The most northwesterly of the volcanic fields of the Hoggar Eggere covers about 100x60 km. It consists essentially of a basalt plateau across which are scattered numerous vents. Lelubre (1952) distinguishes three major volcanic rock features: trachytic plugs, which constitute the largest edifices, basaltic flows, forming the bulk of the plateau and one of which extends along a valley to the northwest for some 40 km, and more recent basaltic cinder cones showing little sign of erosion. Few details of the volcanology or petrography of the Eggere province seem to be available but Kornprobst et al. (1987), in describing primary igneous graphite in ultramafic xenoliths from near Tissemt, in southeastern Eggere, give a few details of the host rocks. Their map shows basaltic flows, trachytic flow domes, phreatomagmatic tuff-rings and maars and strombolian cones. They refer the xenolith host rock to basanite and note the presence of about 5% leucite. Pineau et al. (1987) give carbon isotope data on the xenoliths and host rocks described by Kornprobst et al. (1987), and one xenolith from Eggere is in the suite described by Dupuy et al. (1986). There is little doubt that a thorough study of the province would bring to light a highly alkaline sequence.
DUPUY, C., DOSTAL, J., DAUTRIA, J.M. and GIROD, M. 1986. Geochemistry of spinel peridotite inclusions in basalts from Hoggar, Algeria. Journal of African Earth Sciences, 5: 209-15.KORNPROBST, J., PINEAU, F., DEGIOVANNI, R. and DAUTRIA, J.M. 1987. Primary igneous graphite in ultramafic xenoliths: I. Petrology of the cumulate suite in alkali basalt near Tissemt (Eggere, Algerian Sahara). Journal of Petrology, 28: 293-311.LELUBRE, M. 1952. Recherches sur la géologie de L'Ahaggar central et occidental (Sahara Central). Bulletin du Service de la Carte Géologique de l'Algérie, Ser. 2, 22: 1-385.PINEAU, F., JAVOY, M. and KORNPROBST, J. 1987. Primary igneous graphite in ultramafic xenoliths: II. Isotopic composition of the carbonaceous phases present in xenoliths and host lava at Tissemt (Eggere, Algerian Sahara). Journal of Petrology, 28: 313-22.