Alkaline Rocks and Carbonatites of the World

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Oranais

stripes

Occurrence number: 
003-00-001
Country: 
Algeria
Location: 
Longitude: -1.58, Latitude: 35.28

Between Oran and the border with Morocco to the west are a number of volcanic fields that include both alkaline and non-alkaline rocks and range broadly in composition from basalt to rhyolite. In his extensive account Sadran (1958) distinguishes the following centres: Moyenne Tafna, M'Sirda Fouaga, Souhalia, Basse Tafna and Beni-Saf, Ain Temouchent and Sahel d'Oran. The Ain Temouchent field is the most extensive covering about 20x20 km. In all these areas there are numerous volcanic centres from which lava flows and pyroclastic deposits emanated and in some of them substantial edifices, principally of rhyolite, as well as dykes, which are described in some detail. Sadran's (1958) nomenclature indicates the presence of basalt, andesite, rhyolite, analcime tephrite, limburgite and basanitoid but it is noteworthy that in a section investigating the classification of the rocks according to the systems of Lacroix, von Wolff, Rittmann and others some rocks are indicated as being pantellerite/comendite, phonolite and nephelinite. Plotting the numerous analyses of Sadran (1958) in the IUGS TAS diagram (Le Maitre et al., 1989) indicates the presence of the series picrobasalt-basalt-basaltic andesite-andesite-dacite-rhyolite, basalt-trachybasalt-trachyandesite, tephrite/basanite-phonotephrite with several rocks plotting in the foidite field. The more northerly fields of Sahel d'Oran and Ain Temouchent are clearly the most alkaline. Among the rocks described in some petrographical detail are analcime-rich basanites in the Ain Temouchent field that contain large nepheline crystals. Xenoliths are plentiful in the central fields and include sedimentary, metamorphic and a wide range of igneous rocks with some, including chromite-bearing peridotites and pyroxenites, of upper mantle origin (Sadran, 1958; Dautria and Girod, 1987).

Age: 
Bellon and Guardia (1980), based on K-Ar and palaeomagnetic studies, distinguished five periods of volcanic activity i.e. around 5.1, 4.14-4.25, 3.82-3.41, 2.91-2.47 and 1.91-1.62 Ma.
References: 
BELLON, H. and GUARDIA, P. 1980. Le volcanisme alcalin plio-quaternaire d'Algérie occidentale. Étude radiométrique et paléomagnetique. Révue de Géologie Dynamique et de Géographie Physique, 22: 213-22.DAUTRIA, J.M. and GIROD, M. 1987. Cenozoic volcanism associated with swells and rifts. In: P.H. Nixon (ed), Mantle xenoliths. 195-214. John Wiley, Chichester.SADRAN, G. 1958. Les formations volcaniques tertiares et quaternaires du Tell Oranais. Publications du Service de la Carte Géologique de l'Algérie. Bulletin, N.S. 18: 1-533.SADRAN, G. 1958. Les formations volcaniques tertiares et quaternaires du Tell Oranais. Publications du Service de la Carte Géologique de l'Algérie. Bulletin, N.S. 18: 1-533.
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