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 ma gmas from qz tholeiites to nephelinites. Incompatible element patterns , gradually increasing from tholeiites to nephelinites, share geochemi cal 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 0.7 0287-0.70327 and Nd-143/Nd-144 0.51302- 0.51291 for alkaline lavas. An integrated petrogenetic model based on phase equilibria, major and tr ace element compositions and geothermobarometry of lavas and included mantle xenoliths leads to the following constraints: (1) most of the m agmas were generated within spinel peridotite facies lithospheric mant le from progressively deeper sources (30 to similar to 90 km depth), w ith concomitant decrease in the degree of melting (from 30 to 3%), whi ch is positively correlated with MgO content from tholeiites to nephel inites; (2) alkalinity and incompatible element contents are controlle d by the degree of partial melting and source enrichment related to as thenospheric metasomatizing melts or fluids infiltrating depleted lith ospheric mantle; (3) mantle sources have to be lherzolites bearing met asomatic amphibole +/- phlogopite for tholeiites (S-1), alkali basalts and basanites (S-2), and clinopyroxene-rich lherzolites (or even wehr lites) bearing amphibole + phlogopite + carbonatitic metasomatic compo nents for nephelinites (S-3); the Sr-Nd isotopic differences between a lkaline and sub-alkaline lavas are consistent with a strong alkali-sil icate +/- carbonatitic metasomatism of the deepest lithospheric mantle sources, and with a less intensive enrichment, only by alkali-silicat e agents, of the upper lithospheric mantle; (4) melting processes appe ar to be controlled by the high geothermal gradient in the area (close to the hydrated mantle solidus) and are probably triggered by local d ecompression effects related to the lithospheric transtensive fault sy stem.