crust produced in the Permianhad the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time