Rifting-related magmatic episodes of South-Western Malagasy (Morondava basin)

Greater India and Malagasy underwent a southwards lateral displacement along the Davie ridge. Then, separation between Greater India and Malagasy occurred during the Upper Cretaceous. A voluminous magmatism has been evidenced during or after this rifting, in different of the island, particularly in the Morondava basin, in the western part of the island, Cretaceous lavas are mostly basalts (45-54 % SIO2) of two types: Ti-rich (TiO2 > 2 %) and Ti-poor (TiO2 < 2 %), of tholeiitic to transitional affinities. A few gabbroic intrusions and doleritic veins outcrop too. Later, during the Upper Miocene, basic lavas of transitional to alkaline affinities erupted south of the basin. Several mantellic sources are evidenced, which explain the geochemical diversity of studied lavas: EM I, EM II, and moreover a source close to those described in Etendeka in Namibia. The Cretaceous volcanism seems to have been linked to the activity of the Marion hot spot. (authors)