Abstract

The northern part of the Late Cretaceous flood-basalt province of Madagascar is formed by lava flows, dykes and intrusive complexes. These range from tholeiitic to weakly alkaline and have composition from basalts to basaltic andesites (with rare, more evolved, rocks). The magmatic evolution is dominated by low-pressure crystal fractionation of olivine, Cr-spinel, plagioclase and clinopyroxene. The erupted rocks form two subprovinces, in terms of incompatible element concentrations or ratios and geographical position. The mafic and intermediate samples from the western subprovince have low abundances of high field strength elements (e.g. Nb 2–9 ppm; Zr 50–230 ppm), negative Nb anomalies on mantle-normalized diagrams, and a wide range of initial (88 Ma) 143Nd/144Nd (0.51293–0.51197; εNd +7.9 to −10.9). These elemental and isotopic variations are compatible with melting of predominantly N-MORB-like mantle and variable low-pressure crustal contamination. The mafic samples of the eastern subprovince are mildly enriched in incompatible elements (e.g. Nb 6–25 ppm; Zr 109–330 ppm), with the highest values found in tholeiites from the east Mahajanga basin, Tampoketsa Kamoreen and Tamatave. They also have a comparatively narrow range of initial 143Nd/144Nd (0.51276–0.51258; εNd +5.1 to +1.0), consistent with derivation from incompatible element enriched mantle sources and minor subsequent crustal contamination. The marked differences in the chemical and isotopic characteristics of the samples imply variable degrees of partial melting of hotter than normal, chemically heterogeneous, mantle sources at pressures where spinel to garnet are residual phases. No clear evidence is seen for a chemical component similar to that represented at present by the Marion hotspot lavas.

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