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.
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Tioueine is one of a number of concentrically zoned plutons, known locally as the Taourirt granites, which intrude Upper Proterozoic rocks of the central Pharusian belt of the Hoggar.
From an area of about 4000 km2 of the Pan-African belt of western Hoggar, west of the In Ouzzal granulite block, Dostal et al. (1979) have described a sequence of metamorphosed alkaline intrusions and dyke complexes.
These are two lamproite intrusions of which Kef Hahouner is a steeply-dipping dyke-like body several 10s of metres thick which has been traced for about 1.2 km, and Koudiat el Anzazza, located 16 km to the east-northeast, a pipe some 100 m in diameter (Raoult and Velde, 1971; Kaminskey et al., 1
On the northeastern side of the Hoggar dome, in the In Teria district, which is north-northeast of Illizi, some 23 craters have been identified which extend over 50 km along a northwest-southeast line. They vary from 60 to 800 m in diameter and many have well preserved tuff rings.
The most northeasterly of the Hoggar volcanic fields Adrar n'Ajjer extends over about 90x40 km. No detailed accounts have been located but Fabre (1976) describes the field as consisting of basalts and phonolites with remarkably well preserved volcanoes.
The Anahef region, referred to as Amadror by Remy (1967), lies between the volcanic fields of Adrar n'Ajjer and Atakor and differs from the rest of the Hoggar volcanic province in containing not only extensive areas of extrusive rocks but also numerous intrusions.
The Atakor volcanic field extends over 2150 km2 and has an estimated volume of 250 km3 (Girod, 1971). It lies on a 1 km high basement dome which is 80 km in diameter and culminates in Mont Tahat, the highest point in Algeria. The lavas lie on Precambrian granitic and gneissic rocks.
Tahalra is the most southerly of the volcanic fields of the Hoggar region (Fig. 4) and extends over an area of 80x30 km. The volcanic rocks lie on Precambrian metamorphic and igneous rocks and locally their Palaeozoic cover which are cut by a north-south-trending shear zone.
It has been proposed by Ouzegane et al. (1988) that a group of rocks occurring northwest of the Hoggar, and formerly described as brecciated metamorphic marbles, are carbonatites.