Accretion of arc-oceanic lithospheric mantle in the Mediterranean: evidence from extremely high-Mg olivines and Cr-rich spinel inclusions in lamproites

Si-rich Mediterranean type lamproites (48-56 wt.% SiO₂) are olivine-phyric, mantle-derived volcanics, in which both phenocrystic and xenocrystic olivine are present. Here we demonstrate the phenocrystic origin of the most extremely NiO-MgO enriched olivine in lamproites with Mg# up to 0.95, that host Cr-rich (Cr# around 0.95) spinels. Our comprehensive study of olivine-spinel pairs from Mediterranean lamproites enables us to constrain the extent of depletion of their mantle source. Olivine-spinel pairs from primitive Mediterranean lamproites plot in the most refractory part of the olivine-spinel mantle array diagram, showing even more refractory character than mineral pairs from boninites and cratonic mantle xenoliths. This indicates involvement of an ultra-depleted mantle component which had previously lost up to 40% basaltic components. These characteristics would fit the involvement of depleted subcratonic lithospheric mantle which underwent komatiite extraction in the Archaean, but mantle of such composition and age is known neither from ultramafic xenolith suites nor in tectonically emplaced ultramafic massifs in western and central Europe. Here, we explore the possibility that the extremely depleted component of the mantle source of Mediterranean lamproites is derived from an island-arc oceanic lithosphere accreted during Alpine collisional processes. This may provide compelling evidence for recent accretion of oceanic lithospheric blocks during Mesozoic subduction-collision processes in the Mediterranean.