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 invest igated by cathodoluminescence and dated by ion microprobe (SHRIMP). Most of the zircons show a oscillatory or sector zoned core, which is surrounded b y a weakly zoned or unzoned rim, irregular in shape. The U-Tn-Pb analyses r evealed that, opposite to the cores, the zircon rims have low U- and Th-con tents, 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 th e zircon rims recrystallized and changed their internal zoning as well as t heir chemical and isotopic composition during high-pressure (15-18 kbar) mo derate temperature (550-600 degrees C) metamorphism. In the Sesia-Lanzo Zon e, eclogite-facies zircons from three different localities and four differe nt rock types were analysed. The age of metamorphic zircon rims was obtaine d from an eclogite from Monte Mucrone (65 +/- 5 Ma), an eclogitic micaschis t from the Aosta Valley (65 +/- 3 Ma) and two mafic rocks from Cima di Bonz e (68 +/- 7 Ma). The similar to 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 degrees C. The age results suggest a maximum rate of 0.7 cm/y for the subd uction of the Sesia-Lanzo Zone to eclogite-facies conditions. The Cretaceou s-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 sug gest that the Sesia-Lanzo Zone behaved as a separate continental block at t he time of Alpine convergence. (C) 1999 Elsevier Science B.V. All rights re served.