The volcanism of southern Italy: Role of subduction and the relationship between potassic and sodic alkaline magmatism

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Abstract
The ultrapotassic magmatism of southern Italy (the Roman province) is well known, and recently these highly unusual lavas have been explained in terms of subduction‐related processes. Less well studied are the coeval calc‐alkaline to potassic rocks of the nearby Aeolian Islands, which are situated above a Benioff zone and are therefore demonstrably related to recently active subduction. On a number of geochemical diagrams the Roman and Aeolian provinces define continuous trends, which may be accommodated in a single petrogenetic model involving mixing of three isotopically and elementally distinct components. Two of these are subduction‐related: first, a high Sr/Nd, high Th/Ta component derived largely from basaltic ocean crust and, second, a component with extremely high Th/Ta, but relatively low Sr/Nd derived largely from subducted sediments. These are mixed with mantle wedge material which, prior to subduction, was characterised by highly radiogenic Pb isotope ratios, and is therefore comparable to the mantle source of Mount Etna volcanism. Thus it would appear that midplate tholeiitic to Na‐alkalic magmatism and continental margin calc‐alkaline to ultrapotassic magmas were derived from mantle sources which, prior to subduction, had similar isotopic signatures. This observation has important implications for the potential involvement of trace element and isotope enriched (OIB‐like) mantle in the genesis of subduction‐related volcanism.