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C. Cucciniello, L. Melluso, F. Jourdan, J.J. Mahoney, T. Meisel, V. Morra
40Ar-39Ar ages and isotope geochemistry of Cretaceous basalts in northern Madagascar: refining eruption ages, extent of crustal contamination and parental magmas in a flood basalt province
Geological Magazine 150, 1-17, 2013

The Madagascar Cretaceous igneous province exposed in the Mahajanga basin is represented by basalt and basaltic andesite lavas. New 40Ar-39Ar plateau ages (92.3 ± 2.0 Ma and 91.5 ± 1.3 Ma) indicate that the magmatism in the Mahajanga basin started about 92 million years ago. Four geochemically distinct magma types (Groups A-D) are present. Group A and C rocks have low to moderate TiO2 (1.2-2.6 wt.%), Nb (3-9 mg/g) and Zr (82-200 mg/g), and show large variations in eNdi (+0.1 to -10.5), 206Pb/204Pb (15.28 to 16.33) and gOs (+11.4 to +7378). The large isotopic variations, particularly in Os, Nd and Pb isotopic compositions, are likely due to crustal contamination. The low Pb isotope ratios observed in the Group A and C rocks suggest involvement of continental crust with low m (238U/204Pb). Group B and D rocks have moderate to high TiO2 (2.2-4.9 wt.%), Nb (8-24 mg/g) and Zr (126-317 mg/g). Age-corrected isotopes of Group B and D lavas show small range in eNdi (+1.0 to +4.0) and wide range in gOs (+128 to +1182). Values of 207Pb/204Pb are within the range for Groups A and C, but the Group D 206Pb/204Pb (16.52-17.07) and 208Pb/204Pb (37.51-38.01) values are higher, indicating a different crustal contaminant. Pb isotopic values of the Group B seem reflect the isotopic features of their mantle source. The magma groups of Mahajanga display wide range of trace element and isotopic compositions that cannot be explained only by open-system crystallization processes but, rather, distinct mantle sources.
Keywords: ages, isotope geochemistry, Madagascar, basalts, Cretaceous