Prograde and retrograde evolution of eclogite from Adrar Izzilatène (Egéré-Aleksod terrane, Hoggar, Algeria) determined from chemical zoning and pseudosections, with geodynamic …

Abstract

Adrar Izzilatène in the Egéré-Aleksod terrane of the LATEA metacraton (Hoggar, Algeria) exposes one of the best preserved examples of eclogite facies metamorphism in Hoggar. Three distinct stages of metamorphic development are recognised, namely, the pre-peak stage (M1), characterised by garnet, amphibole, epidote, quartz and rutile, the peak eclogite facies stage (M2), consisting of omphacite, garnet, edenite, epidote, quartz and rutile, and the retrograde stage (M3), where initial decompression resulted in the appearance of plagioclase, the development of pargasite + plagioclase kelyphites and finally the formation of anhydrous plagioclase + diopside coronas. Porphyroblastic omphacite has a jadeite content of up to XJd= 0.36, which is the highest yet observed for eclogite facies rocks fromthe Tuareg Shield. Garnet growth zoning patterns are characterised by flat profiles in the cores (XAlm = 0.55–0.60; XPrp = 0.12–0.16; XGrs = 0.26–0.30) before showing a decrease in almandine to XAlm = 0.45, coupled to an increase in pyrope to XPrp = 0.29 and decrease in grossular to XGrs = 0.26 at the rims. Calculated P–T–MH2O pseudosections show that the prograde M1 assemblage equilibrated at 13–14 kbar and 580 °C, before pressure and temperature increased to 19 kbar and 650–700 °C at fluid-saturated conditions during peakmetamorphism. Retrogression involved near-isothermal decompression to 8–9 kbar and 700–750 °C at fluid-undersaturated conditions. Prograde-to-peak metamorphism of the Izzilatène eclogite could have involved either oceanic or continental subduction, followed by exhumation as the area was obducted towards the LATEA metacraton during the Pan-African orogeny and the assembly ofWestern Gondwana. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The Tuareg Shield represents a part of the Trans-Saharan Pan-African orogenic belt and is composed of 23 displaced terranes that are separated by subvertical strike-slip shear zones or major thrust fronts (e.g., Cahen et al., 1984; Black et al., 1994; Fig. 1a). These terranes, amalgamated together during the Pan-African orogenic cycle between 870 and 540 Ma, have different origins and are composed of either variably reworked Archaean to Paleoproterozoic domains (referred to as metacratons by Liégeois et al., 2013), or of juvenile Neoproterozoic units (Caby, 2003; Fezaa et al., 2010; Liégeois et al., 2003; Ouzegane et al., 2003; Peucat et al., 2003 and references therein). These terranes are further characterised by a marked absence of Mesoproterozoic rocks or events (e.g., Fezaa et al., 2010; Peucat et al., 2003). The Pan-African orogeny is related to the formation of the Gondwana supercontinent, and in the Tuareg Shield, this involved the accretion of several island arcs before the final collisional stage at 630–580 Ma. During final collision, the Shield was involved in northwards-directed tectonic escape as it was being squeezed between the West African craton and the Saharan metacraton (Black et al., 1994; Dostal et al., 2002; Liégeois et al., 2003, 2013). The Tuareg Shield is known for the occurrence of Precambrian rocks that experienced high-pressure eclogite facies metamorphism during the accretionary stage of the Pan-African orogeny (Berger et al., 2014; Bertrand, 1974; Derridj et al., 2010; Doukkari et al., 2014; Liégeois et al., 2003; Sautter, 1985; Zetoutou et al., 2004). These eclogite facies rocks have principally been described in the Central Hoggar (Southern Algeria), but have recently also been discovered in Western Hoggar (Berger et al., 2014). The preservation of the pre-collisional eclogite metamorphic imprint occurred through the obduction of these rocks onto a rigid continental terrane, thereby shielding them from later collisional processes (Liégeois et al., 2003). The subsequent Phanerozoic geological evolution of Hoggar is limited to Cenozoic intraplate volcanism (Liégeois et al., 2005 and references therein) and doming-related Lithos xxx (2014) xxx–xxx ⁎ Corresponding author. E-mail address: sidali.doukkari@gmail.com (S.A. Doukkari). LITHOS-03470; No of Pages 16 http://dx.doi.org/10.1016/j.lithos.2014.12.007 0024-4937/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier .com/locate/lithos Please cite