Alkaline Rocks and Carbonatites of the World

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Jabal Archenu (Arkenu, Arkeno, Archenou, Arknu)

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Occurrence number: 
097-00-012
Country: 
Libya
Location: 
Longitude: 24.72, Latitude: 22.27

The Archenu complex intrudes Precambrian metamorphic basement rocks and overlying Cambro-Ordovician sediments. The 20x14 km intrusion consists of a series of ring-dykes and sheets, which were emplaced about several centres, remnants of pyroclastic rocks and numerous phonolite sills lying within sediments to the north of the central complex (Hunting Geology and Geophysics Ltd, 1974). The volcanic rocks are preserved principally within the eastern part of the complex and consist of phonolitic and trachytic tuffs and agglomerates, some of which include pebbles of sedimentary as well as igneous rocks. The earliest intrusive phases are microsyenite and phonolite ring-dykes which may contain feldspar or nepheline phenocrysts. These earliest rocks are cut by fine- to medium-grained syenite sheets which, however, also cut a syenite that is generally considered to belong to a later phase, indicating an extended period of emplacement of the syenite sheets. The next major event was emplacement of a large nepheline syenite ring-dyke. Subsequent intrusions, also ring-dykes, are syenites and nepheline syenites which reach 1500 and 1750 m in thickness and are variably porphyritic with pegmatitic segregations common. There is almost a continuum from perthosites to nepheline syenites with >20% nepheline. The whole complex is cut by a variety of dykes and shallow-dipping sheets of syenite, microsyenite, trachyte and occasionally basalt. Flinn et al. (1991) give petrographic details of a range of rocks together with data on feldspar, nepheline, analcime, sodalite, zeolites, pyroxene, amphibole, biotite, titanite, lorenzenite, eudialyte, catapleite, lavenite, cancrinite, magnetite and ilmenite. Modal and chemical data for 19 specimens, including trace elements and a full range of REE for eight of them, are presented and discussed by Flinn et al. (1991).

Age: 
K-Ar determinations on two specimens of nepheline (?) syenite gave 42(2 and 45(2 Ma (Mahrholz, 1968) and a further sample 44 Ma (Schurmann, 1974), while determinations on K-feldspar, biotite and aegirine from syenite gave 32(1, 36(1 and 48(2 Ma (Vail, 1976). Klerkx and Rundle (1976) give K-Ar ages on K-feldspar, biotite and aegirine from nepheline syenite of the outer part of the complex of 31.8(1, 35.6(1 and 47.7(1.5 Ma, and Klerkx (1985) a Rb-Sr age of 50(2.4 Ma.
References: 
ALKHAZMI, R.A. 1985. Some aspects of the geology, geochemistry and petrogenesis of Jabal Arkeno ring complex, S.E. Libya. 13th Colloquium of African Geology, 277.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. MAHRHOLZ, W.W. 1968. Geological exploration of the Kufra region. Geological Section Bulletin, Kingdom of Libya, Ministry of Industry, 8: 1-76.SCHURMANN, H.M.E. 1974. The Pre-Cambrian in North Africa. E.J. Brill, Leiden. 352 pp.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: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith