U-Pb zircon data for tuffaceous layers from the sedimentary cover of the Strona-Ceneri Zone and Po Plain: constraints on the Triassic geodynamic evolution of the Southern Alps.

In the western sector of the Southern Alps, tuffaceous levels areoccasionally found in the Triassic sedimentary sequences. The direct agecharacterisation of these layers is very limited: in fact, a U-Pb zircon ageof 245±1 Ma is available only for the tuffites from Mt. San Giorgio area(Lugano, CH: [1]), which results slightly older than that expectedaccording to the stratigraphic position. Besides, no geochemicalinformation is presently available for these layers and the geodynamicsetting of the related volcanism is still matter of debate.Thus, with the aim to place further constraints of the geodynamicevolution of the Southern Alps in Mesozoic times and increase theaccuracy of the age record of the sedimentary sequence, we haveperformed a mineralogical, geochemical and geochronological study ontuffaceous deposits outcropping within the Anisian-Ladinian succession ofthe sedimentary cover of the Strona-Ceneri Zone in the Borgosesia area[2,3] and recovered at the same stratigraphic position in wells of theVillafortuna-Trecate oil field (western Po Plain, Piedmont region, NW Italy:[4]). CO2 concentration was determined by Dietrich-Früling calcimeter.XRD data indicate that such tuffaceous deposits are constituted by avariable mixture of magmatic and sedimentary components. Major andtrace element compostions, assessed by means of XRF analysis, suggest acalc-alkaline affinity for the magmatic component. To provide accurategeochronological constraints, zircons have been separated withconventional methods from four tuffaceaous outcrops, namely "Lembo diSostegno", "Lembo di Crevacuore" and "Lembo di Monte Fenera"outcrops, as well as from a buried tuffaceous sample of theVillafortuna-Trecate oil field. Zircons were mounted in epoxy resin andcharacterised under cathodoluminescence (CL). Based on colour,morphology and internal structure, they have been divided in twopopulations. One group is constituted by light-pink coloured zircons withprismatic habits and tight oscillatory zoning suggesting growth undermagmatic conditions. The zircons from the second group are colourless,rounded in shape and with only relics of magmatic zoning, consistent withmetamorphic recrystallization. U-Pb ELA-ICP-MS data point to ages of237±8 Ma and 229±9 Ma for the magmatic growth of the zircons from thetuffites of "Lembo di Crevacuore" and "Lembo di Sostegno", respectively,which are in agreement with their stratigraphic position. The volcanicactivity producing these layers might be linked to the intrusive Triassicmagmatism documented in the Finero Complex (NE Ivrea-Verbano Zone[5,6]). Conversely, the zircons from the "Lembo di Monte Fenera" andVillafortuna-Trecate oil field produce a very large range ofcrystallisation-recrystallisation ages, which span from Proterozoic toPaleozoic to the Permian-Triassic boundary, thus indicating a dominantcontribution of the crystalline basement to the zircon population.