How old is the Eye of Africa? A polyphase history for the igneous Richat Structure, Mauritania

Abstract

The annular Richat Structure, in Mauritania, is among the most striking geological features on Earth visible from space. Nicknamed “the Eye of Africa”, its intriguing concentric shaping has drawn attention for several decades. Formerly hypothesized as an astrobleme, it is now consensually recognized as a complex igneous intrusion, involving two ring-like gabbroic bodies, a central breccia and carbonatite dykes. However, the coexistence of until now undated tholeiitic gabbros and 99 Ma carbonatites akin to alkaline magmatism remained enigmatic.We provide the first plagioclase 40Ar/39Ar age determination and new geochemical data on the gabbroic bodies. Although no robust age was obtained, numerical modelling of our results suggests that the gabbros were intruded between 230 and 200 Ma. This age bracket is compatible with the ∼200 Ma Central Atlantic Magmatic Province (CAMP). Moreover, the compositions of these tholeiitic gabbros match the two chemical groups of the CAMP the best represented in northwest Africa. Therefore, we argue that the Richat gabbros correspond to two CAMP sills intruded conformably into the Late Proterozoic/Lower Paleozoic sedimentary sequences of the Taoudenni basin. About 100 My later, the gabbros and their country rocks were locally uplifted by an alkaline intrusion (expressed now on surface by the carbonatites), resulting in a circular doming, 40 km in diameter. The subsequent erosion led to the actual flattened concentric structure. Therefore, the Richat Structure represents a two-stage igneous-history, separated by a time lapse of ∼100 My, resulting in two CAMP gabbroic sills mimicking ring-like bodies.