Elsevier

Journal of African Earth Sciences

Volume 147, November 2018, Pages 544-553
Journal of African Earth Sciences

A subducted Neoproterozoic rift assemblage: The Egere Group (central Hoggar, Algeria)

https://doi.org/10.1016/j.jafrearsci.2018.07.006Get rights and content

Highlights

  • Metasediments of the Egere Group were deposited in a rift setting.

  • Serpentinite lenses represent remnants of an assumed ocean-continent transition.

  • Early Pan-African eclogitic conditions relate to W-dipping subduction.

  • The Arechchoum gneisses include reworked basement and Pan-African diatexites.

  • Exhumation was assisted by NW-directed thrusting and ductile flow

Abstract

The metasedimentary Egere Group (central Hoggar, Algeria) mainly comprises abundant marbles, metapelites, metaquartzites and mafic rocks, all affected by eclogitic metamorphism. Field relationships and pre-metamorphic features in the northern Egere region attest that most eclogites represent basaltic metatufs, metabasalts extruded during carbonate sedimentation and mafic sills. A slice of ultramafic rocks includes polymictic ultramafic sedimentary breccias containing blocks of marble, suggesting these represent a preserved piece of ocean-continent transition. Field observations, along with published metamorphic constraints and age determinations suggest that the Egere Group was deposited in a Neoproterozoic rift zone. Following peak eclogitic metamorphism (T = from 700 to 770 °C and P max. around 19 kbar after Doukkari et al., 2014), anatexis affected most rocks at decreasing pressure. Strong high-temperature ductile deformations, NW-verging folds and thrustings relate to exhumation of the lower crust assisted by viscous flow of anatectic domes. The location of the Egere eclogites and the similar age of pre-tectonic calc-alkaline plutons exposed west of the 4°50 fault in the Iskel terrane suggest that eclogitic metamorphism formed in a west-dipping subduction setting.

Introduction

Few remnants of oceanic lithosphere (Bou Azzer, south Morocco, El Hadi et al., 2010) and of ocean-continent transition (Timetrine, Caby, 2014) are exposed in the Trans-Saharan Pan-African belt of West Africa. Identification of these remnants, and their study, is paramount for reconstructing the evolution of the Tuareg Shield that, following Black et al. (1994), represents a collage of terranes now separated by mega-shear zones. In the western part of the Shield for example where possible remnants of ocenic lithosphere are identified (Augé et al., 2012), Statherian rifting (1.85–1.70 Ga, Caby and Andreopoulos-Renaud, 1983), was followed by a Mesoproterozoic rifting event that, in northeast Mali, took place after fragmentation of a carbonate platform (Bertrand-Sarfati et al., 1987; Bosch et al., 2016). The Pharusian Ocean opened east of the West African craton slightly before 795 Ma (El Hadi et al., 2010; Berger et al., 2011) and back arc rifting, related to eastward subduction (Dostal et al., 1996) is dated at around 680 Ma in western Hoggar (Caby and Monié, 2003; Bosch et al., 2016). Proterozoic eclogites have been reported in the central and western part of the Tuareg shield (Fig. 1 A). All occurrences are inserted in passive margin and/or rift assemblages. The central Hoggar eclogites (Liégeois et al., 2003; Arab et al., 2014; Doukkari et al., 2014a, Doukkari et al., 2014b, Fig. 1 B) are located 500 km east of the main Pan-African suture that documents a 2500-km long deep Ediacaran subduction along the Gondwana Orogen (Ganade de Araujo et al., 2014).

This paper is focused on the northern and central part of the Egere area where several occurrences of eclogites have been identified (Arab et al., 2014; Doukkari et al., 2014a, Doukkari et al., 2014b). Further field reconnaissance work and extensive observations of satellite images in this well exposed mountainous eclogitic terrain aim to reconstruct the sedimentary and geodynamic environment of the Egere Group and to put new constraints on the significance of eclogitic metamorphism in central Hoggar.

Section snippets

Overview of LATEA and adjacent terranes

Hoggar is considered as an amalgame of terranes delimited by shear zones (Black et al., 1994). In this mosaic of terranes, the LATEA domain (Fig. 1B) represents a c. 800 km long and c. 200–250 km wide superterrane (Laouni, Azrounfad, Tefedest, Egere-Aleksod terranes). This complex continental block is divided into sub-units by NNW-SSE trending shear zones that branch into the 4°50 mega-shear zone, a nearly rectilinear tectonic line traced up to central Brazil (Ganade de Araujo et al., 2014).

The Egere Group

The Egere Group essentially consists of high-pressure metasediments interlayered with lenses of mafic rocks converted to garnet amphibolites and eclogites, rare ultramafics and pre-metamorphic granitoids. Four distinct formations were distinguished in the Egere region by Duplan (1972). The lithostratigraphic log (E0, E1, E2, E3) presented by this author (Fig. 2) was interpreted as a right way-up metasedimentary pile, up to 5–6 km thick deposited concordantly above the Arechchoum gneisses in

Structure and architecture of the northern Egere region

The structure of the Egere is dominated by N-S trending metasedimentary belts some tens of kilometers wide including plateaus displaying flat-lying lithologies and foliation, separated by elongate domes of the Arechchoum gneisses that occupy about 50% of the exposures (Fig. 2). The gneiss domes display classical features of hot orogens submitted to viscous flow, as depicted by Whitney et al. (2004) and Rey et al. (2011). Many dikes centered on the domes also cut the lower part of the adjacent

Summary of the metamorphic evolution

Most eclogites displaying nearly random fabrics occur towards the cores of amphibolite lenses and boudins of any size. Pluri-centimeter-sized high temperature amphibole, garnet and zoisite also represent post-kinematic blasts in surrounding amphibolites. There are however examples of mylonitic amphibolites cut by plagioclase veins. In most metapelites in contrast, mineral lineations trending NNW are defined by fibrolite, the early garnet-bearing leucosomes being also deformed.

Several detailed

Nature, age of protoliths and sedimentary environment of the Egere Group

The existence of three stratigraphic units displaying similar rock types of quartzites, metapelites sl, carbonates (Fig. 2) as concluded by Duplan (1972), is questionable, owing to the lack of detailed diagnostic petrologic and mineralogic descriptions of lithotypes. Moreover, a great similarity of rock sequences from the Egere Group and from the Arechchoum gneisses has been pointed out by Duplan (1972) who stated that “no criteria allow distinguishing the metasediments from the two groups”

Conclusion

The late Neoproterozoic Egere Group is interpreted as a marine rifted domain in which basaltic volcanism was synchronous with carbonate sedimentation, as well as relics of the Neoproterozoic OCT. Further field studies are required to better understand the tectonic features, the initial tectonic vergence, the timing of exhumation and the exact nature and metamorphic evolution of the basement domes. In the framework of an eroded Neoproterozoic Himalayan-type range (Ganade de Araujo et al., 2014),

Acknowledgments

We are grateful to OPNA, ORGM and COMENA for logistic support during field work. Many thanks to Khadidja Ouzegane and Jean-Robert Kienast for fruitfull discussions.

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