Ivrea Zone and Adjacent Southern Alpine Basement

Abstract

The available literature is critically assessed and existing controversies are emphasized. Based on a synthesis of published ideas the extended discussion proposes the following evolutionary scheme (compare sketches in Fig. 3): (1) an impressive volume of Proterozoic metasediments in the Ivrea zone and Serie dei Laghi has first been juxtaposed during accretionary wedge formation and underplating above a subducted Ivrea oceanic crust during a Late Proterozoic to Early Palaeozoic event (“Cadomian” or Caledonian). (2) A high geotherm was established by Ordovician times (460–480 Ma) as evidenced by anatexis in paragneisses of the Ivrea zone and granitic intrusion associated with granitization of adjacent metasediments in the Serie dei Laghi. This time marks the onset of a long-lasting history of deep burial under elevated temperatures for both Ivrea zone and Serie dei Laghi. (3) Variscan orogeny at around 320 Ma led to crustal thickening and decoupling between lower crust (Ivrea zone) and intermediate crustal levels (Serie dei Laghi). Large parts of the Serie dei Laghi were exhumed during Variscan orogeny, thereby preserving pressure-dominated Variscan mineral assemblages while the Ivrea zone remained in a lower crustal environment resulting in widespread re-equilibration of mineral assembleges to moderate pressures. (4) An important Late Palaeozoic magmatic event of Permian age is unrelated to Variscan orogeny since it post-dates re-establishment of normal crustal thickness. Synchronous magmatic activity is documented at all crustal levels from the still granulite facies metamorphic crust-mantle transition to the sedimentary cover and is best explained in the context of intracontinental wrench tectonics. (5) Crustal thinning associated with passive continental margin formation in the Early Mesozoic leads to considerable uplift of the Ivrea zone and cooling to temperatures below 300 °C. (6) Late Oligocene to Early Miocene deformation is responsible for the final exposure of a vertical section through attenuated continental crust.