CHROMATOGRAPHIC METASOMATISM OF THE ARABIAN-NUBIAN LITHOSPHERE

Trace elements and isotopic ratios of calc-alkaline and tholeiitic dik es from the very last stage of the late Proterozoic, Pan-African oroge ny in the northern Arabian-Nubian Shield (ANS), and alkali basalts fro m the overlying Phanerozoic section are used to constrain the composit ion and model the evolution of the lithospheric mantle in this region. The dikes and basalts are interpreted as lithospheric melts formed du ring the post-orogenic (and post-subduction) history of the shield. Wh ile the mafic member of all suites share a primitive La/Th ratio, the Nb/Th and Ce/Pb are distinct for each suite. The (Nb/Th)(PM) (primitiv e mantle normalized) is similar to 0.2 in the calc-alkaline dikes and 1.4 in the tholeiitic dikes and the Phanerozoic alkali basalts. The (C e/Pb)(PM) ratios are low in the dikes (0.4 in the calc-alkaline and 0. 3 in the tholeiitic) and high in the Phanerozoic basalts (2.8). We sug gest that the Variations in the trace element ratios reflect sampling of different zones in the lithospheric mantle, which were formed by su bduction related metasomatism of the mantle wedge. We constructed a ch romatographic model to explain this zonation. In this model a plume-de rived oceanic lithosphere is subducted and dehydrates at depth. Fluids released from the dehydrating slab metasomatize the overlying wedge a nd form amphibole-rich channels. Nb is preferentially taken by the amp hibole and is enriched only in the lower zones of the column. The othe r elements (U, Th, REE and especially Pb and Rb) behave incompatibly. They are enriched in the fluid and transported efficiently to the melt ing zone in the centre of the wedge. Dehydration of the base of the we dge as it descends below the amphibole stability field depletes this r egion in Pb and Rb. After the end of subduction, the wedge is fossiliz ed and forms the lithospheric mantle. The zone above the Nb concentrat ion front is sampled by the calc-alkaline magmas. The tholeiitic magma s sample the zone below the Nb front. The Phanerozoic alkali basalts s ample the dehydrated base where Nb is high and Pb and Rb low. Trace el ement patterns in amphibole xenoliths from Mount Carmel, Israel, are s imilar to the model's prediction for amphibole in the residual base of the lithosphere. The model correctly reproduces the major features in the isotopic evolution of the Arabian-Nubian lithosphere, and has imp lications for the formation of HIMU-type magmas in mid-plate environme nts. (C) 1997 Elsevier Science B.V.