February 2010 LIP of the Month

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Regional dyke swarms of the Reguibat Shield, Mauritania and Morocco: plumbing systems for Precambrian Large Igneous Provinces

Henry C. Halls, Department of Chemical and Physical Sciences, University of Toronto Mississauga and Department of Geology, University of Toronto, Canada; henry.halls@utoronto.ca

An important component of Large Igneous Provinces (LIPs) is their regional dolerite dyke swarms. Mapping in the Reguibat Shield of Mauritania and Morocco using Google Earth© reveals numerous major regional dyke swarms (Halls 2010) that are thought to be the feeder system for regional flood basalts that were subsequently removed by erosion. The recognition of these regional dyke swarms is the first step toward sorting out the Precambrian LIP history of the Reguibat Shield. The dyke swarms will provide a plumbing system framework to which other components, remnant flood basalts, sills, and other minor intrusions can be linked to define regional LIP events, as has been done in the Canadian Shield and elsewhere (e.g. Ernst and Bleeker 2010).

The Reguibat Shield comprises a typical Archean to Early Proterozoic crystalline metamorphic terrain, covered by Neoproterozoic to Palaeozoic rocks to the north and south (Fig. 1).  Google Earth© has  been used to trace  several major dyke swarms that cut the Shield but not the surrounding  cover rocks, suggesting that most of them are Precambrian in age.

The most extensive swarm is one (shown in red) that strikes about N-S in the eastern part of the Shield. Here individual dykes can be followed for at least 200 km before becoming concealed to the north and south by the cover rocks. They have been studied by Aifa et al (1993) and a Pb-Pb age of 1.95 Ga has been reported. Westwards, dykes continue to strike approximately normal to the elongated, arcuate shape of the shield, so that towards the west they gradually assume NW trends (Fig. 2). Confirmation of the dykes’ existence is obtained from the Mauritanian government aeromagnetic map (Mauritania prism project [www.mautiania.mr/fr/pdf/depliant_geophysique.pdf]) (Fig. 3). Cross-cutting relations between dykes in the western region (Fig. 4) show that the “red” swarm is the oldest of several others (Figs. 5, 6). If they are part of the same swarm found at the eastern end of the Reguiba shield, then a giant radiating dyke swarm with a southerly focus can be defined. 

At the western end of the shield (Fig. 4) a dense swarm (dykes shown in purple) extends in a NE direction for more than 200 km with a width of several hundred kilometres. The full dimensions are concealed by sand. Locally dyke intrusion is so intense that crustal extensions of at least 30% are indicated.  The swarm appears to disappear within the shield at it’s NE end, although “red” NW-trending dykes can still be seen (Fig. 4).  Possible explanations are that the swarm dies out or the dykes, unlike the NW-trending ones, have no relief and are covered by sand. The dykes are younger than the NW-trending swarm on cross-cutting relations (Fig. 5). 

Dykes with a NNE-NE trend give prominent aeromagnetic anomalies where they occur in southern Morocco. The age of the “purple” dyke swarm is unknown. It appears to be older than a widely-spaced set of dykes (shown in yellow on Fig. 4) that runs generally E-W (Figs. 6, 7) but it does not appear to cut (and is therefore older than) a major NNE-trending dyke, “the Great Dyke of Mauritania” (green in Fig. 4), that runs for more than 100 km (Fig. 4) and which is locally more than 1 km wide (Figs. 8-10).  This major dyke is accompanied by other dykes of similar trend. Since neither swarm appears to cut surrounding cover rocks, both are likely to be Precambrian in age.  If the dense NE-trending swarm does die out towards the NE, it may be fed from a source to the SW. Dykes show branching towards the NE (Fig. 11) which would support this idea.

On the aeromagnetic map of the central part of the Reguibat Shield (Fig. 3), a major ENE-trending magnetic anomaly extends for at least 500 km. It may be generated by one of several large Jurassic dykes that trend more NE in Morocco and the Iberian Peninsula (e.g., Touil, 2008). In the Reguibat Shield the dyke (shown in blue on Fig. 2) is only occasionally visible in the satellite imagery.


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Figure 11.

References

Aifa, T., Lefort, Y.-P., Ouddane, M., and Calza, F. (1993). Mise en évidence d’antiformes générées en régime extensif sur la marge orientale du craton ouest-africain (région des Eglab): Arguments paléomagnétique et gravimétriques. Bulletin du Service Géologique de l’Algérie, v. 4, pp. 121-136.

Ernst, R.E., Bleeker, W. (2010) Large Igneous Provinces (LIPs), giant dyke swarms, and mantle plumes: significance for breakup events within Canada (and selected adjacent regions) from 2.5 Ga to present. Canadian Journal of Earth Sciences, in press.

Halls, H.C. (2010) The Reguibat Shield, Mauritania: a dyke swarm bonannza!  Abstract for 6th International Dyke Conference, February 4-7, 2010 Varanasi, India (Dyke Swarms: Keys for Geodynamic Interpretation), http://idc6.igpetbhu.com/

Touil, A. (2008) Petrography, mineralogy and geochemistry of the Ighrem diabase dyke (Anti-Atlas, Southern Morocco). Revista de la Sociedad Geológica de España, v. 21, pp. 23-33.