Constraints on the post-Variscan thermal evolution of the Ivrea crustal section (Italian-Swiss Alps) from U-Pb dating of relict rutile in middle crust amphibolites

The Ivrea crustal section from the Italian-Swiss Alps is a nearly coherent section of the pre-Alpine, lower to upper continental crust, which became part of a passive continental margin in the Middle Jurassic. Here we apply the U-Pb rutile thermochronometer to amphibolites collected along mid levels of the Ivrea section, in order to improve the understanding of the thermal evolution of extending continental margins. Relict rutile in the Monte Gambarogno amphibolite lens from the northeastern Ivrea crustal section provided a Permian age (284 ± 24 Ma). On the other hand, an Upper Triassic (215 ± 6 Ma) age was acquired for relict rutile in the Alpe Morello amphibolite lens, which is exposed along the southwestern Ivrea crustal section. We attribute these ages to U-Pb isotope re-equilibration of rutile crystallized in pre-Permian times, in response to two distinct heating events. The first is related to the Lower Permian post-Variscan extension and is inferred to have reset the U-Pb system of rutile from middle and deep levels of the whole Ivrea crustal section. The subsequent thermal perturbation of Upper Triassic age is most likely related to the onset of the rifting process that ultimately led to crustal breakup in the Middle Jurassic. We propose that the latter heating event was not able to disturb the U-Pb system of middle crustal rutile along the future proximal domain of the extending continental margin.