Unusual evolution of silica-under- and -oversaturated alkaline rocks in the Cenozoic Ambohimirahavavy Complex (Madagascar) : Mineralogical and geochemical evidence

The almost unknown Ambohimirahavavy ring complex in the Cenozoic alkaline province of northwestern Madagascar has recently attracted considerable interest because of the discovery of important rare-metal mineralization. The complex consists of arc-shaped bodies made up of silica-under- and -oversaturated syenites and extremely evolved peralkaline granitic dykes, as well as several mafic to felsic volcanic units, including basalt, phonolite and trachyte, all of which have an alkaline affinity. Uranium-lead zircon ages of 24.2. ±. 0.6. Ma and 23.5. ±. 6.8. Ma have been obtained for nepheline syenites and peralkaline granitic dykes, respectively, which, together with field data and ages of neighboring complexes, support emplacement controlled by regional lithospheric structures, rather than an evolving hot spot.Whole-rock major and trace-element and Sr-Nd isotopic data for the mafic suite suggest that the parental melt of this complex was generated by low degrees of melting of a metasomatized mantle source with residual amphibole. Fractional crystallization of this alkali basaltic melt likely produced the silica-undersaturated suite. We propose that the silica-oversaturated suite evolved from the undersaturated melt after contamination of the latter by crustal material. Further evolution to peralkaline compositions in both suites is attributed mainly to plagioclase and alkali feldspar segregation.Nepheline and feldspar compositions, as well as considerations of mineral equilibria among mafic silicates and Fe-Ti oxide minerals indicate crystallization temperatures of 1000 to 700. °C and an oxygen fugacity of 0.4 to 0.8. log units below the fayalite-magnetite-quartz (FMQ) buffer at 1. kbar for the silica-undersaturated melt, and temperatures of 860 to 570. °C and an oxygen fugacity of 1.5 to 3.8. log units below FMQ for the oversaturated syenitic melt. The undersaturated melt evolved towards a more peralkaline composition. Crystallization of arfvedsonite plus aegirine further reduced the melt the evolution of which ended with fluid exsolution. At late stages of crystallization, the oversaturated melt departed from the reducing trend of the undersaturated melt, evolving towards high oxygen fugacity. Very late exsolution of the fluid permitted concentration of the HFSE in the last stages of magmatic evolution with local production of low-temperature pegmatitic phases extremely enriched in these elements.