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

Abstract

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/Ybn=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., 206Pb/204Pb=18.68–18.77, 87Sr/86Sr=0.704011–0.704207 for the Alaotra–Votovorona districts; 206Pb/204Pb=19.04–19.14, 87Sr/86Sr=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.