Geochemistry and zircon U-Pb geochronology of magmatic enclaves in trachytes from the Euganean Hills (NE Italy): further constraints on Oligocene magmatism in the eastern Southern Alps

Abstract

This study presents the results of the geochemical and geochronological investigation of magmatic enclaves in trachytes from the Euganean Hills (NE Italy). The Euganean Hills magmatic district is the most recent (Eocene-Oligocene) volcanic distric within the Veneto Volcanic Province (VVP), which represents the main magmatic event in the eastern Southern Alps. The studied enclaves include intrusive and crystal mush types, the latter characterised by embayments of the host trachyte towards the core of the enclaves. Both types have zircon U-Pb ages of ~30–32 Ma, indicating that they are contemporaneous with the second Euganean magmatic phase. Amphibole-bearing intrusive enclaves (syenites and quartzo-monzonites) and crystal mushes have similar major and trace element compositions ( e.g. REE, Rb, Ba, K, Nb, Pb, Th and Y contents) as those of the host trachyte. They are interpreted as magma chamber solidification fronts with different degrees of crystallization, which were caught as enclaves by the trachytic magma during its ascent. Amphibole-free intrusive enclaves show peculiar trace element compositions (depletion in Th, U, Nb, Ta, MREE and HREE, and marked Eu positive anomaly), in contrast to the compositions of Euganean lavas and other studied enclaves, and are interpreted as cumulative rocks resulting from concentration of crystals fractionated during trachyte–rhyolite evolution. This study confirms that low-pressure fractional crystallisation played a primary role in the evolution of magma in the VVP. The first zircon U-Pb ages from the Euganean rocks prove a Lower Oligocene age for the second Euganean magmatic phase. The use of this robust chronometer should be extended to the Euganean lavas, to better constrain the chronological sequence of eruptive events.