Nature and Evolution of the Lithospheric Mantle Beneath the Hoggar Swell (Algeria): Deformation, Melt-Rock Interactions and Olivine Seismic Properties Recorded by Mantle Xenoliths

The Hoggar mantle xenoliths provide insights into the evolution of the subcontinental lithospheric mantle in response to the geodynamic events that shaped NW Africa since, at least, the Panafrican orogeny. The Pan-African heritage is preserved in deformed lherzolite xenoliths (porphyroclastic to equigranular) from peripheral Hoggar localities (Tahalgha and Eggéré districts, Kourim et al., Journal of Petrology 55:2249–2280, 2014; Kourim et al., Tectonophysics, Special Issue: Constraints on Composition, Structure and Evolution of the Lithosphere 650:18–33, 2015; this study). These samples are distinguished by only incipient annealing, LREE-depleted clinopyroxene compositions, well-preserved olivine preferential crystallographic orientations (axial-[010]) consistent with transpressional regime, and low equilibrium temperatures (750–900 °C) achieved after lithosphere thermal relaxation. They are considered to represent the sub-continental lithosphere after the rejuvenation processes that took place during the late stages of the Pan-African orogeny, likely associated with igneous refertilization. Extensive lithospheric rejuvenation occurred either regionally, as a result of lithospheric delamination or thermo-mechanical erosion after thickening, or more locally along meridian shear zones. The Cenozoic events are marked by partial to complete annealing of pre-existing deformation microstructures, increased equilibrium temperatures (up to 1150 °C), extensive olivine-, clinopyroxene- (±amphibole-) forming metasomatic reactions, and changes in olivine CPOs and seismic properties. These modifications are observed either at the scale of magma conduits and their wall rocks (Kourim et al., Journal of Petrology 55:2249–2280, 2014; Kourim et al., Tectonophysics, Special Issue: Constraints on Composition, Structure and Evolution of the Lithosphere 650:18–33, 2015) or at the whole Hoggar scale, as shown by increasing degree of textural annealing and metasomatism from Tahalgha and Eggéré to Manzaz (i.e. from outer to central Hoggar). Our data show little changes at intermediate scale, as might have been expected, in particular, near or across major shear zones such as the 4°35′. This finding favors relatively large-scale asthenospheric upwelling related to upper mantle instabilities or local convections, rather than a process involving merely the reactivation of pan-African lithospheric faults.