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

Advection of heat by the intrusion of mantle-derived magma is frequently invoked as the driver of high-temperature metamorphic events in the deep crust. One of the best places worldwide to study and evaluate this process is the deep crustal section of the Ivrea-Verbano Zone of northwest Italy. The Ivrea–Verbano Zone and the adjacent Serie dei Laghi 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. The relationship between magmatism and granulite-facies metamorphism in this area has been addressed by a number of investigators, some of whom inferred that the Permo- Carboniferous intrusions provided the heat for partial melting of deep crustal rocks and their granulite-facies metamorphism, while others questioned this interpretation, noting the largest mafic intrusion cross cuts the regional structural grain and metamorphic gradient and is mantled by a relatively thin metamorphic aureole. We present new in-situ zircon U–Pb age data, which significantly extend the record of igneous activity in this crustal section, 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 paragneisses overlaps a gap from ca. 314 to 283 Ma between igneous and reset zircons in a closely associated mafic–ultramafic sill. On the other hand, U/Pb zircon ages demonstrate intrusive activity section upwards in the middle and upper crust during this time gap. We thus infer that prolonged maintenance of temperatures above the zircon stability field or zircon open-system behaviour in the granulite-facies paragneisses 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.