The Cenozoic igneous activity of Sardinia
- ISSN: 03698963
During the Cenozoic, the island of Sardinia was the location of two different magmatic episodes: 1) a Oligocene to Miocene (hereafter OM) cycle approximately 32-15 Ma and 2) a Pliocene to Quaternary (hereafter PQ) cycle approximately 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 SiO (sub 2) -oversaturated and SiO (sub 2) -undersaturated evolved types (rhyolites and phonolites); moreover, for a given SiO (sub 2) , OM rocks have higher CaO, lower TiO (sub 2) and lower Na (sub 2) O compared to the PQ rocks; 5) trace element abundance 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 isotopic composition (the OM rocks have (super 87) Sr/ (super 86) Sr generally >0.7047, whereas the PQ rocks have (super 87) Sr/ (super 86) Sr generally <0.7050). (super 143) Nd/ (super 144) Nd and (super 206) Pb/ (super 204) Pb ratios 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 approximately 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 (super 87) Sr/ (super 86) Sr (0.7043-0.7051), low (super 143) Nd/ (super 144) Nd (0.5124-0.5126), and is characterised by the least radiogenic Pb isotopic composition so far recorded in Italian (and Circum-Mediterranean) Cenozoic igneous rocks (super 206) Pb/ (super 204) Pb = 17.36-18.07); these are the most widespread volcanic rocks and crop out in central and northern Sardinia.