Complex geometry of the Cenozoic magma plumbing system in the central Sahara, NW Africa

Abstract

Topographic uplifts in the central Sahara occur in the Hoggar-Aïr and Tibesti-Gharyan swells that consist of Precambrian rocks overlain by Cenozoic volcanic rocks. The swells and associated Cenozoic volcanism have been related either to mantle plumes or to asthenospheric upwelling and to partial melting due to rift-related delamination along pre-existing Pan-African mega-shears during the collision between Africa and Europe. The Cenozoic volcanic rocks in the Hoggar generally range from Oligocene tholeiitic/transitional plateau basalts, which occur in the centre of the dome, to Neogene alkali basalts characterized by a decrease in their degree of silica undersaturation and an increase in their La/Yb ratios. The alkali basaltic rocks occur mainly along the margins of the dome and typically have less radiogenic Nd and Sr isotopic ratios than the tholeiitic/ transitional basalts. The geochemistry of the most primitive basaltic rocks resembles oceanic island basalt (OIB) tholeiitic -in particular high-U/Pb mantle (HIMU)-type -and is also similar to those of the Circum-Mediterranean Anorogenic Cenozoic Igneous (CiMACI) province. These characteristics are consistent with, but do not require, a mantle plume origin. Geophysical data suggest a combination of the two mechanisms resulting in a complex plumbing system consisting of (a) at depths of 250-200 km, an upper mantle plume (presently under the Aïr massif); (b) between 200 and 150 km, approximately 700 km northeastward deflection of plume-derived magma by drag at the base of the African Plate and by mantle convection; (c) at approximately 150 km, the magma continues upwards to the surface in the Tibesti swell; (d) at approximately 100 km depth, part of the magma is diverted into a low S-wave velocity corridor under the Sahara Basin; and (e) at approximately 80 km depth, the corridor is tapped by Cenozoic volcanism in the Hoggar and Aïr massifs that flowed southwards along reactivated Precambrian faults.

Crustal Thickness Beneath Libya and the Origin of Partial Melt Beneath AS Sawda Volcanic Province From Receiver Function Constraints

“…For example, Hegazy () argues that the Libyan intraplate volcanism is related to hot mantle plumes in the underlying asthenosphere. Keppie et al () propose a conceptual model, using available geophysical and geological constraints, outlining the geometry of a channel plume underlying North Africa, which laterally feeds geographically discrete volcanic fields, similar to a model of the Afar plume (cf. Ebinger & Sleep, ).…”

Geologic and Metallogenic Framework of North Africa

“…1) (Gourgaud and Vicent 2004;Duggen et al 2005;Farahat et al 2006;Lustrino and Wilson 2007;Cvetkovic et al 2010;Keppie et al 2011;Lucassen et al 2013). The resulting volcanic rocks are part of the large Circum-Mediterranean Anorogenic Cenozoic Igneous province (CiMACI; Lustrino and Wilson 2007), whereas in northeast Africa the coeval igneous activity relates to formation of the East African Rift and the Red Sea (Chorowicz 2005;Lucassen et al 2013).…”