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

Abstract

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#<63; Ni<150 ppm and Cr<270 ppm) different from typical primitive mantle liquids, suggesting low pressure fractional crystallization processes. Alkaline lavas (mainly hawaiite plus rare alkali basalt and mugearite) are commonly characterized by the presence of mantle xenoliths and have higher Mg# (up to 71), Ni (up to 340 ppm) and Cr (up to 420 ppm) than the tholeiitic rocks. Both alkaline and tholeiitic lavas show sub-parallel patterns in primitive mantle-normalized diagrams, with peaks at Ba, Pb and Sr and relatively low abundances of Nb and Ta, resulting in high Ba/Nb ratios (generally >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.