Petrology of two continental alkaline intraplate series at Emi Koussi volcano, Tibesti, Chad

Tibesti volcanoes (Chad) are poorly documented, as much for volcanological evolution as for mineralogy and geochemistry. Emi Koussi volcano is a wide ignimbritic shield-like volcano that results from the succession of three eruptive sequences, each one related to a caldera collapse. Activity occurred from 2.4 Ma to the recent Quaternary. The continental alkaline volcanics of Emi Koussi exhibit two bimodal lava series, silica-saturated (up to 11% normative quartz) and silica-undersaturated (up to 29% normative nepheline). Basanite and tephrite, trachyte and phonolite are dominant while trachyandesite is very scarce. The basanites and tephrites contain olivine, diopside, plagioclase, nepheline, kaersutite and titanomagnetite. The trachytes contain alkali feldspar, plagioclase, Na–Ca amphibole (kataphorite, richterite), mica, augite, aegirine augite, titanomagnetite and ilmenite. In phonolites, alkali feldspar, andesine, nepheline or sodalite, hedenbergite, aegirine augite, amphibole (ferropargasite and kaersutite), titanite and titanomagnetite occur. Trachyandesite contains olivine, augite, edenite, phlogopite, andesine, alkali feldspar, titanomagnetite and ilmenite. The two suites exhibit a wide compositional gap of about 10% SiO2. Trachytes are peralkaline and comenditic. Major and trace element data are compatible with differentiation dominated by fractional crystallisation. The fractional crystallisation has been modelled using major-element least-squares mass balance models. Crystal fractionation involving olivine, clinopyroxene, amphibole and plagioclase is the best way to explain the petrogenesis of the undersaturated suite. The silica-saturated suite recorded removal of olivine, pyroxene, amphibole, mica, plagioclase, alkali feldspar, magnetite and apatite. Trace element data, including rare earth elements, calculated using the Rayleigh equation, confirm the models for the undersaturated suite, because there is a close agreement between the observed and calculated data. But the petrogenesis of the silica-saturated suite is probably related to other processes, such as crustal contamination.