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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 The Central Atlantic Magmatic Province (CAMP)and Birth Ksour Basin (Saharan Atlas, Algeria) 270 Amar Meddah 1* , Hervé Bertrand 2 , Miloud Benhamou 1 1. Laboratoire Magmatisme et géodynamique des bassins algériens, université d’Oran Es-Sénia, BP 1524, campus II, 31100 Es-Sénia Oran, Algérie (E-mail: amar.meddah@gmail.com; m_benhamou@hotmail.com) 2. UMR CNRS 5570, Ecole normale supérieure de Lyon et université Claude-Bernard–Lyon-1, 46, allée d’Italie, 69364 Lyon cedex 07, France (E-mail: Herve.Bertrand@ens-lyon.fr) The Saharan Atlas is a Meso-Cenozoic intra- cratonic chains formed during the orogeny Atlas from subsiding basins. In the Ksour mountains (Western Saharan Atlas, Algeria), the structuring of the basin is due to the Triassic-Liassic rifting. During upper Triassic, it was filled with sedimentary series generally capped by a volcanic pile (Aït Ouali and Delfaud, 1995; Yelles-Chaouche et al., 2001). These outcrops are associated with Triassic diapirs controlled by NE-SW anticlines(Galmier, 1972). We investigate the volcanism of the Ksour Mountains, based on a detailed mapping, in order to reconstitute the Triassic-Liassic volcano-evaporitic sequence(Meddah et al., 2006; Meddah et al., 2007). Three volcanic units, of basaltic composition, have been recognized: the lower unit (B1) is 4 to 6 m thick and is associated with red oligist argillites; the intermediate basaltic unit (B2), 2.5 m thick, overlies 10 m of variegated gypseous argillites and is associated with 2 m of argillites and grey stromatolitic limestones; the upper unit (B3), 5 m thick, is overlain by black stromatolitic limestones (1.5 m thick) covered by evaporitic argillites and halite and, upwards, by dolomites. In Djebel Melah Rhaetian-Hettangian has been attributed to the black stromatolitic limestones by a lamellibranches Gervillia praecursor QUENSTEDT (Flamand, 1911; Bassoulet, 1973). In Ain-Ouarka Het- tangian has been attributed to the top of these dolomites by an ammonite Caloceras sp. of the Planorbis Zone (Mékahli, 1995). In El-Hendjir, Chelalla Dahrania, and Tiout, the upper unit is directly overlain by bluish-grey stromatolitic dolomitic limestones. This formation is considered of Rhaetian (Aït Ouali and Delfaud, 1995) or Rhaetian-Hettangian on the basis of lamellibranches(Bassoulet, 1973). The middle and upper limestones show Hummocky Cross Stratification (HCS), synsedimentary folding and emersion surfaces, indi- cating a continental lagoon to supratidal environment preceding the setting up of the Liassic carbonated platform. The synsedimentary basaltic lava-flows locally flooded wet sediments, as attested by the silicified basis of carbonated and argillaceous layers. This sequence describes the syn-rift volcano-sedimentary filling and the transition from an evaporitic cycle to the first open marine incursions heralding the post-rift installation of the Liassic carbonate platform, in the Ksour basin. In this context, the volcanic units witness the different stages of the birth Ksour basin. In these volcanic units, the primary paragenesis, represented by olivine, plagioclase, clinopyroxene and Ti-magnetite, is homo- geneous and typical of the continental tholeiites forming the CAMP province, particularly in Morocco (Bertrand, 1991). The olivine occurs as micropheno- crysts in the lower unit, more occasionally in the intermediate unit and is absent in the upper unit: This represents a good stratigraphic marker. A granophyric interstitial phase has been observed once in the lower unit (Tiout locality). The volcanic rocks of Ksour Mountains