Metasomatism in the lithospheric mantle and its relationships to magmatism in the Veneto Volcanic Province, Italy

A detailed textural and geochemical study was carried out on a suite of anhydrous, spinel-bearing peridotitic xenoliths hosted in alkaline basic lavas from the Veneto Volcanic Province (VV P). Lherzolites and harzburgites show metasomatic textures superimposed on primary, protogranular textures. On the basis of textural relationships, relative modal percentages and grain size between primary and secondary parageneses, three metasomatic textures were distinguished: Type A characterized by reaction areas around orthopyroxene and spinel, generating olivine, clinopyroxene and spinel secondary crystals; Type B by the abundant presence of glassy patches closely intergrown with olivine, clinopyroxene, spinel and feldspar; and Type C by reaction areas around clinopyroxene, giving rise to spongy or completely recrystallized clinopyroxenes. Unmetasomatized lherzolites display flat REE patterns at about 2-3 x chondrite, while metasomatized lherzolites and harzburgites show a classic spoon-shaped light (L) REE-enriched patterns. Major and incompatible element analyses of primary and secondary minerals (including glass), together with textural features and whole-rock data, allow the definition of at least two distinct events which affected the VVP lithospheric mantle. Geochemical features (including trace elements) of primary clinopyroxenes and spinels, as well as whole-rock geochemistry, indicate that the initial process was of partial melting, followed by enrichment due to infiltration of a strong undersaturated Na-alkali silicate melt as suggested by secondary clinopyroxenes and coexisting glass. The measured Nd and Sr isotopic compositions of whole-rock and separated clinopyroxenes plot in a distinctive array originating from isotopically depleted compositions in the ⁸⁷Sr/⁸⁶Sr vs ¹⁴³Nd/¹⁴⁴Nd diagram (⁸⁷Sr/⁸⁶Sr = 0.702214-0.702543; ¹⁴³Nd/¹⁴⁴Nd = 0.51325-0.51329), close to the DM component, and trending towards the enriched low Nd - high Sr components (HIMU and EMI) recorded by the metasomatized peridotites ( ⁸⁷Sr/⁸⁶Sr = 0.702951 -0.70365, ¹⁴³Nd/ ¹⁴⁴Nd 0.51297-0.51299). Late Cretaceous lamprophyric dykes, cutting the South Alpine Basement at Calceranica (Trento) and well reproducing the geochemical features of the inferred metasomatic melt, plot at the end of the enrichment trend of the VVP peridotites. These isotopic features, coupled with the other geochemical parameters, strongly suggest that a metasomatic agent interacted with the primary mantle assemblage, forming glass and feldspar (plus recrystallized olivine, clinopyroxene and spinel) as reaction products. In the same diagram, VVP basic lavas lie within a field encompassing both the metasomatized xenoliths and the inferred metasomatic agent. On the basis of their incompatible element contents, the presence of amphibole and phlogopite in the VVP magma sources could be invoked. The incompatible element budget related to these minerals was compared with the metasomatic contribution in the mantle xenoliths, calculating the elemental distribution between primary and secondary phases. The strict analogy between incompatible element contents in VVP magma sources and VVP mantle xenoliths, together with their similar isotopic ratios, strongly suggest that the metasomatic agent responsible for the enrichment of the VVP magma sources was very similar to that affecting the VVP xenoliths. It is conceivable that a continuum metasomatic enrichment, starting at least from Late Cretaceous, progressively affected the upper portion of the VVP lithospheric mantle.