Siroua (Sirwa, Jebel Siroua)

Siroua is a strato volcano 25 km in diameter set directly on Precambrian rocks, principally schists and granites. It is located on a major fault zone which separates the Atlas from the Anti-Atlas tectonic zones. The volcano comprises both tuffs and lavas, the former being the more abundant with air fall tuffs predominant but ignimbrites also occurring. The volcanic suite, which is characterised by the rarity of basic rocks, consists of hawaiite, mugearite, benmoreite, nepheline-bearing trachyte, phonolite, quartz trachyte and comendite. Although for the western part of Siroua Berrahma and Delaloye (1989) were able to recognise six volcanic episodes, based on cross-cutting relationships, there is considerable overlap in time of eruption of the various rock types, although basalts only appear in the last stages. The hawaiite and mugearite, which form rare dykes as well as flows, contain phenocrysts of augite, plagioclase and sparse altered olivine, brown amphibole and titanomagnetite in a mesostasis of augite, magnetite, alkali feldspar and nepheline. Benmoreite forms flows and is generally altered; it comprises 78% phenocrysts of alkali feldspar (Or29-39) together with augite and titanomagnetite in a matrix of the same minerals together with rare plagioclase and nepheline. The principal rock type of Siroua is nepheline-bearing trachyte which contains alkali feldspar (Or33-41) and rare clinopyroxene phenocrysts in a matrix of alkali feldspar microlites, apatite, titanite, iron oxides and rare clinopyroxene which is somewhat sodic. Phonolite forms flows in the northwest of the volcano and consists dominantly of alkali feldspar (Or29-47) with nepheline, nosean, sodic pyroxene and aenigmatite phenocrysts in a matrix of alkali feldspar, aegirine, nepheline and nosean. The quartz trachytes and comendites form domes and necks and consist of alkali feldspar, aegirine, arfvedsonite and aenigmatite; narsarsukite from this locality has been described in some detail by Wagner et al. (1991). Rock analyses are given by Berrahma and Hernandez (1985).