Petrogenesis and geodynamic control of intraplate Cenozoic volcanism in Italy

Gianluca Bianchini, Luigi Beccaluva, Costanza Bonadiman, Claudio Natali, and Franca Siena

The Cenozoic evolution of the Mediterranean area has been characterized by various subduction processes and related volcanism (calcalkaline, shoshonitic and ultrapotassic series), in the framework of a general convergence between the African and Eurasian plates and interposed microplates.

Intraplate volcanism closely following or accompanying (in space and time) the subduction-related magmatism also occurred in three main Italian provinces along rift systems: a) Veneto, within the Adria microplate (an “African Promontory”); b) Iblei (Sicily) within the northernmost part of the African lithosphere; c) Sardinia, a drifted fragment of the European lithosphere. In Veneto (Paleogene) and Iblei (Neogene-Quaternary), transtensional rift volcanism developed as foreland reaction to collisional processes along the Alpine and Maghrebian chains respectively, generating basic magmas ranging in composition from tholeiites to Na-alkali basalts and nephelinites. In Sardinia, Neogene-Quaternary volcanism - related to general tensional tectonics of the central Mediterranean - produced comparatively more potassic magmas ranging in composition from subalkaline basalts, alkali basalts/trachybasalts to basanites, locally associated with rhyolitic and trachyphonolitic differentiates.

A review of petrogenetic studies, based on incompatible element and Sr-Nd-Pb isotope systematics for both lavas and associated mantle xenoliths for the three volcanic provinces, leads to the following constraints: 1) the primary magmas, from tholeiites, alkali basalts to basanites and nephelinites, were generated by decreasing melting degrees (30% to 3%) of progressively deeper lithospheric mantle sources (ca. 30 to 110 km depth); 2) extensive and multiple enrichment processes by OIB-type alkali-silicate metasomatizing melts widely affected all mantle sources; 3) a previously depleted lithospheric mantle (DM) is enriched by prevailing HIMU metasomatic geochemical components in both Veneto and Iblean provinces in analogy with other north-african volcanic districts, whereas in Sardinia distinctly more potassic EMI signature predominates (in addition to HIMU), as commonly observed in the European lithosphere.

Regional studies on the associated mantle xenoliths and peridotite massifs suggest that the OIB-type metasomatic agents were possibly active at least since the Mesozoic (Wilson and Bianchini, 1998; Beccaluva et al., 2001b; 2005b) resulting from the remobilization of older components (long-term isolated crustal lithologies recycled within the mantle by previous orogenic cycles), plausibly trapped in the deep transition zone (410-660 km depth). Activation of deep mantle material, ultimately resulting in the studied intraplate volcanism could be interpreted as a far-field dynamic response of the neighbouring Cenozoic subductions, which could have produced localized mantle upwellings. Therefore, the intraplate volcanism of the Central Mediterranean area may reflect a physical (not compositional) effect of the coeval Cenozoic subductions which could have triggered convective instabilities further remobilizing deep mantle domains.