Rare Earth and 3d Transition Element Geochemistry of Peridotitic Rocks: I. Peridotites from the Western Alps

Trace element analyses have been obtained employing RNAA andINAA techniques for 23 bulk-rock specimens and for five pairsof mechanically separated opx and cpx from Western Alpine peridotites.Investigated rocks include 5 garnet lherzolites from Alpe Arami,and spinel (+plagioclase) lherzolites from Finero (2), Balmuccia(7), Baldissero (6) and Lanzo (3). Three pyroxene pairs wereanalysed from Balmuccia and two from Baldissero. All rocks exhibit marked LREE depletions relative to chondriticabundances except for the two Finero samples which appear tobe HREE depleted. Separated minerals also show LREE depletionsand HREE enrichments relative to chondrites. However, intermediaterare earths are markedly depleted in opx whereas they are enrichedin coexisting cpx. Higher overall concentrations and patternssimilar to those of the bulk rocks indicate that REE distributionsin lherzolites are dominated by clinopyroxene chemistry. Incontrast, both opx and cpx appear to contribute equally to the3d transition element geochemistry of the investigated peridotites. Most of the investigated rocks show the effects of early partialmelting of a pre-existing mantle source material characterizedby ‘chondritic’ REE fractionation and by a 3d transitionelement composition near the estimated values of Jagoutz etal. (1979). The melting process probably developed in a closed system (equilibriummelting) and at temperatures which, for the spinel peridotiteprotolith, seemingly were compatible with estimates of Presnallel al. (1979) for the ‘melting at the cusp’ process(T = 1200–1250 °C). In some cases the residual rocksunderwent a further contamination event. This is particularlyevident for the Lanzo peridotites, but possibly also for singleBaldissero and Balmuccia specimens. During ascent to the surface, the rocks underwent subsolidusannealing which occurred at temperatures around 900–1000°C under more or less closed system conditions.     

Rare Earth and 3d Transition Element Geochemistry of Peridotitic Rocks: II. Ligurian Peridotites and Associated Basalts

Peridotites and associated basalts of the External (EL) andInternal (IL) Ligurian terrains of the Northern Apennines havebeen investigated geochemically. The REE and 3d transition elementcompositions of IL peridotites reflect partial fusion of a spinelperidotite protolith probably in the spinel-plagioclase transitionzone. EL peridotites underwent only incipient melting underclosed system conditions. Basalts associated with EL and ILperidotites have petrography and chemistry akin to MORB. Basedon REE, they can be regarded as the products of fractional melting(IL to some extent at higher F with respect to EL). PositiveEu anomalies in most of the studied IL basalts are due to plagioclasefractional crystallization; also, on the basis of Sc, Co, Nicompositions, fractional crystallization of olivine can be recognizedfor both EL and IL lavas. IL basalts are possibly cogeneticwith IL peridotites, but the same cannot be said for EL lavas.