Mineral chemistry from the Late Cretaceous peralkaline rhyolite at Hadjer el Khamis, Chad: constraints on magmatic conditions

The Late Cretaceous (74.4 ± 1.3 Ma) peralkaline rhyolite from Hadjer el Khamis erupted during a period of rifting within the Bornu and Termit basins of the Lake Chad region. The rhyolites are representative of a residual silicic magma that was derived by fractional crystallization of mantle derived basaltic parental magma. The primary phenocrysts of the rocks are quartz, alkali feldspar, arfvedsonite, aegirine, hedenbergite, and ilmenite. From the whole rock and mineral compositions, we conclude that the pre-eruptive temperatures were between 800 °C and 900 °C, but probably closer to 850 °C to 900 °C. The Fe-rich mafic mineral compositions and absence of magnetite indicate that the relative oxidation state of the melt was below the fayalite-magnetite-quartz buffer. The estimated relative oxidation state is calculated to range from ∆FMQ –1.1 to ∆FMQ – 2.8. The water content of the magma was likely close to or at saturation (i.e. H₂O ≈ 3.2 wt%) whereas the fluorine content was likely less than saturation (i.e. F_meltwt% < 0.6 wt%) due to the absence of magmatic fluo-rite. The pre-eruptive magma was likely volatile-rich, but underwent a period of significant degassing prior to or during eruption that led to a drier lava with higher viscosity, lower effusivity that allowed for slow cooling, the development of columnar joints, and the crystallization of hypersolvus feldspar.