Nephelinitic to tholeiitic magma generation in a transtensional tectonic setting: an integrated model for the Iblean volcanism, Sicily

The Pliocene-Pleistocene volcanism of the Iblean area developed along a NE-SW lithospheric wrench fault system with a wide range of basic magmas from qz tholeiites to nephelinites. Incompatible element patterns, gradually increasing from tholeiites to nephelinites, share geochemical characteristics with within-plate sodic magmas, and show analogies to HIMU and, to a lesser extent, EM II ocean-island basalts (OIB), in agreement with their isotopic signatures: Sr-87/Sr-86 0.70271-0.70302 and Nd-143/Nd-144 0.51325-0.51299 for subalkaline and Sr-87/Sr-86 0.70287-0.70327 and Nd-143/Nd-144 0.51302- 0.51291 for alkaline lavas. An integrated petrogenetic model based on phase equilibria, major and trace element compositions and geothermobarometry of lavas and included mantle xenoliths leads to the following constraints: (1) most of the magmas were generated within spinel peridotite facies lithospheric mantle from progressively deeper sources (30 to similar to 90 km depth), with concomitant decrease in the degree of melting (from 30 to 3%), which is positively correlated with MgO content from tholeiites to nephelinites; (2) alkalinity and incompatible element contents are controlled by the degree of partial melting and source enrichment related to asthenospheric metasomatizing melts or fluids infiltrating depleted lithospheric mantle; (3) mantle sources have to be lherzolites bearing metasomatic amphibole +/- phlogopite for tholeiites (S-1), alkali basalts and basanites (S-2), and clinopyroxene-rich lherzolites (or even wehrlites) bearing amphibole + phlogopite + carbonatitic metasomatic components for nephelinites (S-3); the Sr-Nd isotopic differences between alkaline and sub-alkaline lavas are consistent with a strong alkali-silicate +/- carbonatitic metasomatism of the deepest lithospheric mantle sources, and with a less intensive enrichment, only by alkali-silicate agents, of the upper lithospheric mantle; (4) melting processes appear to be controlled by the high geothermal gradient in the area (close to the hydrated mantle solidus) and are probably triggered by local decompression effects related to the lithospheric transtensive fault system.