An isotopic and geochronological investigation of the younger igneous rocks of the Seychelles microcontinent

Rb/Sr and K/Ar dating of the younger igneous rocks of the Seychelles Islands reveals a peak of magmatic activity at 63 Ma. This corresponds very well with the timing of continental rifting between the Seychelles microcontinent and the Indian plate deduced from ocean floor magnetic anomaly patterns.18O/16O,87Sr/86Sr,143Nd/144Nd and Pb isotope compositions of Cretaceous/Tertiary dolerite dykes indicate strong contamination by continental basement, but there is evidence for only limited contamination of Tertiary alkaline differentiated plutons. The alkaline magmas have Nd/Sr isotope compositions near the Kerguelen field and Pb isotope compositions intermediate between Indian Ocean ridges and islands. They are attributed to a mantle plume which initiated sea-floor spreading on the Carlsberg Ridge. The data suggest that the distinctive chemistry of the Indian Ocean mantle has persisted for at least the Tertiary period.