Seminari di Scienze della Terra
Universita’ di Genova
30 Gennaio 2007

 Multidisciplinar Approach to Natural Triggers of Lithosphere Evolution

It has been suggested that the volcanic activity of Ustica (together with that of the Prometeo seamount and Mt. Etna) can be explained without the need to invoke a deep-rooted mantle plume, but considering instead flow of African mantle below Sicily, channeled along the SW edge of the Ionian oceanic plate which is undergoing subduction roll-back. MT. ETNA: The volcanic activity of Mt. Etna started about 0.6 Ma ago characterized by the eruption of tholeiitic to transitional lavas (580-250 ka), and evolved with time towards more sodic alkaline magma compositions (hawaiites, mugearites, benmoreites, trachytes; <250 ka). The scarcity of near-primitive melts is mainly attributable to shallow-level fractionation processes. Moderate assimilation of crustal lithologies and heterogeneity of the mantle source is suggested by the variability of Sr and Nd isotope and trace element ratios (87Sr/86Sr = 0.7030-0.7038; 143Nd/144Nd = 0.51293-0.51283; 206Pb/204Pb = 19.47-20.01; 208Pb/204Pb = 39.11-39.59), which are similar to the EAR mantle reservoir and HIMU OIB. Volcanic activity at Mt. Etna has been considered to be related to asthenospheric flow through a slab window created as consequence of differential roll-back of the subducted Ionian (Mesogean) oceanic lithosphere relative to the neighbouring African and Adrian plates: passive upwelling of asthenospheric mantle through such a slab-window would cause partial melting as consequence of adiabatic decompression. Such asthenospheric “suction” might also mobilise subduction-related fluids coming from the neighbouring Ionian oceanic plate. However, in contrast with this model (that implies a shallow origin for Mt. Etna melts), on the basis of a detailed global seismic tomographic study, other authors have suggested the presence of a deep mantle plume beneath Mt. Etna. HYBLEAN MTS.: Unlike most of the other circum-Mediterranean volcanic provinces, the Hyblean Plateau represents a long-lived magmatic system (~200 Myr), whose early products are late Triassic basaltic rocks while the youngest are Pleistocene. The Tertiary-Quaternary volcanic products include rare strongly alkaline sodic mafic rocks (nephelinites and ankaratrites; ~1%), transitional basalts (~3 %), mildly alkaline sodic rocks (basanites, alkali basalts and hawaiites; ~32 %) and abundant tholeiitic basalts (~62 %).

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