Primary igneous graphite in ultramafic xenoliths: I petrology of the cumulate suite in alkali basalt near tissemt (Eggéré, Alegerian Sahara)

Ultramafic xenoliths (harzburgite, olivine-orthopyroxenite, orthopyroxenite, websterite and clinopyroxenite) in a Plio-Quaternary strombolian cone near Tissemt (Egǵré, Hoggar, Algerian Sahara) contain large (up to 1 mm in diameter) euhedral flakes of graphite. These xenoliths are associated with mafic granulites free of graphite. Petrological, mineralogical, and geochemical data indicate that these rocks have been scavenged from a Precambrian layered intrusion emplaced in the deep crust. Textural evidence suggests that the graphite could have crystallized relatively early from a silica-saturated melt: following cumulus crystallization of olivine and orthopyroxene, the graphite crystallized, together with olivine, orthopyroxene, and spinel, as a component of the intercumulus assemblage. The crystallization of graphite directly from the melt is related to relatively high pressure (c. 5 kb) of carbon-rich fluid (CO+CO2+H2O) at relatively low oxygen fugacity (-logfo2, ∼ 10 at 1200 °C). © 1987 Oxford University Press.