are basalts and basaltic andesite, poorly differentiated basalts ([Mg] (= Mg/(Mg + Fe)) from 0.57 to 0.71) of sub-alkaline field. Significant alteration is reflected by a loss on ignition varying from 1.93 to 4.31 wt%, in accordance with the hydrothermal phases observed. The alkaline elements are highly mobilized (K2O up to 7.55%) and cannot be used as magmatic markers. Nevertheless, the tholeiitic nature of the three volcanic units is clearly attested by immobile elements: low Ti (TiO2: 1.13-1.63 wt%), P (P2O5: 0.1-0.2 wt%) and Zr (87ppm < Zr < 151ppm) contents, Y/Nb and Zr/P2O5*104 ratios varying from 1.7-4.6 and 0.07-0.09, respectively(Bassoulet, 1973). Trace element patterns, enriched in light rare-earth (La/Yb(n) = 2.5-5.9) and other incompatible elements (Th(n) = 57-124), dis- playing an anomaly negative Nb, are characteristics of continental tholeiites. These chemical characteristics, along with the stratigraphic position of the lava pile, allow to assign this volcanism to the Central Atlantic Magmatic Province (CAMP). Moreover, a decreasing of incompatible elements is observed from the base to the top of the lava pile: Average values TiO2: 1.46 wt%, 1.34 wt%, 1.18 wt%; P2O5: 0.17 wt%, 0.16 wt%, 0.12 wt%; La/Yb(n): 5.24, 3.96, 2.52, in the lower, intermediate, and upper units, respectively. The basalts from the Ksour Mountains are geochemically akin to the CAMP low-Ti tholeiites identified in West Africa. These results highlight the geodynamic evolution of the Atlasic domain at the Triassic-Jurassic (Tr-J) boundary and the CAMP extension in Algeria. The volcanism of the Ksour Mountains represents one of the easternmost witnesses of CAMP, associated with the central Atlantic rifting, at the T-J boundary. Key words: Tholeiitic basalt; CAMP; T-J boundary; Saharan Atlas; Algeria
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 References: Aït Ouali R., Delfaud J. Les modalités d’ouverture du bassin des Ksour au Lias dans le cadre du rifting jurassique au Maghreb. Comptes Rendus de l’Académie des Sciences, Paris, 1995, t. 320, série IIa: 773-778. Bassoulet J.P. Contribution à l'étude stratigraphique du Mésozoïque de l'Atlas saharien occidental (Al- gérie). Thèse Sci. Nat. Univ. Paris 6, 1973: 2. Bertrand H. The Mesozoic tholeiitic province of Northwest Africa: A volcano-tectonic record of the early opening of central Atlantic. In: Kampunzu A.B., Lubala R.T. (ed). Magmatism in Extensional Structural Settings, the Phanerozoic African Plate. Springer-Verlag, 1991: 147-191. Flamand G.B.M. Recherches géologiques et géographi- ques sur le haut pays de l'Oranie et sur le Sahara (Algérie et territoire du sud). Thèse Sci., Lyon, 1911, n47 et A. Rey. 22 cartes. Galmier D. Photogéologie de la région d’Aïn Séfra (Algérie). Publications du Services Géologique de l’Algérie. Nouvelle Série, 1972: n42. Meddah A., Bertrand H. Elmi, S. Nouvelles données sur le volcanisme du Trias des Monts des Ksour (Atlas saharien occidental, Algérie). XIIème séminaire National Science de la Terre. Oran, 2006. Meddah A., Bertrand H., Elmi S. La province magmatique de l’Atlantique central dans le bassin des Ksour (Atlas saharien, Algérie). Compte Rendus Geoscience, 2007, 339: 24-30. Mékahli L. Hettangien-Bajocien supérieur des Monts des Ksour: Biostratigraphie, sédimentologie, évolution paléogéographique et stratigraphique séquentielle. Thèse doctorat d’état Univ. d’Oran, 1995. Yelles-Chaouche A.K., Ait-Ouali R., Bracene R., et al. Chronologie de l’ouverture du bassin des ksour (Atlas saharien, Algérie) au début du Mésozoïque. Bulletin de la Société Géologique de France, 2001, t. 172, n3: 285-293. 271
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