Subcontinental lithosphere reactivation beneath the Hoggar swell (Algeria)

Abstract

In the Tahalgha district (southwestern Hoggar, Algeria), the Cenozoic volcanism has sampled subcontinental mantle beneath two crustal terranes that collided during the Pan-African orogeny: the "Polycyclic Central Hoggar" to the east and the "Western Hoggar" to the west. Two major lithospheric shear zones separate these terranes: the "4 degrees 35" and the "4 degrees 50" faults. Mantle xenoliths were collected between the two faults and across the 4 degrees 35 fault. In addition to a range in equilibrium temperatures and chemical compositions reported elsewhere, the samples show variations in their microstructures and crystallographic preferred orientations. Equilibrium temperatures and geochemical characteristics allow dividing them into low - (LT; 700-900 degrees C), intermediate - (IT; 900-1000 degrees C), and high-temperature (HT; 1000-1100 degrees C) xenoliths. The LT and IT peridotites occur on both sides of the 4 degrees 35 fault; they are usually coarse-grained. HT xenoliths are present only east of the 4 degrees 35 fault, in the narrow domain stuck between the two faults; they are fine-grained and extensively affected by annealing and melt-rock reactions. Microstructures and crystallographic textures indicate that deformation in the LT- and IT-xenoliths occurred through dislocation creep under relatively high-temperature, low-pressure conditions, followed by post-kinematic cooling. The fine-grained HT-xenoliths were deformed under relatively high-stress conditions before being annealed.Combining microstructural and CPO data with petrological and geochemical informations suggests that: (1) the LT xenoliths are remnants of the Neoproterozoic lithospheric mantle that preserved microstnictural and chemical characteristics inherited from the Pan-African orogeny, and (2) the HT xenoliths record localized Cenozoic deformation associated with melt channeling through feed-back processes that culminated in the formation of high-permeability porous-flow conduits. Limited grain-growth in HT xenoliths suggests that advective heating of melt conduits was transient and rapidly followed by thermal relaxation due to conductive heat loss into wall-rock peridotites represented by the IT xenoliths, then by exhumation due to volcanic activity. (C) 2014 Elsevier B.V. All rights reserved. Show more