Geochemical and isotopic constraints on the petrogenesis of Plio-Pleistocene basaltic rocks from the Middle Atlas (Morocco)

During the Plio-Pleistocene, a diffuse volcanic activity occurred in the Moroccan Middle Atlas, producing alkali basalts, basanites, with minor tholeiitic basalts and rarer clinopyroxene-rich nephelinites with overall OIB-like geochemical characteristics. The rocks show rather primitive compositions (Mg# = 60-74) and coherent major oxide variations with SiO2 that could be interpreted at a first sight with clinopyroxene +/- olivine fractionation, without any plagioclase involvement. However, the continue decrease of LILE and HFSE, as well as LREE/HREE ratios with increasing SiO2 from clinopyroxene-rich nephelinites to tholeiitic basalts is interpreted as the result of increasing degrees of partial melting of a similar mantle source with variable spinel/garnet ratios. The investigated rocks exhibit relatively limited Sr-Nd-Pb isotopic variations, with the most differentiated compositions displaced towards more radiogenic Sr and less radiogenic Nd and Pb isotopic ratios. The Sr-Nd isotopic systematics indicate derivation from depleted mantle sources, while Pb isotopes are compatible with derivation from sources with moderately enriched U/Pb, approaching the composition of the HiMu-OIB endmember. The overall major oxide and trace element variations, coupled with isotopic ratios are compatible with little or no interaction with crustal lithologies of alkali basaltic magmas, while the tholeiitic basalts seem to have experienced minor crustal contamination. The presence of negative K and Pb anomalies, enrichment in Ba and Nb, and depletion in Rb in primitive mantle-normalised diagrams may reflect the presence of restitic phlogopite and amphibole, which may serve as indicators of metasomatic processes that have affected the source regions in their geological past. It is proposed that the heterogeneously metasomatized lithospheric mantle with enriched incompatible elements plays an important role in melt generation, without requiring the presence of any thermal anomaly in the form of a local mantle plume or a channelled one centred beneath Canary Islands.