(super 40) Ar/ (super 39) Ar 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

The Madagascar Cretaceous igneous province exposed in the Mahajanga basin is represented by basalt and basaltic andesite lavas. New (super 40) Ar- (super 39) Ar plateau ages (92.3 + or - 2.0 Ma and 91.5 + or - 1.3 Ma) indicate that the magmatism in the Mahajanga basin started about 92 Ma ago. Four geochemically distinct magma types (Groups A-D) are present. Group A and C rocks have low to moderate TiO (sub 2) (1.2-2.6 wt%), Nb (3-9 mu g g (super -1) ) and Zr (82-200 mu g g (super -1) ), and show large variations in epsilon Nd (sub i) (+0.1 to -10.8), (super 206) Pb/ (super 204) Pb (15.28 to 16.33) and gamma (sub Os) (+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 mu ( (super 238) U/ (super 204) Pb). Group B and D rocks have moderate to high TiO (sub 2) (2.2-4.9 wt%), Nb (8-24 mu g g (super -1) ) and Zr (120-327 mu g g (super -1) ). Age-corrected isotopes of Group B and D lavas show a small range in epsilon Nd (sub i) (+1.0 to +4.0) and a wide range in gamma (sub Os) (+128 to +1182). Values of (super 207) Pb/ (super 204) Pb are within the range for Groups A and C, but the Group D (super 206) Pb/ (super 204) Pb (16.52-17.08) and (super 208) Pb/ (super 204) Pb (37.51-38.01) values are higher, indicating a different crustal contaminant. Pb isotopic values of the Group B rocks seem to reflect the isotopic features of their mantle source. The magma groups of Mahajanga display a wide range of trace element and isotopic compositions that cannot be explained only by open-system crystallization processes but, rather, by distinct mantle sources.