Comparative study of ultramafic xenoliths and associated lavas from South-Eastern Sicily: nature of the lithospheric mantle and insights on magma genesis

Abstract

Isotope analyses of lavas from the Hyblean Plateau (SE Sicily) provided first order constraints for the characterization of the relative magma sources, contributing to a better understanding of the tectono-magmatic evolution of the northernmost part of the African plate. To integrate this research, we are currently studying ultramafic xenoliths of mantle provenance exhumed by the Hyblean alkaline volcanics. These mainly consist of anhydrous spinel-facies peridotites and subordinate pyroxenites/websterites. The paragenesis of the latter is extremely variable in terms of modal proportions and mineral composition, with clinopyroxene composition ranging from Cr-diopside to Al-augite, and variable amount of spinel ± garnet. New Sr-Nd isotopic analyses carried out on hand-picked (and leached) clinopyroxenes indicate that peridotites have 87Sr/86Sr ranging from 0.70288 to 0.70309, and 143Nd/144Nd from 0.51287 to 0.51292. This Sr-Nd isotopic fingerprint approaches that of the HIMU mantle end-member, regionally referred as EAR (European Astenospheric Reservoir) to emphasize a connection with a sub-lithospheric metasomatic component ubiquitous throughout Europe, the Mediterranean area and North Africa. The Sr-Nd analyses of pyroxenites also reveal an EAR affinity, but are slightly distinct from those of peridotites. They display 87Sr/86Sr ranging between 0.70305–0.70326 and 143Nd/144Nd between 0.51292–0.51299, overlapping the composition typical of the Hyblean alkaline lavas. This possibly implies that pyroxene-rich domain significantly contributed to the genesis of the Hyblean magmas. Therefore, the presented Sr-Nd isotopic ratios suggest that the role of the pyroxene-rich mantle portions was more important than commonly considered in the petrogenetic models of Hyblean magmas. This scenario would be coherent with recent evidences highlighting the importance of pyroxenites (together with peridotites) as contributing sources to basalt generation.