Origin and evolution of Cenozoic magmatism of Sardinia (Italy). A combined isotopic (Sr–Nd–Pb–O–Hf–Os) and petrological view

Abstract

The Cenozoic igneous activity of Sardinia is essentially concentrated in the 38–0.1Myr time range. On the basis of volcanological, petrographic, mineralogical, geochemical and isotopic considerations, two main rock types can be defined. The first group, here defined SR (subduction-related) comprises Late Eocene–Middle Miocene (~38–15Ma) igneous rocks, essentially developed along the Sardinian Trough, a N–S oriented graben developed during the Late Oligocene–Middle Miocene. The climax of magmatism is recorded during the Early Miocene (~23–18Ma) with minor activity before and after this time range. Major and trace element indicators, as well as Sr–Nd–Pb–Hf–Os–O isotope systematic indicate complex petrogenetic processes including subduction-related metasomatism, variable degrees of crustal contamination at shallow depths, fractional crystallization and basic rock partial melting. Hybridization processes between mantle and crustal melts and between pure mantle and crustally contaminated mantle melts increased the isotopic and elemental variability of the composition of the evolved (intermediate to acid) melts. The earliest igneous activity, pre-dating the Early Miocene magmatic climax, is related to the pushing effects exerted by the Alpine Tethys over the Hercynian or older lower crust, rather than to dehydration processes of the oceanic plate itself.The second group comprises volcanic rocks emplaced from ~12 to ~0.1Ma. The major and, partially, trace element content of these rocks roughly resemble magmas emplaced in within-plate tectonic settings. From a Sr–Nd–Pb–Hf–Os isotopic point of view, it is possible to subdivide these rocks into two subgroups. The first, defined RPV (Radiogenic Pb Volcanic) group comprises the oldest and very rare products (~12–4.4Ma) occurring only in the southern sectors of Sardinia. The second group, defined UPV (Unradiogenic Pb Volcanic), comprises rocks emplaced in the remaining central and northern sectors during the ~4.8–0.1Ma time range. The origin of the RPV rocks remains quite enigmatic, since they formed just a few Myr after the end of a subduction-related igneous activity but do not show any evidence of slab-derived metasomatic effects. In contrast, the complex origin of the mafic UPV rocks, characterized by low 206Pb/204Pb (17.4–18.1), low 143Nd/144Nd (0.51232–0.51264), low 176Hf/177Hf (0.28258–0.28280), mildly radiogenic 87Sr/86Sr (~0.7044) and radiogenic 187Os/188Os ratios (0.125–0.160) can be explained with a mantle source modified after interaction with ancient delaminated lower crustal lithologies. The strong isotopic difference between the RPV and UPV magmas and the absence of lower crustal-related features in the SR and RPV remain aspects to be solved.