Depletion Events, Nature of Metasomatizing Agent and Timing of Enrichment Processes in Lithospheric Mantle Xenoliths from the Veneto Volcanic Province
Journal of Petrology
analogues of the modelled metasomatizing agent/s occur in the Mantle xenoliths included in the alkaline basic lavas from the South Alpine domain as late Cretaceous lamprophyric dykes, as Paleogene Veneto Volcanic Province (VVP) consist of predominant well as VVP alkaline basic lavas. An estimate of timing of the spinel lherzolites (21-6% clinopyroxene) and minor spinel harzenrichment processes was achieved by a REE inward diffusion burgites (4-2% clinopyroxene), mainly protogranular textured.
... ranular textured. Most model in primary and spongy clinopyroxenes, assuming LREEof the xenoliths show superimposed textural evidence of metasomatic enriched secondary clinopyroxenes as the boundary surface comprocesses, consisting of reaction patches and spongy clinopyroxenes, position. Model calculations imply a complete chemical revariably associated with glass and secondary olivine, clinopyroxene, homogenization of a 1 mm crystal in a time span of 4·8-16 kyr. spinel and feldspar. Whole-rock and mineral major and trace element Consequently, the observed REE zoning of clinopyroxenes, together data indicate a complex history of depletion and enrichment processes with the presence of glassy patches, indicates that the most recent undergone by the continental lithospheric mantle beneath a withinmetasomatic processes occurred shortly before the entrainment of plate region. Protogranular-textured clinopyroxenes from lherzolites mantle xenoliths by the host lavas. 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], KEY WORDS: mantle xenoliths; metasomatic melts; trace elements; clinowith much lower HREE contents (>3 times chondrite). Most pyroxenes; glasses 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, INTRODUCTION require more complex processes possibly involving multistage melting Metasomatic processes recorded in mantle xenoliths of garnet-to spinel-bearing sources, or diffusion-controlled melt- (Menzies et al., 1987; Menzies, 1990; Siena et al., 1991; peridotite interactions. The slightly LREE-depleted to LREE-Hauri et al., 1993; Ionov et al., 1996) have been the object enriched patterns of protogranular-textured clinopyroxenes in harzof increasing interest for earth scientists in recent decades, burgites and spongy clinopyroxenes in lherzolites [(La/Yb) N = as they represent one of the most important mechanisms 0·27-5·67] can be accounted for by metasomatic enrichment events. in modifying mantle compositions and, by implication, Major and trace element mass balance calculations were successfully the sources of basic magmas. In this context, the quancarried out to quantitatively model the metasomatic parageneses by titative evaluation of the metasomatic reactions and identification of their causative agents (Ionov et al., 1994 ; the addition of 1-6% of Na-alkaline basic melt/s.