New Lithological, Petrographic and Geochemical Data from the Karthala Lava Massif, Grande Comore, Indian Ocean

The Karthala massif, which covers the entire central and southern part of the island of Grande Comore, is made up of massive basalt flows, both vesicular and non-vesicular, pahoehoe and aa-type lavas and oceanite boulders. Petrographic observations of massive basalt flows show porphyritic and vacuolar microlitic textures marked by successive crystallization of opaque minerals, olivines, clinopyroxenes, orthopyroxenes and plagioclases in little or no mesostasis. This crystallization sequence is characteristic of a high-pressure geodynamic environment. The chemical variation of major and trace elements shows that the basalts studied are characterized by an under-saturation in silica (47.44%) and an enrichment in alkalis (1.95%) and titanium (2.33%). The rare-earth spectra of these basalts show a subparallel pattern, suggesting the same source. In the rare earth and multi-element diagram, the Karthala basalts show an alkaline nature, characterized by enrichment in light rare earths (LREE) and large ion lithophile elements (LILE) and depletion in heavy rare earths (HREE) and high field strength elements (HFSE). They also show positive Nb and Ta anomalies, typical of oceanic island basalts (OIBs) emplaced by mantle plume dynamics. These geochemical characteristics are compatible with an enriched, deep-seated garnet lherzolite-type source.