The Pliocene Montiferro, volcanic complex (central-western Sardinia, Italy): geochemical observations and petrological implications

The Montiferro volcanic complex is one of the several districts of Middle Miocene-Quaternary magmatism of Sardinia. It was active between 3.9 and 1.6 Ma, with a climax at 3.6 Ma. Erupted rocks have a wide spectrum of compositions, ranging from basic and ultrabasic to highly evolved lithotypes, belonging to three different magmatic suites: 1) strongly alkaline sodic series; 2) mildly alkaline sodic series; 3) tholeiitic series. The products of the strongly alkaline series are mainly basanites, erupted both at the first and at the last stages of Montiferro activity. Strongly alkaline magmas are likely to have risen rapidly to the surface, as testified by the common occurrence of mantle xenoliths and by the absence of differentiated products. The mildly alkaline series ranges from hawaiites to trachytes and phonolites through very abundant mugearites and few benmoreites. Primitive magmas of this series are likely to have ponded at crustal levels, experiencing both differentiation and crustal contamination processes. Finally, the tholeiitic series is mainly represented by rare basaltic andesites and a few basaltic trachyandesites straddling the alkaline/subalkaline boundary. Incompatible trace elements, Sr-87/Sr-86 and Nd-143/Nd-144 isotopic ratios are consistent with the derivation of these products from a common mantle source, represented by a spinel-lherzolite. Primitive magmas of three magmatic suites may have generated by different degrees of partial melting of this source, progressively increasing from strongly alkaline (similar to 1-3% partial melting) to mildly alkaline (similar to 5-7%) and tholeiitic products (similar to 10-12%). Geochemical modellings suggest that this source is represented by a DMM-like lithospheric mantle, metasomatised by small inputs of various crustal components.