The Cretaceous Richat complex (Mauritania): A peri-Atlantic alkaline process

The Richat structure is a Cretaceous alkaline complex in the Mauritanian part of the Sahara Desert in West Africa. Due to its level of erosion, Richat provides a rare opportunity to study the summit of a complex, particularly its dissolution and fracture processes, and allows its volcanic and magmatic evolution to be established. Its age and geographical location were used to determine the cause of its emplacement and its position within the dynamic context of the Atlantic Ocean opening.;The Richat structure presents a wide variety of extrusive and intrusive rocks from different erosional levels. Several analyses were performed on these igneous rocks, including geochemical, petrological, isotopic (oxygen and carbon), structural, scanning electron microscope (SEM) and X-ray diffraction (XRD) studies. Magnetic and satellite data were also used. Our work shows that Richat represents the superposition of a bimodal tholeiitic series and crosscutting undersaturated carbonatitic and kimberlitic magmas. The alkaline magmas may have been derived from the asthenosphere, whereas the tholeiitic magmas could have come from metasomatized sub-continental lithospheric mantle. In such a scenario, pre-existing anisotropies could have acted as a pathway for ascending asthenospheric and subcontinental melts, allowing for the coexistence of alkaline and tholeiitic magmas within the same igneous complex. The erosional contrast between the central and outer parts of the Richat complex is explained by a piston-like collapse that preserved the central extrusive felsic facies and the hydrothermal karst infilling.;In order to provide a satisfactory explanation for the Richat occurrence, the hot spot hypothesis must be tested. The trajectory of a hypothetical hotspot beneath the Richat structure at 100 Ma until recent time (0 Ma) was calculated with a time interval of 10 Ma using PointTracker v4c. Results show that Richat is unlikely to be related to the passage of the African plate over a mantle plume. To find alternative solutions, a regional compilation of structural features, magmatic intrusions and geophysical anomalies was completed. A Proterozoic aulacogen, probably related to the oceanization stage in the Mauritanides, was identified and recognized as being spatially associated with the Richat structure. It is proposed that the reactivation of such pre-existing structures may have played a role in the emplacement of the Richat complex during mid-Cretaceous tectonic re-equilibrations along continental margins around the peri-Atlantic zone.;The Richat complex was part of the unusual geodynamics processes operating during the breakup of Pangea, specifically at the moment of final separation between the African and South American plates. This period is characterized by abnormal alkaline activity along the continental margins of the Atlantic Ocean, generally associated with the development of various individual hot spots. In order to place the Richat within the dynamic framework of the Atlantic opening, it was necessary to complete a spatial and temporal compilation of Mesozoic alkaline occurrences around the edges of the ocean. The structural characteristics of the various alkaline districts, along with the criteria commonly associated with hot spot models, were also compiled and processed. The results demonstrate that direct links exist between the alkaline districts. Their spatial and temporal distributions were used to define a mid-Cretaceous Peri-Atlantic Alkaline Province (PAAP) with a strong structural control. The PAAP presents few of the classic deep hot spot characteristics, and appears incompatible with the origin of a mantle plume. It is more likely linked to the reactivation of pre-existing lithospheric weaknesses acting as accommodation zones for intraplate stresses during specific episodes in the tectonic evolution of the Atlantic Ocean. A shallow asthenospheric origin is favoured over a deep mantle source. The Richat structure thus bears witness to this peri-Atlantic event.;Geochemical, petrological and fractal analyses, as well as 40 Ar/39Ar age dating, were performed on samples from the central breccia of the Richat structure. Results indicate the presence of a large-scale hydrothermal event responsible for breccia formation by dissolution and collapse of the host sedimentary unit. The breccia core was genetically related to plutonic activity because doming and the production of hydrothermal fluids must have been instrumental in creating a favourable setting for dissolution.;Keywords. karst, hydrothermal, breccia, silicification, Africa, alkaline complex, caldera, mantle plume, Atlantic opening, Cretaceous, reactivation.