The Sesia Magmatic System

Gabriella Demarchi, James E. Quick, Silvano Sinigoi, and Gabriella Peressini

The Ivrea-Verbano Zone (IVZ) and Serie dei Laghi (SdL) of northwest Italy, are two lithostratigraphic units that constitute the deep- and the middle- to upper-crustal components, respectively, of a tilted and exposed section through the pre-Alpine crust of northwest Italy. Both units were significantly affected by a Permian igneous event, leading to underplating of the Mafic Complex in the deep-crustal IVZ and intrusion of granitic bodies in the upper-crustal SdL, which is in turn capped by volcanic, predominantly rhyolitic rocks. SHRIMP U-Pb zircon ages of volcanic rocks (288 ± 2 to 282 ± 3 Ma), formation of granitic plutons in the SdL (289 ± 3 to 275 ± 5 Ma), and gabbro in the IVZ (289 ± 3 to 286 ± 6 Ma) indicate that the onset of bimodal volcanism and granitic plutonism was coincident with and probably triggered by intrusion of mantle-derived mafic melt in the deep crust, and that volcanic activity and presence of granitic melt at depth persisted after underplating had ceased. Collectively, all these coeval igneous rocks are grouped in the “Sesia Magmatic System”, whose activity culminated with the collapse of a caldera at least 13 Km across. In this paper we resume the most significant processes which affected the emplacement and igneous evolution of the IVZ Mafic Complex, briefly discussing its relationships with the upper-crustal igneous activity. Also, in the appendix we report a field guide for a two-days classic excursion across the entire sequence.