Miocene subduction-related magmatism in southern Sardinia: Sr–Nd- and oxygen isotopic evidence for mantle source enrichment

Subduction beneath southern Sardinia in Miocene times gave rise to a suite of basalts, basaltic andesites, high-K andesites and rhyolitic ignimbrites. The Monte Arcuentu volcanic complex is a typical product of this activity. Although the volcanic rocks display some tholeiitic affinities, in terms of K2O–SiO2 relations they resemble calc-alkaline lavas of the Aeolian and Aegean arcs. The most mafic members of the suite have moderately high Ni (164 ppm) and Cr (550 ppm) contents and high MgO (10 wt.%) and are therefore mantle-derived. Mantle-normalised trace element patterns show high K, Rb, Ba and Th, but low Nb and Ta, characteristic of subduction-related magmas. Sr contents in almost all samples are particularly low (165–265 ppm). 6Sri values range from 0.7054 to 0.7113, whilst ϵNdi is between +0.9 and −8.8. The Sr–Nd isotopic field overlaps those of the high-K rocks of Roccamonfina and the Roman Volcanic Province, and extends that of the Aeolian Islands and the Aegean arc. Correlations of radiogenic isotope ratios with SiO2 values in bulk rocks are much stronger than with MgO. Conventionally analysed δ18O whole rock values range from +6.4‰ in basalt to +12.4‰ in a high-K andesite. However, δ18O values for clinopyroxene phenocrysts, analysed by a laser fluorination method, range from +6.17 to +7.46‰. Good correlations are seen between δ18Ocpx values and both MgO and SiO2. Differences between δ18Owhole rock and δ18Ocpx values are probably due to late stage alteration of the groundmass, and cast doubt on the validity of petrogenetic interpretations based solely on whole rock oxygen isotope analysis. –Nd–O-isotopic variation can be modelled either by contamination of mantle-derived magmas by Hercynian continental crust, or by extensive enrichment of the mantle source by subducted sediments. In the δ18Ocpx vs 87Sr/86Sri diagram, crustal contamination hyperbolae are concave-upward because of the low Sr contents of the Monte Arcuentu magmas, so both crustal contamination trends and mantle-enrichment trends can generate the same isotopic relationships. However, the strong correlations of radiogenic isotopic ratios with SiO2 and the weak correlations with MgO are most easily explained by extensive (2–10%) mantle enrichment by subducted siliceous sediments. The correlation between SiO2 and 87Sr/86Sri for the Monte Arcuentu magmas is very unusual within subduction-related magmatism, whereas that between δ18O and 87Sr/86Sr appears to show that the suite forms an end-member composition for many island-arc and active continental margin volcanic suites.