Geodynamic evolution of the Hoggar: Contribution of the serpentinites and mafic rocks from the western Hoggar

Résumé

This doctoral thesis focus on the In-Tedeini domain in the Hoggar shield, a privileged region to constrain the geodynamic evolution of the Gondwana supercontinent. This work deals with multidisciplinary approaches combining the first structural, petrographic, geochemical and geochronological data performed on serpentinites and associated mafic rocks exposed on In-Tedeini domain. Results of this work show that the serpentinites preserved the main characteristics of their parent peridotites attesting to a highly depleted mantle wedge protolith involved in a subduction zone. This is corroborated by the chemistry of the Cr-spinels that constitute the only pristine minerals in these rocks coupled to the geochemical modelling, concerning the Variable trace elements compositions between the North and South In-Tedeini serpentinites. The latter reveals intense and similar fluid-induced dynamic melting processes involving 20 to 22% partial melting of a depleted mantle. The evolution of these two units has diverged with the Southern In-Tedeini serpentinites being particularly refertilized by a volume of 0.02-1.2% of island-arc basaltic melts generated in the mantle wedge. These results also show that these mantle peridotites have been serpentinized in the environment in which they have evolved. A static and complete serpentinization took place in the mantle wedge by interaction with FME-rich fluids derived from both subducted slab and oceanic sediments at high temperature corresponding to amphibolite-facies conditions. It was followed by processes involving localized Si-, then carbonate- and iron-rich fluids. Fieldwork indicate that the emplacement of massive serpentinite lenses in the crust occurred along NNW-SSE major sinistral shear zones steeply dipping eastward, assisted by talc-schists that wrap them and highly localized transpressive deformation between the massive serpentinites and magmatic and sedimentary formations. Petrography and geochemical analysis on the associated magmatic rocks allowed the characterization of basalt-derived amphibolites and their N-MORB and E-MORB signatures. This work also identify metasomatic mafic chloritites dikes in serpentinite bodies and obtain the first U-Pb zircon ages. They may have recorded the subduction related Panafrican island arc melts (770 ± 5 Ma) and the emplacement of In-Tedeini serpentinites within the crust (631 ± 10 Ma); or they may rather correspond to the hydrothermal events endured by the rocks. Finally, this work infers the presence of a major suture zone, oriented NNW-SSE. It is outlined by the presently found In-Tedeini mantle wedge serpentinites, exhumed in a collisional accretionary wedge that likely corresponds to an ancient oceanic lithosphere. This suture connects the western and the central Hoggar, following a Panafrican east-dipping subduction as a result of the West Gondwana assembly. Overall, this PhD research shows that the In-Tedeini domain preserves a key fingerprint of the Neoproterozoic geodynamic evolution of the Hoggar shield.

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