Wadi Natash

In the area of Wadi Natash several syenitic stocks 1-4 km in diameter, numerous volcanic plugs and 800 km2 of volcanic rocks are distributed over a wide area, only the southern half of which has been mapped in detail (Crawford et al., 1984). The plugs and stocks cut Precambrian basement rocks but the volcanic rocks are intercalated with Nubian sandstones which form the western boundary of the field. The stocks consist essentially of peralkaline quartz syenites (El Ramly et al., 1971). The volcanic pile forms three units separated by intercalations of volcaniclastic sediments. The volcanic rocks consist of lava flows of olivine basalt, hawaiite, mugearite, benmoreite and trachytes, with the volcaniclastic sediments consisting of tuffs with sparse wood fragments and leaf imprints. Hashad et al. (1982) also distinguished rhyolites and rhyodacites, based on rock chemistry, but Crawford et al. (1984), using a larger chemical database, did not find silica oversaturated rocks. Individual flow units grade upwards from alkali olivine basalt through hawaiite to mugearite, but benmoreite and trachyte were only found by Crawford et al. (1984) in the eastern part of the area. The basalts contain phenocrysts of olivine, and in some varieties Ti-augite, and microphenocrysts of olivine are present in the mugearites. The latter rocks, together with the benmoreites and trachytes, have a groundmass of aligned feldspar prisms that become more sodic through the series. Fayalitic olivine is present in the benmoreites and may be rimmed by aegirine-augite, which is an abundant phase of the groundmass. The trachytes comprise essentially alkali feldspar and aegirine-augite and aegirine. Hashad et al. (1982) and Crawford et al. (1984) give numerous rock analyses but neither include trace element data. Average values of a range of trace elements in alkali olivine basalt, hawaiite-mugearite and benmoreite are tabulated by Hubbard et al. (1987). Moghazi et al. (1997) have described two trachyte plugs from the Hamash area, which is just to the north of the Wadi Natash volcanic field. The plugs are 200 m high and 300 m in diameter. The trachytes consist of alkali feldspar, interstitial aegirine, riebeckite, biotite and nepheline and the chemistry indicates two populations, one of which plots in the phonolite field of a classification diagram.