Petrogenesis and Nd-Pb-Sr- isotope geochemistry of the olivine melilitites and olivine nephelinites (“ankaratrites”) in Madagascar

The Cenozoic ankaratrites of the Alaotra, Takarindoha-Vatomandry and Votovorona (NE Ankaratra) volcanic fields, Madagascar, range from olivine (+/- monticellite) melilitites, through olivine-melilite nephelinites to olivine ( leucite) nephelinites. The rocks show significant compositional ranges in their coexisting magmatic minerals (olivine-group minerals, melilite, clinopyroxene, nepheline, leucite. Ba-phlogopite, perovskite, ilmenite, spinels, apatite), and evidence of distinct parental magmas, often in different facies of the same vent. Primitive compositions (high Mg#, Cr and Ni concentrations) are found in each volcanic district, and a few lavas contain mantle xenoliths or xenocrysts. The rocks show enrichment in the most strongly incompatible elements (e.g., Ba and Nb up to 200 times primitive mantle, La/Yb(n) =24 to 40), with troughs at K and smooth, decreasing patterns towards the least incompatible elements in mantle-normalized diagrams. The Nd-Pb-Sr isotope geochemistry indicates a marked heterogeneity of the mantle sources of the various districts (e.g., (206)Pb/(204)Pb = 18.68-18.77, (87)Sr/(86)Sr = 0.704011-0.704207 for the Alaotra-Votovorona districts; (206)Pb/(204)Pb = 19.04-19.14, (87)Sr/(86)Sr = 0.703544-0.704017 for the Takarindoha-Vatomandry districts), with significant differences to other Cenozoic mafic volcanic rocks of northern Madagascar. The genesis of the Madagascan ankaratrites is related to rifting events which triggered low-degree partial melting of a garnet peridotite enriched in dolomite and incompatible-element-rich phases, in the lowermost lithosphere. Despite marked geochemical similarities, the source of the Madagascan melilitites bears no isotopic similarity to the HIMU-related sources of melilitites of eastern and southern Africa.