A rare case of subduction-related metasomatism in mantle xenoliths from the Betic area (South Spain)

Within mantle xenoliths from Tallante (South Spain) incompatible elements and Sr-Nd isotopes record in most samples widespread enrichments produced by percolation of Na-alkaline melts (by both porous flow and open-conduit mechanisms) leading to variably veined xenoliths. On the other hand, 3 samples out of 55, containing traces of amphibole and phlogopite and characterised by relatively high SiO2 content (and high modal abundance of opx) display enrichments characterised by a completely different geochemical fingerprint. Their REE distribution is quite unusual, showing chondrite-normalised patterns characterized by pronounced enrichment in MREE (SmN/YbN up to 6.8), coupled with an Eu negative anomaly and a LREE depletion (LaN/YbN down to 0.69). Modelling indicates that these peculiar patterns may be accounted for by infiltration of LREE-enriched silica oversaturated melts, with the subsequent crystallisation of opx. With regard to Sr-Nd isotopes, most of ultramafic xenoliths from Tallante show a compositional field between DM and EM mantle end-members (87Sr/86Sr= 0.70213- 0.70476; 143Nd/144Nd= 0.51339-0.51250), as commonly observed for the European subcontinental mantle. On the other hand, the opx-rich xenoliths considered in this study are characterized by 87Sr/86Sr= 0.70672-0.70856 and 143Nd/144Nd= 0.51213-0.51211. Their composition is therefore very different from all the other recorded in the studied suite, and represents the most extreme EM isotopic signature ever found in anorogenic basic magmas and entrained ultramafic xenoliths throughout the whole Mediterranean area. These values recall those typical of the subduction-related volcanism (Miocene calc-alkaline lavas) occurred in the Betic-Alboran domain. These opx-rich xenoliths could therefore be considered mantle material recording metasomatic effects inherited by percolation of subduction related melts.