The Cenozoic igneous activity of Sardinia

During the Cenozoic, the island of Sardinia was the location of two different magmatic episodes: 1) a Oligocene to Miocene (hereafter OM) cycle ∼32-15 Ma and 2) a Pliocene to Quaternary (hereafter PQ) cycle (∼5-0.1 Ma). These two volcanic cycles differ in many aspects: 1) geographic occurrence [the OM rocks occur almost exclusively in a graben structure called the Fossa Sarda (Sardinian Trough) that cuts the entire island from north to south, whereas the PQ rocks are scattered throughout the island]; 2) petrography (the OM rocks are mostly porphyritic, whereas the PQ rocks are mostly aphyric); 3) geochemical affinity (the OM rocks are mostly subalkaline with a tholeiitic to calcalkaline character, whereas the PQ rocks are mostly sodic alkaline with fewer tholeiitic types); 4) major element compositions [the OM rocks are mostly dacites to rhyolites with fewer basaltic andesites, andesites and rare basalts while the PQ rocks are mostly hawaiites, mugearites and basaltic andesites, with both SiO2-oversaturated and SiO2-undersaturated evolved types (rhyolites and phonolites); moreover, for a given SiO2, OM rocks have higher CaO, lower TiO2 and lower Na2O compared to the PQ rocks]; 5) trace element abundances and ratios (the OM rocks have lower HFSE and REE contents and higher La/Nb and Zr/Nb ratios compared to the PQ samples); 6) Sr isotopie composition (the OM rocks have 87Sr/ 86Sr generally > 0.7047, whereas the PQ rocks have 87Sr/86Sr generally < 0.7050). 143Nd/ 144Nd and 206Pb/204Pb rations of the OM rocks (from 0 5127 to 0.5122 and from 18.52 to 18.71, respectively) fall within the range of the PQ samples (from 0.5129 to 0.5122 and from -17.5 to 19.42, respectively). The OM rocks show geochemical features typical of magmas emplaced in subduction-related settings. They are believed to have been generated within the mantle wedge developed above a west-directed subduction of (Mesogean?) oceanic lithosphere below the southern continental margin of Europe. On the other hand, the PQ volcanic rocks were emplaced concurrent with the formation of the Tyrrhenian Sea and share some geochemical similarities with magmas emplaced in within-plate (anorogenic) tectonic settings, although they exhibit peculiar characteristics. The PQ rocks can be divided into two groups: one group (Unradiogenic Pb Volcanics = UPV) has relatively high 87Sr/ 86Sr (0.7043-0.7051), low 143Nd/144Nd (0.5124-0.5126), and is characterised by the least radiogenic Pb isotopie composition so far recorded in Italian (and Circum-Mediterranean) Cenozoic igneous rocks (206Pb/204Pb = 17.36-18.07); these are the most widespread volcanic rocks and crop out in central and northern Sardinia. The other group (Radiogenic Pb Volcanics = RPV) has chemical and Sr-Nd-Pb isotopie ratios indicative of a markedly different source (87Sr/ 86Sr = 0.7031-0.7040; 143Nd/144Nd = 0.5127-0.5129; 206Pb/204Pb = 18.8-19.4), and crop out only in the southern part of the island. The less differentiated rocks of the two groups also show distinct trace element contents and ratios (e.g. J3a/Nb>14, Nb/U 24 for the RPV). The transition from igneous cycles with orogenic (s.l.) to anorogenic (s.l.) geochemical features is relatively common throughout the entire circum-Mediterranean area; in other places this shift of chemical compositions has been related to «slab detachment» and/or the development of «slab window» processes. In this paper, to explain the geochemical differences between the OM and PQ volcanic products of Sardinia, we propose the involvement of different mantle sources: an asthenospheric mantle source slightly modified by subduction-related metasomatism for the OM rocks and a hthospheric mantle source strongly modified during ancient times (possibly during Hercynian orogenesis) for the great majority of the PQ volcanic rocks.