The transition from alkaline to tholeiitic magmas: a case study from the Orosei-Dorgali Pliocene volcanic district (NE Sardinia, Italy)

During the Pliocene, simultaneously with the opening of the Tyrrhenian Sea, mafic magmas were erupted in NE Sardinia (Orosei-Dorgali area). These range from mildly alkaline with sodic affinity (about 80% of exposure) to tholeiitic (about 20%). The tholeiitic rocks (basaltic andesite) are slightly more evolved than the alkaline ones and show geochemical features (e.g., Mg#20). Similar incompatible element ratios for both alkaline and tholeiitic rocks suggest different degrees of melting of a single mantle source. Mathematical modeling indicates ∼4–6% and ∼10–15% partial melting for alkaline and tholeiitic lavas, respectively. Trace element abundances of the Orosei-Dorgali volcanic rocks are typical of Plio-Pleistocene volcanic rocks of Sardinia but differ strongly from other Cenozoic anorogenic volcanic rocks of Europe. Similarly, Sr (87Sr/86Sr=0.70442–0.70455), Nd (143Nd/144Nd=0.512465–0.512558) and Pb (206Pb/204Pb=17.74–17.86; 207Pb/204Pb=15.53–15.60; 208Pb/204Pb=37.89–38.02) isotopic ratios are very unusual when compared with other Cenozoic European volcanic rocks. Trace element abundances and isotopic composition of the Orosei-Dorgali volcanic rocks suggest a lithospheric mantle origin.