Oulmes Area

In an area extending from approximately 20 km northwest of Oulmes to about 70 km to the southeast there are occurrences of recent volcanic rocks of widely differing extent which have emanated from about 20 volcanic centres. Basalts are represented but the majority of the lavas are strongly alkaline and include basanite, nephelinite and phonolite, many of which contain hauyne and/or leucite, while melilite-bearing varieties have also been described. Basalts and basanites gave rise to scoria cones from which flows up to 8 km in length emanated. Phonolites form intrusions as well as flows. A characteristic feature of the more basic lavas, according to Rachdi et al. (1985), is the scarcity of olivine, being always subordinate to clinopyroxene. The basanites are distinguished by the presence of kaersutite phenocrysts together with Ti-augite or salite phenocrysts in a matrix of labradorite, olivine (Fo78-85), clinopyroxene, analcime and glass. The nephelinites contain phenocrysts of salite, rare olivine (Fo77-79), apatite and some varieties plentiful kaersutite. There is abundant normative nepheline (15-22%) and it has been identified in the matrix together with olivine, rare phlogopite and perovskite. Some ‘nephelinites’ are free of olivine but contain leucite, melanite, sodic pyroxene, melilite and K-feldspar. One such rock from Djebel Targou also contains nosean, and is described in detail by Velde and Rachdi (1988) who give rock and mineral analyses and Sr and Nd isotopic data. The phonolites contain phenocrysts of hauyne and rare brown amphibole in a matrix of iron oxides, sodic pyroxene, sanidine and titanite. The above petrography is based principally on Rachdi et al. (1985) but descriptions of many rocks from this area are given by Gentil (1916) and particularly by Gentil (1936) and Jérémine (1955); the last two references also include rock analyses.