In Teria (Ilizzi)

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 pyroclastic rocks are mixtures of lava lapilli up to 2 mm in diameter and up to 50% of fragments of sedimentary rocks, from less than one millimetre diameter to several centimetres, cemented by calcite and/or zeolite. The lapilli contain phenocrysts of olivine, melilite (Ak67), nepheline, opaques, calcite and rare clinopyroxene in a glassy matrix. Analyses of this rock are given by Bossière and Megartsi (1971), who consider it to be similar to rushayite (olivine melilitite). Mantle peridotite (30%) and pyroxenite (70%) xenoliths up to 30 cm across have been found at 11 centres (Bossière and Megartsi, 1982) and are very abundant in some of them. The principal peridotite type is a spinel-phlogopite harzburgite and phlogopite-bearing garnet peridotite is found at one crater. The pyroxenites include phlogopite-bearing and biotite-amphibole-bearing varieties (Bossière and Megartsi, 1982); these authors describe the xenolith suite in some detail. The mineralogy of 10 xenoliths is also fully described by Dautria et al. (1992) who present chemical data, including REE and other trace elements, for five melilititic host rocks.