Metasomatism of the upper mantle beneath the Hyblean Plateau (Sicily): Evidence from pyroxenes and glass in peridotite xenoliths

Coarse-grained, protogranular spinel peridotite xenoliths in the Upper Miocene Valle Guffari diatreme display a complex history for the shallow mantle underneath the Hyblean Plateau (SE Sicily). The mineral assemblage and composition (olivine Fo_89-91, orthopyroxene En_88-91, Cr-diopside En_48-49 Fs_4-6 Wo_45-48, Cr-rich spinel with cr-number = 25-39) record at least one depletion event caused by melt extraction, followed by metasomatic enrichment. One of these samples (HYB40) hosts a fresh glass vein. Rare earth elements (REE) in clinopyroxenes from these peridotites show three patterns: (1) light REE-enriched (La_n/Yb_n = 7-17); (2) spoon-shaped (La_n/Yb_n = 18-20; La_n/Sm_n = 21-34; Sm_n/Yb_n < 1); (3) nearly flat (La_n/Yb_n ∼3). Whole-rock and clinopyroxene trace elements indicate that these patterns are associated with more or less complete equilibration with at least two distinct metasomatic melts: an alkaline silicate melt resembling the host basalt and a hawaiitic melt (for peridotite HYB40). P-T estimates yield 0.9-1.2 GPa and 870-1050 °C, suggesting that refertilization by metasomatizing melts occurred at the Crust-Mantle boundary or just below. In addition, the P-T data coincide with the palaeogeotherm reported by an earlier worker that is consistent with a high geothermal gradient. However, this thermal regime does not fit with the occurrence of an active mantle plume beneath the Hyblean area because of the deduced mantle potential temperatures, which are almost 200°C lower than those typical for a mantle plume. fO₂ calculation gives a redox state above the fayalite-magnetite-quartz buffer FMQ (up to +1.7 Δlog units) related to melt-driven metasomatism.