Dating of eclogite-facies zircons: The age of Alpine metamorphism in the Sesia-Lanzo Zone (Western Alps)

Zircons from eclogite-facies rocks of the Sesia-Lanzo Zone have been investigated by cathodoluminescence and dated by ion microprobe (SHRIMP). Most of the zircons show a oscillatary or sector zoned core, which is surrounded by a weakly zoned or unzoned rim, irregular in shape. The U-Th-Pb analyses revealed that, opposite to the cores, the zircon rims have low U- and Th-contents, very low Th/U ratios and that their U-Pb systems were reset or they lost lead at the time of metamorphism. This study provides evidence that the zircon rims recrystallized and changed their internal zoning as well as their chemical and isotopic composition during high-pressure (15-18 kbar) moderate temperature (550-600°C) metamorphism. In the Sesia-Lanzo Zone, eclogite-facies zircons from three different localities and four different rock types were analysed. The age of metamorphic zircon rims was obtained from an eclogite from Monte Mucrone (65 ± 5 Ma), an eclogitic micaschist from the Aosta Valley (65 ± 3 Ma) and two mafic rocks from Cima di Bonze (68 ± 7 Ma). The ~65 Ma age is further supported by the ages of the youngest zircon rims in three other samples. These results constrain the age of eclogite-facies metamorphism in the Sesia-Lanzo Zone. An age of 76 ± 1 Ma determined on zircons from a hydrothermal vein is interpreted as dating the prograde formation of the vein at temperatures well below 550°C. The age results suggest a maximum rate of 0.7 cm/y for the subduction of the Sesia-Lanzo Zone to eclogite-facies conditions. The Cretaceous-Tertiary age for the eclogite-facies metamorphism in the Sesia-Lanzo Zone implies that this continental unit was not subducted together with either portions of the Adriatic margin or the neighbouring Mesozoic Tethys. We suggest that the Seisa-Lanzo Zone behaved as a separate continental block at the time of Alpine convergence.