A suture related accretionary wedge in the Gondwana assembly: Insights from serpentinites in the Hoggar shield, Algeria

The assembly of West Gondwana supercontinent involved several complex processes that led to the formation of the Hoggar shield during the Panafrican-Brasiliano event. We present the first petro-geochemical, geochronological and field data of serpentinite lenses exposed along the In-Tedeini domain, within deformed talc-schist and magmatic-sedimentary formations. The serpentinites preserved the main characteristics of their parent peridotites (Al/Si: 0.004-0.03; Mg/Si: 1.14-1.62; Al2O3: 0.15-1.37 wt%; Mg#: 85.3-94; Ti: 9.34-120 ppm; Nb: 0.007-0.46 ppm), attesting to a highly depleted mantle wedge protolith involved in a subduction zone. This is in agreement with the high Cr# (0.55-0.6), low to moderate Mg# (0.36-0.65) and low TiO(2 )contents (< 0.1 wt%) of their constitutive Cr-spinels. Geochemical modelling suggest that both the North and South In-Tedeini serpentinite units have experienced intense and similar fluid-induced dynamic melting episodes. The evolution of these two units has diverged, with the Southern In-Tedeini unit being refertilized by a small volume of island-arc basaltic melts generated in the mantle wedge. Serpentinization of these rocks probably occurred under static conditions at high temperature corresponding essentially to amphibolite-facies conditions. Field relations suggest that the exhumation of the massive serpentinites occurred along major sinistral shear zones steeply dipping eastward, assisted by talc-schists that highly localized transpressive deformation. First U-Pb zircon ages obtained from a metasomatic chloritite in North In-Tedeini serpentinites; they may have recorded the age (770 +/- 5 Ma) of the subduction related Panafrican island arcs and the emplacement (631 +/- 10 Ma) of In-Tedeini serpentinites within the crust; or they may rather correspond to the serpentinization events endured by the rocks. All together, the reported results support the presence of a major suture zone, oriented NNW-SSE. This suture is outlined by mantle serpentinite lenses exhumed in a collisional accretionary wedge, which connects the western and the central Hoggar, following a Panafrican east-dipping subduction. This tectonic system would have contributed to the closure of the Goias-Pharusian ocean.