U-Pb ages, Pb-Os isotope ratios, and platinum-group element (PGE) composition of the west-central Madagascar flood basalt province

The Mailaka lava succession (central-western Madagascar) forms part of the Madagascar large igneous province and is characterized by basaltic to picritic basalt lava flows and minor evolved flows. In situ U-Pb dating of zircon in rhyodacites yields concordant ages of 89.7 \ub1 1.4 and 90.7 \ub1 1.1 Ma. Therefore, the capping rhyodacitic unit of the Mailaka lava succession was emplaced just after the underlying basalt sequence (dated paleontologically at Coniacian-Turonian). Two geochemically different lava series are present. A transitional series ranging from picritic basalts to basalts has incompatible element abundances and Pb, Os, and Nd isotope ratios within the range of mid-ocean ridge basalts. In addition, the concentrations of platinum-group elements (Ir<0.35 ng/g, Ru