Ultramafic xenoliths from the Veneto Volcanic Province (Italy): petrological and geochemical evidence for multiple metasomatism of the SE Alps mantle lithosphere

摘要

Ultramafic (mg# > 88) xenoliths from the Tertiary Veneto Volcanic Province (SE Alps, Italy) are characterized by variable depletion due to removal of a basaltic component and show significant trace element and isotopic (87Sr/86Sr: 0.703031—0.704356; 143Nd/144Nd: 0.512817-0.513085; 206Pb/204Pb: 18.539—19.694; 207Pb/204Pb: 15.608—15.658; 208Pb/204Pb: 38.412—39.660; δ18O: 6.6—11.2) ) heterogeneity. Variable large-ion lithophile and light rare earth/high field strength element ratios and isotopic compositions occur in samples showing comparable mineralogy and major element content. Petrologic and geochemical characteristics of whole rocks and minerals may be explained by metasomatism of the local mantle lithosphere induced by both alkaline magmas upwelling to the surface (OIB-like mantle diapirism) and interaction with slab-derived material. In comparison with the geochemistry of xenoliths carried by alkaline magmas in typical withinplate tectonic environments, the Veneto xenoliths record a major heterogeneity of the local lithosphere. This appears to be related to a geodynamic scenario that was previously dominated by active continental collision, in which plume-like magmatism subsequently manifested itself.