Alkaline Rocks and Carbonatites of the World

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Uweinat (Jebel Uweinat, Al Uwaynat, Al Awaynat, Awenat)

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Occurrence number: 
097-00-013
Country: 
Libya
Location: 
Longitude: 24.92, Latitude: 21.92

This is by far the largest intrusive complex in Libya covering 430 km2. The occurrence extends eastwards into Egypt and Sudan, but the greater part is located within Libya. The complex comprises a large, sub-circular ring intrusion up to 25 km in diameter flanked on the northern side by three overlapping subsidiary rings. Parts of the eastern side of the complex are capped by Lower Palaeozoic sediments. The main complex has a high, circular outer rim of coarse-grained quartz syenite inside which granite has weathered to an annular depression; both rock types contain aegirine. An inner circular ridge, up to 8 km wide, consists of a series of both inward and outward dipping cone sheets, and slopes inwards to a central syenite. The central syenite, which is cut locally by trachyte dykes, contains up to 12% quartz, but quartz is not present in the surrounding cone sheets. The preceding account is based on a report of Hunting Geology and Geophysics (1974) but Klerkx (1985) says that foidal syenites occur in the inner part of the structure, while the centre is composed of trachytic breccias and phonolites. A lens of distinctive riebeckite microgranite, which is cut by aplitic and granophyric dykes, is located 8 km southwest of the centre of the complex. Exposure of the three rings north of the main complex decreases northwards. The more southerly consists of a central porphyritic syenite which is surrounded by a coarse-grained quartz syenite and this in turn by a trachytic intrusion breccia. The central ring comprises medium to coarse syenites cut by trachytic cone sheets, while the poorly exposed most northerly ring is of quartz syenite. Extensive mineralogical data are given by Flinn et al. (1991) and Atherton et al. (1991) give modal and chemical analyses of 14 rocks including a full set of REE for three rocks.

Age: 
Vail (1976) reports K-Ar dates on K-feldspar from granite of 42(1 and 45(1 Ma, and on aegirine, also from granite, of 42(1 Ma. K-Ar determinations on K-feldspar from peralkaline granite of 45(1 and 41.7(1 Ma and on aegirine of 42.0(1 are reported by Klerkx and Rundle (1976); they also give K-Ar whole rock dates of 45.6(1 Ma for phonolite from a plug north of Uweinat and 32.0(1 Ma for dolerite cutting the Uweinat granite. Andre et al. (1991) report a K-Ar age of 40-45 Ma and 55(1.5 Ma by Rb-Sr. A U-Pb age on zircon from an alkali granite of 42.3(0.8 Ma was obtained by Flinn et al. (1991).
References: 
ANDRE, L., KLERKX, J. and BUSREWIL, M.T. 1991. Geochemical and Rb-Sr isotopic data on felsic rocks from Jabal al Awaynat alkaline intrusive complex (SE Libya). In M.J. Salem, M.T. Busrewil and A.M. Ben Ashour (eds) The geology of Libya. 7: 2511-27. Elsevier, Amsterdam.ATHERTON, M.P., LAGHA, S and FLINN, D. 1991. The geochemistry and petrology of the Jabal Arknu and Jabal al Awaynat alkaline ring complexes of SE Libya. In M.J. Salem and M.T. Busrewil (eds) The geology of Libya, 7: 2559-76. Elsevier, Amsterdam.FLINN, D., LAGHA, S., ATHERTON, M.P. and CLIFF, R.A. 1991. The rock-forming minerals of the Jabal Arknu and Jabal al Awaynat alkaline ring complexes of SE Libya. In M.J. Salem and M.T. Busrewil (eds) The geology of Libya, 7: 2539-57. Elsevier, Amsterdam.HUNTING GEOLOGY and GEOPHYSICS LTD, 1974. Geology of the Jabal Al Uwaynat area, Libyan Arab Republic. Report to Industrial Research Centre, Department of Geological Research and Mining, Tripoli. Boreham Wood, U.K. (unpublished). 156 pp.KLERKX, J. 1985. The alkaline ring complexes of Jebel Uweinat (SE Libya). Journal of African Earth Sciences, 3: 275 (Abstract).KLERKX, J. and RUNDLE, C.C. 1976. Preliminary K/Ar results on different igneous rocks from the Jebel Uweinat region, (S.E. Libya). Rapport Annuel l’Année 1975. Départment de Géologie et de Minéralogie du Musée Royal de l’Afrique Centrale, Tervuren, 105-11. VAIL, J.R. 1976. Location and geochronology of igneous ring-complexes and related rocks in north-east Africa. Geologisches Jahrbuch, B 20: 97-114.
Location: 
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