Affiliation: | aDipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, 35137 Padova, Italy bDipartimento di Scienze della Terra, Università di Milano, Via Botticelli 23, 20133 Milano, Italy cCNR, Istituto di Geoscienze e Georisorse, Via Moruzzi 1, 56127 Pisa, Italy dCNR, Istituto di Geoscienze e Georisorse, Corso Garibaldi 37, 35137 Padova, Italy eResearch School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia |
Abstract: | Mafic alkaline lavas from the Venetian Volcanic Province (NE Italy) contain orange–brown zircon megacrysts up to 15 mm long, subhedral to subrounded and showing equant morphology, with width-to-length ratios of 1:2–1:2.5. U–Pb ages of zircon (51.1 ± 1.5 to 30.5 ± 0.51 Ma) fit the stratigraphic age of the host lava (Middle Eocene and Oligocene) and their oxygen isotope composition (δ18O = 5.31–5.51‰) is similar to that of zircon formed in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g. depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that centre-to-edge zircon zoning is not consistent with evolution of the melt by fractional crystallization. All the above features, together with the fact that zircon and host basalts are coeval, indicate that the studied Zr megacrysts crystallised from a primitive alkaline mafic magma, which later evolved to the less alkaline host magma. |