NEW STRUCTURAL AND PETROGRAPHIC DATA ON T HE MAGMATISM SOUTH OF THE MORONDAVA BASIN (SOUTHWESTERN MADAGASCAR)

New petrological and geochemical data are presented on basic igneous r ocks from the south of the Morondava basin located in the southwestern part of Madagascar. The structural setting is well defined and, with the aid of a schematic map, the spatial relationships between the magm atic centres and major faults can be deduced from seismic data. On the other hand, new petrographic and geochemical data enable the differen t types of basaltic magmatism manifested in this area to be distinguis hed. It appears that tholeiitic magmatism evolved towards transitional magmatism from the south to the north of the Morondava basin. This ge ochemical evolution of basaltic magmatism can be linked to the initiat ion of rifting between Madagascar and Africa along the Mozambique chan nel and the length of the Davie ridge associated with the opening of t he Indian ocean. These observations, together with structural data and recently obtained geochronological data, contribute to a better under standing of the mechanism and evolution of Madagascan rifting. Copyrig ht (C) 1996 Elsevier Science Ltd