Madagascar volcanic provinces linked to the Gondwana break-up: Geochemical and isotopic evidences for contrasting mantle sources

Abstract

After the Gondwana break-up and Greater India splitting off, Madagascar suffered volcanic episodes. The Cretaceous Morondava flood basalt (CFB) province was emplaced ca. 93 Ma ago. Two coeval high-Ti–P (HTP) and low-Ti–P (LTP) suites are distributed within the alkaline Ankilizato (central) and tholeiitic Manamana (southern) sub-provinces. Sr–Nd isotope compositions display a trend from depleted ( ε Nd = + 7.5; Sr i = 0.7030; Antsoha end-member) to highly enriched products ( ε Nd = − 17; Sr i = 0.7228; Manamana end-member). Antsoha end-member is considered to be located at the lithosphere–asthenosphere boundary while the composition of Manamana end-member implies a location within the Archean to Proterozoic continental lithosphere. Oligocene Ankaratra HTP alkaline province, Miocene Ankilioaka transitional province, Pliocene Ambre Mountain and Pleistocene Nosy Be Island alkaline volcanoes display a restricted isotopic range ( ε Nd = + 4 to 0; Sr i = 0.7032 to 0.7048) forming an array distinct from the Cretaceous trend, implying the enriched pole did not correspond to Manamana end-member. This Cenozoic array could result from mixing between Antsoha end-member and BSE or a pole beyond. The exceptional variation of mantle sources involved in the Cretaceous episode contrasts with the more homogeneous near-BSE source during the Cenozoic. Several lines of evidence dismiss the mantle plume model. Cretaceous volcanism resulted from reactivation of a lithospheric scale shear zone due to plate reorganisation that eventually led to the Madagascar–India continental break-up. The lower lithospheric/asthenospheric melt (Antshoha end-member) was able to melt the most enriched/fusible parts of an upper lithospheric mantle (Manamana end-member); these two melts partly mixed. Long-lasting Neogene volcanic activity was less voluminous. The melting source was restricted to a lower part of the lithospheric mantle.