U–Pb baddeleyite and zircon ages of 2040 Ma, 1650 Ma and 885 Ma on dolerites in the West African Craton (Anti-Atlas inliers): Possible links to break-up of Precambrian supercontinents
Highlights
► U–Pb baddeleyite and zircon ages on dolerites in the West African Craton (Anti-Atlas inliers). ► Possible links to break-up of Precambrian supercontinents. ► Evidence of a 2040 Ma bimodal magmatic event in the Anti-Atlas. ► First evidence of an extensive ca. 1.65 Ga magmatic event in the Anti-Atlas. ► Evidence of ca. 885 Ma intraplate magmatic event in the Anti-Atlas.
Introduction
The West African Craton hosts the remnants of extensive events of tholeiitic/alkaline magmatism represented by mafic sill and dyke swarms in most inliers of the Anti-Atlas chain of southern Morocco. These types of intrusive rocks are potentially important for the reconstruction of Precambrian continental blocks into their original relative positions using dyke swarm patterns, radiometric ages and palaeomagnetic signatures (e.g., Bleeker and Ernst, 2006, Buchan et al., 1998, Ernst and Buchan, 1997, Ernst and Bleeker, 2010, Fahrig, 1987, Halls, 1982, Li et al., 2008).
On the basis of geochronological data available prior to this study (e.g., Gasquet et al., 2004 and references therein), the Precambrian magmatism in the Anti-Atlas is represented by at least four generations of dykes: (i) tholeiitic dykes of Palaeoproterozoic age such as those in the Tagragra of Tata inlier which have been dated at 2040 ± 6 Ma using the SHRIMP U–Pb method on zircons (Walsh et al., 2002); (ii) microgranite dykes from the Kerdous inlier dated at 1760 ± 3 Ma (Pb–Pb evaporation on zircon, Gasquet et al., 2004), which are cartographically and structurally associated with the mafic dykes in this inlier and in the Tagragra of Akka inlier; (iii) Cryogenian mafic dykes with tholeiitic and alkaline affinities, coeval with the opening of a back-arc oceanic basin in the Central Anti-Atlas (Clauer et al., 1982). A U–Pb zircon age of 761 ± 2 Ma has been obtained for plagiogranites associated with the Siroua ophiolites (Samson et al., 2004); (iv) upper Neoproterozoic dykes corresponding to 575 ± 4 Ma mafic dykes intruding the Taourgha granite in the Bas Drâa inlier (U–Pb zircon, Aït Malek et al., 1998) and 600 ± 5 Ma dacitic dikes intruding the schists and the granite basement of the Tagragra of Akka inlier (U–Pb SHRIMP on zircon, Gasquet et al., 2004).
Here, we report zircon and baddeleyite U–Pb ages for four mafic dykes and two sills cross-cutting the Palaeoproterozoic basement of the Zenaga, Agadir Melloul and Iguerda-Taïfast inliers. These results contribute to the (i) refinement of the magmatic barcode for the West African Craton, (ii) reappraisal of the age of some of the lithological units of the central Anti-Atlas, (iii) comparison of dolerite ages between the three inliers, and (iv) discussion of the geodynamical evolution of the northern margin of the West African Craton during Proterozoic time and provisional suggestions for its position in Precambrian supercontinents.
Section snippets
The West African Craton
The West African Craton (WAC), stable since 2 Ga, is located on the northwestern margin of African continent (Fig. 1). The basement of the WAC is composed of three Archaean and Palaeoproterozoic shields: to the south is the Leo-Man shield, to which the smaller Kayes (Mali) and Kenieba (Senegal) inliers are associated; to the north is the Reguibat shield; and to the extreme north is the Anti-Atlas belt (studied area; Figs. 1 and 2). In between are the vast Taoudeni and Tindouf intra-cratonic
The Zenaga inlier
The Zenaga inlier (Fig. 4), with an area of about 800 km2, is located 80 km southwest of the city of Ouarzazate. Along with the Agadir Melloul, Iguerda-Taïfast, Bou Azzer El Grâara inliers and the Siroua Window, they define the Central Anti-Atlas. The Zenaga inlier is a depression containing mainly Palaeoproterozoic rocks unconformably overlain by a Neoproterozoic formation affected by the major phase of the Pan-African orogeny. Five main tectonostratigraphic units were identified in the Zenaga
Sample description
The six samples dated by U–Pb geochronology were collected from three separate inliers in the central Anti-Atlas (discussed above): Zenaga, Agadir Melloul and Iguerda-Taïfast inliers.
Sample DZ49 is from a WNW trending, 50–80 m thick dyke of the Ifzwane dolerite Suite collected in the central part of the Zenaga inlier (Fig. 4). It is less altered in comparison to dykes intruding the eastern part of the inlier, and the original texture and the primary mineralogy are preserved in most outcrops. The
Analytical procedures
Approximately 100–300 g of a sample were crushed to powder using standard procedures. Zircon and baddeleyite were extracted using the Wilfley water-shaking table following the protocol of Söderlund and Johansson (2002) in the Geological Department of Lund University, Sweden. Zircon and baddeleyite fractions were split into 2 to 5 fractions per sample and analyzed by ID-TIMS (isotope dilution thermal ionization mass spectrometry) at the Laboratory of Isotope Geology at the Swedish Museum of
Evidence of a 2040 Ma bimodal magmatic event in the Anti-Atlas
In the Zenaga Inlier, the 2040 ± 2 Ma age of sample DZ49 dates the emplacement of dykes of the Ifzwane Suite, and is the first date from a Precambrian dolerite in the central Anti-Atlas. It matches the age of the Ierhourtane dolerite dyke of the Tagragra of Tata inlier in the western Anti-Atlas (2040 ± 6 Ma, Walsh et al., 2002), located ca. 80 km away (Fig. 2). Dyke DZ49 has a WNW trend (Fig. 7A). The trend of the other dyke dated at ca. 2040 Ma (in the Tagragra of Tata inlier) is on a short (~ 0.5 km)
Conclusions
Six new U–Pb ages have been produced for dolerite dykes and sills from various inliers in the Anti-Atlas belt which marks the northern margin of the West African craton (WAC). U–Pb ages of 2040 ± 2 Ma, 1656 ± 9 Ma, and ca. 1650 Ma (Zenaga inlier), 1654 ± 16 Ma (Agadir Melloul inlier), and 885 ± 13 Ma (Iguerda-Taïfast inlier) and ca. 885 (Zenaga inlier) of dykes and sills provide constraints on the position of the WAC in three supercontinent situations. The 2040 Ma dykes, and coeval Eburnean granites,
Acknowledgments
This work was carried out as part of PhD theses of Djiky Kouyaté, M'hamed El Janati, and Abdelhakim El Bahat to be submitted to (a) the Department of Geology of the Faculty of Sciences-Semlalia, Cadi Ayyad University of Marrakech (DK and MJ) and (b) the Department of Geology, Faculty of Sciences, Ibnou Zohr University of Agadir (AB). This research is being funded by the following research projects: (i) International Collaborative Research Grant, Swedish-MENA Research Links Programme to Ulf
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