The geochemical peculiarity of "Plio-Quaternary" volcanic rocks of Sardinia in the circum-Mediterranean area

Late Cenozoic volcanic rocks on the island of Sardinia are mildly alkaline- transitional lavas, dominantly hawaiites, mugearites, and transitional basalts with minor phonolites and trachytes, which form ∼80% of the entire sample population. Tholeiitic basaltic andesites form the remaining 20% of the analyzed rocks. The oldest lavas, the ca. 6.6-4.4 Ma radiogenic Pb volcanic group, are in southern Sardinia; they have geochemical characteristics very similar to most Circum-Mediterranean Anoro- genic Cenozoic Igneous Province rocks. After a gap of ∼0.5 m.y., volcanism occurred in central and northern Sardinia, from ca. 3.9 to ca. 0.1 Ma. These products, the unradio- genic Pb volcanic group, are geochemically very different. Their geochemical characteristics (relatively high SiO2, low CaO, and CaO/AI2O3, relatively high Ni, relatively low high field strength elements, low heavy rare earth elements, high Ba/Nb and La/Nb, slightly high 87Sr86Sr, and unradiogenic 143Nd/144Nd and 206Pb/204Pb ratios) are considered to be derived from an orthopyroxene-rich lithospheric mantle source. The origin of this enrichment in orthopyroxene is a consequence of SiO 2-rich melt derived from delaminated and detached ancient lower continental crust reacting with mantle peridotite. The presence of two distinct groups of rocks (unradiogenic Pb volcanics and radiogenic Pb volcanics) in a very close geographic position is related to the existence of a lithospheric discontinuity running roughly E-W in southern Sardinia. © 2007 Geological Society of America.