Tahalra

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. Three major volcanic phases have been distinguished (Dautria et al., 1988). The first activity was in the Miocene and comprises extensive basanitic and possibly nephelinitic flows which make up a plateau and reach 100 m in thickness in the eastern part of the field. The second phase, of Pliocene and lower Pleistocene age, involved the building of about 100 small basaltic Strombolian stratovolcanoes on the Miocene basanitic plateau in the eastern and central areas and on the basement to the west. A further 12 differentiated lava domes and spines of alkali rhyolite and trachyte were emplaced in the central part. During the Palaeolithic and Neolithic a third phase was erupted along the northern margin of the volcanic field consisting of about 20 basaltic maars and cones. The basanitic rocks usually contain phenocrysts of olivine (Fo80-87), which are frequently zoned, salitic clinopyroxene and, rarely in the lavas but abundantly in pyroclastic rocks, amphibole. The groundmass consists of olivine, Ti-salite, plagioclase (An50-65), Ti-magnetite, ilmenite, apatite and interstitial phlogopite, anorthoclase (Ab65-44Or25-41), nepheline, glass and occasionally analcime. Some of the Pliocene and Pleistocene basanites also contain phlogopite and nepheline as interstitial phases, while in some pyroxene phenocrysts occur having greenish cores of partly resorbed acmitic ferrosalite and rims of salite. Pyroclastic rocks of the second phase commonly contain megacrysts of amphibole up to 15 cm in diameter, clinopyroxene up to 5 cm and rare plagioclase, ilmenite and zircon. The megacrysts are generally associated with the presence of ultrabasic xenoliths, mainly spinel peridotites, clinopyroxenites and amphibole-rich rocks, but Leblanc et al. (1982) have also described monomineralic magnesian ilmenite xenoliths. The size and abundance of the xenoliths and megacrysts increase with decreasing age and from east to west (Dautria et al., 1988). Analyses of xenoliths and their minerals are given by Girod et al. (1981) and Dautria et al. (1988) and the latter paper also gives host rock analyses, including much trace element data, as do Dupuy et al. (1986) for spinel peridotite xenoliths. Dautria et al. (1987) describe amphibole-rich xenoliths and give mineralogical and chemical details of these and their host lavas. Pb, Nd and Sr isotope data on three lavas are given by Allègre et al. (1981). The rocks are classified as basanite with normative Ne 3-15 or nephelinite in which Ne ranges from 15 to 24. Such data suggest that they are all basanites and that true nephelinites do not occur.