Petrographic and geochemical investigations of the southern part of Anjouan Island (Comores archipelago) reveal a diversity of volcanic facies, including alkaline basalts, basanites, phonolitic nephelinites, and phonolites. Petrography analysis highlights the occurrence of both vesicular and non-vesicular structures, as well as porphyritic and fluidal microlitic textures. The mineralogical composition reflects a sequential crystallization of opaque minerals, olivine, pyroxene, sanidine, and nepheline. Geochemistry, for its part, highlights the alkaline nature and silica undersaturation of the rocks from the southern part of Anjouan Island. It also reveals magmatic differentiation of the lavas through fractional crystallization, evolving from alkaline basalts to phonolites, via basanites and phonolitic nephelinites, respectively. These lavas are also enriched in LREE and depleted in HREE with steeply sloping spectra. These geochemical characteristics are consistent with an enriched mantle source located at great depth in the lower mantle, in which garnet and amphibole occur as residual. This source, derived from a mantle plume, underwent a low degree of partial melting, generating a magma that likely ascended through lithospheric fractures associated with hotspot activity, thereby producing oceanic island basalts (OIBs); this interpretation is supported by the presence of positive Nb anomalies.
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