U-Th-Pb “multi-phase” approach to the study of crystalline basement: application to the northernmost sector of the Ivrea-Verbano Zone (Alps)

In situ U-Pb geochronology was carried out on amphibolites and siliciclastic metasediments of the Kinzigite Formation exposed in the northernmost sector of the Ivrea-Verbano Zone (Finero area). The aim is to shed light on the tectono-metamorphic evolution of this intermediate-lower crustal section and its bearing with the evolution of the southern and better known sectors of the Ivrea-Verbano Zone. Based on eld observation and petrography, a metamorphic gradient gently increasing from amphibolite to upper amphibolite facies (from SE to NW) characterizes the whole metamorphic sequence. Granulite facies conditions are reached only by the slivers (septa) of the Kinzigite Formation into the External Gabbro unit of the Finero Ma c Complex. Metapelites consist mainly of biotite, quartz, plagioclase, garnet, and brolitic to prismatic sillimanite; muscovite progressively disappears as K-feldspar appears and becomes abundant. Amphibolites are made of green-brown hornblende and plagioclase and may contain clinopyroxene de ning thin layers together with plagioclase and titanite. Both metapelites and amphibolites show mylonitic deformation which is more intense towards NW, i.e. towards the lower structural levels. The mylonitic deformation strongly affected the lower crustal metabasic rocks of External Gabbro unit (Finero Ma c Complex). Zircon, monazite and titanite U-Pb geochronology was carried out with laser ablation (LA)- ICP-MS on amphibolites, migmatitic paragneiss and septa. The multi-chronological approach allowed recognizing three discrete tectono-metamorphic events, at Permian, Triassic and Jurassic. Zircon and monazite yielded Permian ages suggesting (re)crystallization during an high temperature event characterized by both metamorphism and magmatism. Titanite dating provided Triassic and Jurassic ages, that were interpreted as U-Pb resetting ages. A Triassic perturbation of the U-Pb system was also recorded by zircon and monazite as rare domains. The tectono-metamorphic reconstruction of the evolution of the northernmost Ivrea- Verbano Zone, as revealed by the new geochronological data, is only partially in agreement with the temperature-time evolutions depicted for the southern sectors of the Ivrea-VerbanoZone. Permian ages indicating magmatism and high temperature metamorphism are common throughout the Ivrea-Verbano Zone, as well as the Jurassic ages related to local thermal pulses likely related to lithospheric thinning and associated lithospheric advection. Conversely, the occurrence of Triassic ages is peculiar of the Finero area. Two possible explanations may account for this Triassic event: Triassic ages are possibly related to the thermal effect and uid circulation during the emplacement of the External Gabbro unit; or alternatively, they are the response to the ductile deformation largely recognized in the whole area. This study is a further evidence of the necessity of approaching crystalline basement with multiple geochronometers in order to unravel the complete tectono-metamorphic evolution.