The Cretaceous Morondava volcanic province (West Madagascar): mineralogical, petrological and geochemical aspects

Upper Cretaceous continental break-up between Madagascar and Greater India resulted in intense volcanic activity throughout the island, but mostly in the east coast. The Morondava Basin, southwest Madagascar, was capped 88 Ma ago by volcanic formations triggered by this event. Mineral and bulk rock compositions reveal two groups of suites. The high Ti-P group is composed o f olivine basalts and a ferrobasalt-basalt-andesitic basalt series. The low Ti-P group comprises genetically unrelated basalts and andesitic basalts. All rocks display the (olivine+plagioclase+clinopyroxene+Fe-Ti oxides) four phase assemblage. Rock-forming minerals of the high Ti-P group are composi tionally unimodal, while rocks of the low Ti-P groups contain olivine and p lagioclase, each yielding bimodal populations of compositions, probably due to transient low Ti-P magmas storage within a deep magma chamber emplaced at the crust-mantle boundary. In the southern area of the basin near Manamana, low Ti-P andesitic basalts were produced by partial melting of subcontinental lithospheric mantle, re ferred to as the Manamana end member. The nearby high Ti-P series can be ex plained by partial melting of a mixed source comprising the Manamana end me mber and deeper materials and subsequent fractionation. In the central area of the basin near Ankilizato, the lithospheric source of high Ti-P olivine basalt, the Ankilizato end member, differs from the Manamana end member. T he nearby low Ti-P basalts can be explained by a complex mixture of sources . (C) 2001 Elsevier Science Limited. All rights reserved.