IGNEOUS PETROGENESIS IN THE STRAIT OF SICILY RIFT ZONE

The Strait of Sicily Rift Zone (SSRZ) is a northwest-southeast trending transtensional rift situated in the Mediterranean Sea between Sicily and north Africa. The SSRZ consists of three basins: the Pantelleria Trough, the Linosa Basin, and the Malta Trough. Volcanoes are situated in or adjacent to all except the Malta Trough, and include two islands (Pantelleria and Linosa) and several seamounts. The thickness of the crust throughout the region is 25-35 km, but thins to 16-18 km in the basins of the SSRZ. The Pantelleria Trough is characterized by high average heat flow values and a strong positive Bouguer anomaly, which suggest asthenospheric upwelling to ~60 km and the presence of abundant basaltic material at depth. Mafic lavas ranging in composition from alkaine and transitional basalt to hawaiite (~45-50 wt% SiO₂) comprise the dominant volume of eruptive products in the SSRZ; evolved rocks (trachyte and pantellerite) and common only on Pantelleria, where they crop out over ~94% of the island’s surface. There are two distinct groups of mafic lavas: Trend A is characterized by higher TiO₂, K₂O, P₂O₅, LILE, and HFSE for a given MgO as well as higher Ce/Y and LREE/HREE ratios than Trend B. Both trends have similar MREE/HREE ratios and Sr-Nd-Pb isotpic ratios. Melting conditions in the SSRZ are constrained by integrating three different geochemical modelling techniques: INVMEL, pMELTS, and recalculation of primary magma composition and olivine thermobarometry. The results suggest that both groups of mafic lavas originated form a simliar, primitve mantle source with a mantle potential temperature of ~1443°C and evolved via isentropic partial melting from depths of ~100-110 km to 60 km, largely within the garnet-spinel transition zone. Lower degrees (~1-2%) of partial melting resulted in Trend A, consisting of discrete batches of magma with high, but variable LREE/HREE that did not form high level magma chambers. Higher degrees (~3-4%) of partial melting resulted in magma with lower and less variable LREE/HREE (Trend B) that did form high level magma chambers from which the felsic magmas in the SSRZ evolved, primarly via polybaric (basalt) and isobaric (trachyte-rhyolite) fractional crystallization processes.