Duration of igneous activity in the Sesia Magmatic System and implications for high-temperature metamorphism in the Ivrea–Verbano deep crust

The Ivrea–Verbano Zone and Serie dei Laghi of northern Italy constitute a virtually complete section of the Adriatic continental crust, which was intruded at different levels by coeval magmas of Permo-Carboniferous age, collectively referred to as the “Sesia Magmatic System”. At the deepest levels of the section, the relationship between magmatic underplating and granulite-facies metamorphism in the Ivrea–Verbano Zone may be studied. We present new zircon U–Pb age data, which significantly extend the record of igneous activity in this terrane, and which demonstrate that igneous pulses began intruding the deep and middle crust no later than 314 Ma, an age corresponding to that inferred for regional granulite-facies metamorphism and predating by more than 20 m.y. the well-documented, main magmatic pulse at about 292 to 282 Ma. A gap in age information from 316 ± 3 Ma to 276 ± 4 Ma, observed in the deep-crustal granulite-facies paragneiss of the Ivrea–Verbano Zone overlaps a gap from circa 314 to 283 Ma between igneous and reset zircons in a closely associated mafic– ultramafic sill. However, zircon ages demonstrate intrusive activity in the middle and upper crust during this time gap. We infer that prolonged maintenance of temperatures above zircon saturation in the deep-crustal paragneiss could explain the gap in zircon ages in those rocks. Viewed in this light, a gap in a geochronological record within high-grade terranes might not necessarily mean an absence of events, but instead may record the occurrence and duration of a thermal peak.