Depletion events, nature of metasomatizing agent and timing of enrichment processes in lithospheric mantle xenoliths from the Veneto Volcanic Province

Mantle xenoliths included in the alkaline basic lavas from the Paleogene Ve neto Volcanic Province (VVP) consist of predominant spinel lherozlites (21- 6% clinopyroxene) and minor spinel harzburgites (4 2% clinopyroxene), mainl y protogranular textured. Most of the xenoliths show superimposed textural evidence of metasomatic processes, consisting of reaction patches and spong y clinopyroxenes, variably associated with glass and secondary olivine, cli nopyroxene, spinel and feldspar. Whole-rock and mineral major and trace ele ment data indicate a complex history of depletion and enrichment processes undergone by the continental lithospheric mantle beneath a within-plate reg ion. Protogranular-textured clinopyroxenes from lherzolites show heavy rare earth element (HREE) contents similar to 10 times chondrite and strong lig ht REE (LREE) depletion [(La/Yb)(N) = 0.002-0.025)], whereas those in harzb urgites vary from slightly LREE depleted to LREE enriched [(La/Yb)(N) = 0.6 4 4.00], with much lower HREE contents (similar to 3 times chondrite). Most HREE patterns can be reproduced by a simple fractional melting model and e xtraction of similar to 5-22% basic melts, starting from the most fertile V VP spinel lherzolite. The anomalously low HREE contents of clinopyroxenes i n a single lherzolite sample, however require more complex processes possib ly involving multistage melting of garnet- to spinel-bearing sources, or di ffusion-controlled melt-peridotite interactions. The slightly LREE-depleted to LREE-enriched patterns of protogranular-textured clinopyroxenes in harz burgites and spongy clinopyroxenes in lherzolites [(La/Yb)(N) = 0.27-5.67] can be accounted for by metasomatic enrichment events. Major and trace elem ent mass balance calculations were successfully carried out to quantitative ly model the metasomatic parageneses by the addition of 1 6% of Na-alkaline basic melt/s. Magmatic analogues of the modelled metasomatizing agent/s oc cur in the South Alpine domain as late Cretaceous lamprophyric dykes, as we ll as VVP alkaline basic lavas. An estimate of timing of the enrichment pro cesses was achieved by a REE inward diffusion model in primary and spongy c linopyroxenes, assuming LREE-enriched secondary clinopyroxenes as the bound ary surface composition. Model calculations imply a complete chemical rehom ogenization of a 1 mm crystal in a time span of 4.8 16 kyr. Consequently, t he observed REE zoning of clinopyroxenes, together with the presence of gla ssy patches, indicates that the most recent metasomatic processes occurred shortly before the entrainment of mantle xenoliths by the host lavas.