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Journal of Petrology Volume 42 Number 1 Pages 173-188 2001
© Oxford University Press 2001
Depletion Events, Nature of Metasomatizing Agent and Timing of Enrichment Processes in Lithospheric Mantle Xenoliths from the Veneto Volcanic Province
ISTITUTO DI MINERALOGIA, UNIVERSITÀ DI FERRARA, CORSO ERCOLE I° d’ESTE 32, 44100 FERRARA, ITALY
Mantle xenoliths included in the alkaline basic lavas from the Paleogene Veneto Volcanic Province (VVP) consist of predominant spinel lherzolites (21–6% clinopyroxene) and minor spinel harzburgites (4–2% clinopyroxene), mainly protogranular textured. Most of the xenoliths show superimposed textural evidence of metasomatic processes, consisting of reaction patches and spongy clinopyroxenes, variably associated with glass and secondary olivine, clinopyroxene, spinel and feldspar. Whole-rock and mineral major and trace element data indicate a complex history of depletion and enrichment processes undergone by the continental lithospheric mantle beneath a within-plate region. Protogranular-textured clinopyroxenes from lherzolites show heavy rare earth element (HREE) contents 
10 times chondrite and strong light REE (LREE) depletion [(La/Yb)N = 0·002–0·025)], whereas those in harzburgites vary from slightly LREE depleted to LREE enriched [(La/Yb)N = 0·64–4·00], with much lower HREE contents (3 times chondrite). Most HREE patterns can be reproduced by a simple fractional melting model and extraction of 5–22% basic melts, starting from the most fertile VVP spinel lherzolite. The anomalously low HREE contents of clinopyroxenes in a single lherzolite sample, however, require more complex processes possibly involving multistage melting of garnet- to spinel-bearing sources, or diffusion-controlled melt–peridotite interactions. The slightly LREE-depleted to LREE-enriched patterns of protogranular-textured clinopyroxenes in harzburgites and spongy clinopyroxenes in lherzolites [(La/Yb)N = 0·27–5·67] can be accounted for by metasomatic enrichment events. Major and trace element mass balance calculations were successfully carried out to quantitatively model the metasomatic parageneses by the addition of 1–6% of Na-alkaline basic melt/s. Magmatic analogues of the modelled metasomatizing agent/s occur in the South Alpine domain as late Cretaceous lamprophyric dykes, as well as VVP alkaline basic lavas. An estimate of timing of the enrichment processes was achieved by a REE inward diffusion model in primary and spongy clinopyroxenes, assuming LREE-enriched secondary clinopyroxenes as the boundary surface composition. Model calculations imply a complete chemical rehomogenization of a 1 mm crystal in a time span of 4·8–16 kyr. Consequently, the observed REE zoning of clinopyroxenes, together with the presence of glassy patches, indicates that the most recent metasomatic processes occurred shortly before the entrainment of mantle xenoliths by the host lavas.
KEY WORDS: mantle xenoliths; metasomatic melts; trace elements; clinopyroxenes; glasses
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