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

 Multidisciplinar Approach to Natural Triggers of Lithosphere Evolution

The trace element (e.g., relatively high Nb-Ta contents) and Sr-Nd-Pb isotopic characteristics (87Sr/86Sr = 0.7027-0.7036; 143Nd/144Nd = 0.51325-0.51290; 206Pb/204Pb = 18.87-20.00; 207Pb/204Pb = 15.57-15.70; 208Pb/204Pb = 38.60-39.58) are EAR-like with similarities to HIMU OIB. Widely varying normalised abundances of the most incompatible elements reflect varying degrees of partial melting of a heterogeneous mantle source. The origin of the most recent volcanic activity in the Hyblean Plateau has been related to extensional tectonics and the development of pull-apart basins, with subsequent adiabatic decompressive melting or to partial melting of a depleted mantle source, variably modified by asthenosphere-derived metasomatizing melts or fluids, with both alkali-silicate and carbonatitic compositions, capable of stabilizing metasomatic phases (e.g., amphibole, phlogopite, apatite and carbonate). TYRRHENIAN SEA: The Tyrrhenian Sea represents a back-arc basin that started to open behing the Apennine-Maghrebide Chain at about 15 Ma. The northern sector opened east-ward at a rate of about 1.3 cm/a, much more slowly than the southeast-ward moving southern sector (~4.8 cm/a). A limited portion of the Tyrrhenian Sea is floored by igneous rocks ranging in age from ~12 Ma to the Present. The back-arc opening of the Tyrrhenian reached an oceanization stage near two basins: the northern Vavilov basin (8-2 Ma) and the southern Marsili basin (2-0.1 Ma). Within these two sub-basins, two large volcanic structures have been identified: the Vavilov (~3-2 Ma) and Marsili seamounts (~1-0.1 Ma), whose composition deviates from N-MORB types towards E-MORB-calc-alkaline basalt. These compositions are thought to be related to mantle sources metasomatized by interaction with the retreating Calabria subduction zone. Magmatism in the northern Tyrrhenian Sea is characterized by emplacement of granitoids plutons (e.g., Vercelli seamount) and minor mafic to felsic volcanic rocks related to the Tuscan Magmatic Province. Southern Tyrrhenian Sea rocks range in compositions from N-MORB to E-MORB (e.g., DSDP sites 373 and 655) plus minor OIB-like (e.g., Magnaghi, Vavilov, Quirra, Etruschi, Prometeo and Aceste seamounts) and “orogenic” (arc-like) composition (andesitic to potassic; e.g., DSDP sites 650-651, Marsili, Palinuro, Anchise and Cornacya seamounts). A southeast-ward migration of “orogenic” magmatism from Sardinia (~32-15 Ma) to the Aeolian Islands (1-0 Ma) has been pointed out.

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