Mantle-derived corundum-bearing felsic dykes may survive only within the lower (Refractory/inert) crust: Evidence from zircon geochemistry and geochronology (Ivrea-Verbano zone, southern Alps, Italy)

Corundum-rich (up to 55 vol.%) felsic dykes formed with albite, ±K-feldspar, ±hercyniteand ±biotite-siderophyllite cut the lower crustal rocks exposed in the Ivrea-Verbano Zone (NW Italy).Zircon is an abundant accessory mineral and its investigation through laser ablation-inductivelycoupled plasma (multi-collector)-mass spectrometer (LA-ICP-(MC)MS) has allowed results to directlyconstrain the timing of emplacement, as well as petrology and geochemistry of parental melts.Zircons are characterized by very large concentration in rare earth elements (REE), Th, U, Nb and Ta,and negative Eu anomaly. U-Pb analysis points to Norian emplacement ages (223 ± 7 Ma and 224± 6 Ma), whereas large positive ?Hf(t) values (+13 on average) indicate a derivation from depletedto mildly enriched mantle source. The mantle signature and the corundum oversaturation werepreserved thanks to limited crustal contamination of the host, high-temperature refractory granulitesand mafic intrusives. According to the geochemical data and to the evidence of the development ofviolent explosions into the conduits, it is proposed that dykes segregated from peraluminous meltsproduced by exsolution processes affecting volatile-rich differentiates during alkaline magmatism.This work provides robust evidence about the transition of the geochemical affinity of Southern Alpsmagmatism from orogenic-like to anorogenic during Norian time, linked to a regional uprising of theasthenosphere and change of tectonic regime.