The relationship between potassic, calc-alkaline and Na-alkaline magmatism in South Italy volcanoes: A melt inclusion approach

Abstract

The present-day tectonic setting of the Tyrrhenian Sea is dominated by the eastward migration of the Tyrrhenian–Appenines subduction system and the existence of a contemporaneous and parallel extensional–compressional regime. This complex setting leads to the occurrence of a wide spectrum of magma-types in the South Italy volcanoes. Here, major and trace-element data for primitive melt inclusions preserved in olivine phenocrysts have been obtained in order to add constraints on the origin of the calc-alkaline magmas from the Aeolian arc (Stromboli and Vulcano islands), the potassic magmas from the Campania Province (Vesuvius and Phlegraean Fields) and the Na-alkaline magmas from Ustica Island. The approach used to determine the possible mantle sources of the trapped melts for each population of melt inclusions is based on the determination of the trace-element incompatibility sequence taken as the relative order of increasing bulk partition coefficients, which depends on the mineralogy of the source and gives direct information about minerals residual at the time of melting. Compositional similarities between the melt inclusions and their host lavas suggest that shallow-level magma contamination did not contribute significantly to the geochemical characteristics of the magma-types encountered in the region. Results of the trace-element modelling indicate that the melt inclusions from the Aeolian Islands and Campania Province volcanoes originate from mantle sources strongly affected by subduction-related metasomatic processes. Trace-element relationships of melt inclusions from Vulcano and Stromboli reflect melting of peridotitic sources that have been enriched by a slab-derived, aqueous fluid formed during dehydratation of K-free phases at shallow to intermediate depths. The negative high-field strength elements (HFSE) anomalies of these inclusions were generated in the absence of any residual phase in which HFSE might be compatible. In addition, their major element characteristics require the involvement of a clinopyroxene-rich source component in their genesis. With regard to the Vesuvius and Phlegraean Fields melt inclusions, their calculated incompatibility sequences point to a common phlogopite-bearing mantle source likely to be the result of interaction and hybridisation reactions with K2O and H2O-rich phases released from the slab at larger depths. Finally, trace-element characteristics of melt inclusions from Ustica Island imply the derivation from an upwelling enriched intraplate mantle source with a HIMU-type signature reflecting an ancient basaltic source component. Moreover, the source seems to be contaminated by a slab-derived fluid due to the rollback motion of the Ionian slab, in a process similar to that operating beneath Mount Etna, South of Ustica.