Ar-40-Ar-39 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

作者：Cucciniello C; Melluso L^*; Jourdan F; Mahoney J J; Meisel T; Morra V

来源：Geological Magazine, 2013, 150(1): 1-17.

DOI：10.1017/S0016756812000088

• ages
• isotope geochemistry
• Madagascar
• basalts
• Cretaceous

The Madagascar Cretaceous igneous province exposed in the Mahajanga basin is represented by basalt and basaltic andesite lavas. New Ar-40-Ar-39 plateau ages (92.3 +/- 2.0 Ma and 91.5 +/- 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 TiO2 (1.2-2.6 wt %), Nb (3-9 mu g g(-1)) and Zr (82-200 mu g g(-1)), and show large variations in epsilon Nd-i (+0.1 to -10.8), Pb-206/Pb-204 (15.28 to 16.33) and gamma(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 (U-238/Pb-204). Group B and D rocks have moderate to high TiO2 (2.2-4.9 wt %), Nb (8-24 mu g g(-1)) and Zr (120-327 mu g g(-1)). Age-corrected isotopes of Group B and D lavas show a small range in epsilon Nd-i (+1.0 to +4.0) and a wide range in gamma(Os) (+128 to +1182). Values of Pb-207/Pb-204 are within the range for Groups A and C, but the Group D Pb-206/Pb-204 (16.52-17.08) and Pb-208/Pb-204 (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.

• 出版日期2013-1