À12, e Hf = À11) accounts for the enrichment of K and other large-ion-lithophile elements in the Italian volcanics. As shown by the relationship in e Hf -e Nd space and the lower-thanchondritic Hf/Sm ratio, this crustal component is dominated by pelagic sediments rather than terrigenous material. The overall scarcity of calc-alkaline compositions in the Italian volcanics and the presence of a HIMU component, which is the hallmark of hot spot basalts, raise the question of how plume mantle source contributes to volcanism in a subduction environment. At about 13 Ma, the Apennine collision terminated the westward subduction of the Adria plate under the European margin and rotated the direction of convergence to the northwest. The cumulative differential of subduction between the fossil plate under Tuscany and the active plate under Sicily since the opening of the Tyrrhenian Sea amounts to at least 300 km and is large enough to rift the dipping plate and open a plate window beneath the southern part of the peninsula. This model is consistent with recent high-resolution seismic tomography. We propose that the counterflow of mixed upper and lower mantle passing the trailing edge of the rifted plate is the source of Italian mafic volcanism. Alternatively, material from a so-far unidentified plume may be channeled through the plate window. The crustal signature is probably acquired by interaction of the mantle advected through the window with the upper part of the subducted plate.

No abstract

Up to 10 per cent of the ocean floor consists of plateaux--regions of unusually thick oceanic crust thought to be formed by the heads of mantle plumes. Given the ubiquitous presence of recycled oceanic crust in the mantle source of hotspot basalts, it follows that plateau material should also be an important mantle constituent. Here we show that the geochemistry of the Pleistocene basalts from Logudoro, Sardinia, is compatible with the remelting of ancient ocean plateau material that has been recycled into the mantle. The Sr, Nd and Hf isotope compositions of these basalts do not show the signature of pelagic sediments. The basalts' low CaO/Al2O3 and Ce/Pb ratios, their unradiogenic 206Pb and 208Pb, and their Sr, Ba, Eu and Pb excesses indicate that their mantle source contains ancient gabbros formed initially by plagioclase accumulation, typical of plateau material. Also, the high Th/U ratios of the mantle source resemble those of plume magmas. Geochemically, the Logudoro basalts resemble those from Pitcairn Island, which contain the controversial EM-1 component that has been interpreted as arising from a mantle source sprinkled with remains of pelagic sediments. We argue, instead, that the EM-1 source from these two localities is essentially free of sedimentary material, the geochemical characteristics of these lavas being better explained by the presence of recycled oceanic plateaux. The storage of plume heads in the deep mantle through time offers a convenient explanation for the persistence of chemical and mineralogical layering in the mantle.