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