JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B4, PAGES 4589-4601,APRIL 10, 1989 The Volcanism of Southern Italy' Role of Subduction andthe Relationship Between Potassic and Sodic AlkalineMagmatism R. M. ELLAM, • C. J. HAWKESWORTH, 2M. A. MENZmS 3 • N. W. ROGERS 2 The ultrapotassic magmatism of southern Italy (the Roman province) is well known,and recently these highlyunusual lavas havebeen explained in terms of subduction-related processes. Lesswell studied are the coeval calc-alkaline to potassic rocks of the nearby Aeolian Islands, which are situated above a Benloft zone and are therefore demonstrably relatedto recentlyactive subduction. On a number of geochemical diagrams the Roman and Aeolianprovinces definecontinuous trends, which maybe 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 mixedwith mantle wedge material which, priorto subduction, wascharacterised by highly radiogenic Pb isotope ratios, and is thereforecomparable to the mantle source of Mount Etna volcanism. Thus it would appear that midplatetholeiitic to Na-alkalic magmatism and continental margin calc-alkalineto ultrapotassic magmas were derivedfrom 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. INTRODUCTION The Cenozoic volcanism of central and southern Italy has intrigued several generations of geologists and volcanologists. Not only are volcanoes such as Vesuvius, Etna and Stromboli notorious for their influential, and often catastrophic effects on civilisations throughout recorded history, but their names have become synonymous with their unusual and diverse range of magmatic products. In particular, the potassic to ultrapotassiclavas of the Roman and Campanian regions have attracted much attention. Recently, this K-alkaline magmatism has been attributed to subduction-related mantle enrichment processes[Thompson, 1977; Rogers et al., 1985]. In contrast, but associated with the potassicmagmatism in both space and time, are a number of Na-alkalic volcanoes. The best known of these is Mount Etna [Carter, 1976; Carter and Civetta, 1977], but they also occur on the island of Ustica [Beccaluva et al., 1982] and, albeit somewhat older (about 60 Ma), at the mainland centre of Pietre Nere [Vollrner, 1976]. We present new data from the calc-alkaline (Salina, Stromboli) to potassic (Stromboli, Vulcano) volcanoes of the Aeolian archipelago(Figure 1) and review the subductionconnection and the relationship between K-alkaline and Na-alkaline volcanism elsewhere in southern Italy, in the light of these recently aquired data fEllam, 1986; Ellam et al., 1988]. 'Department of Earth Sciences, Oxford University, Oxford, United Kingdom. 2Departmentof Earth Sciences, The Open University, Milton Keynes, United Kingdom. 3Department of Geology, Royal Holloway and Bedford New Col- lege, Egham, United Kingdom. Copyright 1989 bythe American Geophysical Union. Paper number 7B7148. 0148-0227/89/007B-7148505.00 This region has remained relatively poorly studied, despite the similarity of some of its lavas to those of the Roman province low-potassium series (LKS) and notwithstanding the demonstrable association with recently active subduction. Clearly, the Aeolian Island lavas provide an interesting example of alkalic subduction zone volcanism in their own right, and study of the Aeolian lavas may also shedlight on the origin of the Romanprovince magmatism. Corsica Sardinia Stromboli •o •' •,•-P ...... '...Tuscan .."•k Vulsini & Rome Campi Tyrrhenian Sea Ernici Pietre Nere• -t) Roccamonfina • Naples • • / 13oo / / / / / / // / / Aeolian/Islands Us[ica Sicily Etna Fig. 1. Distribution of Italian volcanoes. Dashed lines are depth contours (in kilometres) to Benioff zone [Anderson and Jackson, 1987]. Solid lines indicate major faults [Gasparini et al., 1982]. Inset: details of Aeolian Island localities. 4589