Melting and metasomatism in the lithospheric mantle of NE Spain: Geochemical and Sr[sbnd]Nd isotopic characteristics

Abstract

The study of major and trace element compositions of whole rocks and minerals, along with SrNd isotopes on separated clinopyroxene crystals, from mantle xenoliths in Neogene–Quaternary basaltic rocks, serves to evaluate the processes which have affected the subcontinental lithospheric mantle in the Catalan Volcanic Zone of NE Spain, and to compare it to other mantle domains in Europe. These xenoliths are mainly anhydrous spinel lherzolites and harzburgites, with rare olivine websterites. Concentrations of Al2O3, CaO, TiO2 and mildly incompatible trace elements decrease gradually from lherzolites to harzburgites. Light rare earth elements (LREE) and other incompatible trace elements display scatter vs. depletion indexes and usually increase from depleted lherzolites to much enriched harzburgites, indicating that metasomatism affected mainly the latter. Clinopyroxene rare earth element (REE) patterns show slight LREE depletion in most lherzolites and LREE–middle rare earth element (MREE) enrichment in harzburgites. The exceptions are a few clinopyroxene patterns from lherzolites with significant LREE–MREE depletion or LREE enrichment and u-shapes. The SrNd isotopic compositions of clinopyroxene also show a gradual variation from depleted MORB mantle (DMM) lherzolites to very enriched mantle (EM) harzburgites (87Sr/86Sr: 0.702486–0.709772; 143Nd/144Nd: 0.513359–0.512411). Most of these compositions match those for off-craton xenoliths. They are also similar to compositions observed in xenoliths of anhydrous spinel lherzolites and harzburgites from the neigbouring subcontinental lithospheric mantle of SE Spain and the southern part of the Massif Central, France. Fractional melting estimates from a Primitive Mantle source indicate that melt extraction was under 12% and 26% for lherzolites and harzburgites respectively. Melting took place in the spinel lherzolite field except for a few lherzolites that may have started to melt in the presence of garnet and continued in the spinel field. The formation of DMM lherzolites as refertilisation products, via percolation and reaction of N-MORB type basalts with refractory and ancient isotopically enriched harzburgites, cannot be ruled out. However, there is no definitive compositional evidence of such a process. It is suggested that harzburgites would have undergone multi-stage metasomatism. An earlier episode would have been caused by subduction-related hydrous fluids or melts related to the Variscan orogeny or an earlier orogenic episode. This is deduced from the time integrated low 143Nd/144Nd and very enriched 87Sr/86Sr values of harzburgite clinopyroxene. A later metasomatic episode is mainly cryptic and would have been related to the percolation of alkaline silicate melts and carbonatite derivatives. It would have affected lherzolites and especially harzburgites, causing opposite isotopic trends in the former compared to the latter. The SrNd isotopic compositions of the metasomatic agents would be close to that of a websterite xenolith. The age of this second metasomatism is also uncertain, since magmatic alkaline episodes have been registered since the Permo-Triassic periods.