Todra And Tin Taralle Volcanics

The products of relatively recent volcanic activity are widespread over the southern part of the Air massif and around Tin Taralle to the northeast. The volcanic rocks, which include basalts, trachytes and phonolites, are most extensively developed around Todra and generally take the form of flows but some tuffs also occur close to Todra. Over 130 small cones occur, from which the lavas emanated, again concentrated in the vicinity of Todra and Tin Taralle, and erosion of these has given rise to some upstanding plugs. The cones rarely reach a kilometre in diameter and are up to 200 m high. The basaltic rocks are generally nepheline normative and pass into basanites with modal nepheline (Pouclet and Karche, 1988), ubiquitous olivine, spinel, augite, labradorite and kaersutite, with a matrix of plagioclase, pyroxene, olivine, opaque minerals and sometimes alkali feldspar, biotite and analcime (Black et al., 1967). In the Todra area mugearites have been mapped and, according to Pouclet and Karche (1988), differentiated rocks include sodic hawaiite, mugearite, benmoreite, trachyphonolite, trachyte and comendite. They also indicate the presence of nephelinites. The felsic rocks are dominantly trachytes which have sanidine phenocrysts and either hedenbergite plus biotite or aegirine-augite to aegirine and katophorite-aenigmatite (Black et al., 1967). The phonolites comprise alkali feldspar, nepheline, analcime, kaersutite, aegirine-augite and aegirine and nosean. Hazell et al. (1988) discuss briefly the chemistry and give some Sr, Nd and Pb isotopic data.