Un plume mantellico pulsante nel Mediterraneo e nella Tetide Alpina.

We interpret the Oligocene to Recent geodynamic evolution of the western and central Mediterranean asins and surrounding mountain chains in the frame of a plume model. A wide plume head would have progressively grown eastward within the transition zone (410 to 670 km depth), determining asthenospheric asymmetric expansion and consequent stretching of the overlying lithosphere. Rift push forces would have generated compression at the outer border of the thinned lithospheric zone, leading to the nucleation of the Apennine-Maghrebian fold-and-thrust belt system. The development of the Plio-Quaternary Tyrrhenian and peri-Tyrrhenian volcanism was also strictly controlled by the lithospheric extension and asthenospheric unloading, but the very aged isotopic signatures of the parental melt source, characterised by C-H-O and K metasomatic enrichments, reflects a contribution from deep mantle fluids and materials possibly carried by the plume. This contribute is very different from any typical asthenospheric or lithospheric reservoir, being consistently marked by two isotopic end-members, named ITEM and FOZO (for their definitions see BELL et alii, 2004). Basing on the recognition of these geochemical signals also in Late Cretaceous-Eocene Italian igneous products, we deduce that the plume rise must date back to the Alpine Tethys extensional phase. Possibly, during the early Cenozoic, the plume was relatively quiescent allowing the Alpine orogen to form, but still sending tiny fingers of low viscosity ultramafic magmas towards the surface, thus generating a number of isolated lamprophyric occurrences. Starting with early Oligocene times, there was a new pulse of material from the deep mantle into the plume which initiated its growth within the transition zone giving birth to the Mediterranean rift basins and associated magmatism. Two discrete plume pulses would have lead to the opening of the Ligurian and Balearic basins between 30 and15 Ma ago and of the Tyrrhenian Sea between 13 My and recent.