Petrogenesis of a basanite–tephrite–phonolite volcanic suite in the Bobaomby (Cap d’Ambre) peninsula, northern Madagascar

The Late Cenozoic Bobaomby volcanic field is located in the northernmost Madagascar, in the area north of the Massif d’Ambre. It comprises widely scattered outcrops of lava flows, dykes, scoria cones, tuff rings and plugs, emplaced in the Meso-Cenozoic sedimentary rocks of the Diego Basin. The Bobaomby rocks range in composition from Mg-rich, sodic basanite to phonolite (MgO from 13 to 0.15 wt.%), with a marked compositional gap between mafic and relatively evolved compositions (phonotephrites and tephritic phonolites). The volcanic rocks form a clear differentiation trend, that can be modelled by fractional crystallization of olivine, clinopyroxene, opaque oxides and kaersutitic amphibole (i.e. the observed phenocryst phases). The phonolites have alkali feldspar and nepheline phenocrysts (±sodalite and mafic phases), plot very close to or at the phonolite minimum in the Petrogeny’s Residua system and are the result of about 90% fractional crystallization, starting from basanite.The most mafic basanites, often carrying xenoliths of mantle-derived spinel lherzolites, show typical enrichment in the most strongly incompatible elements, have a peak at Nb in the mantle-normalized diagrams, and have a trough at K, a feature of within-plate basalts with HIMU-like geochemical affinity. Their composition can be modelled after ca. 4% partial melting of an enriched mantle source, possibly located in the lowermost lithospheric mantle. This source (that may be amphibole-bearing) is very similar in composition with the source of the Nosy Be basanites, located some 170 km southwest of the study area.