Origin of Cretaceous alkaline annular structures in the peri-cratonic terranes of the Reguibat Rise in Morocco: New constraints from carbonatite–silicate pairs and subsolidus processes

Abstract

We report an integrated field, petrographic, and geochemical study of three major Cretaceous carbonatite annular occurrences from the Oulad Dlim massif (Adrar Souttouf, NW margin of the West African Craton, south Morocco): Twihinate, Lamlaga, and Lahjayra. These complexes are commonly considered isolated carbonatite structures partially to completely overlain by silica breccias and iron oxide rocks. The carbonatites include two calcio‑carbonatites facies with distinct textural features and bulk-rock and mineral geochemistry: the “white” facies present characteristics consistent with a primary magmatic stage, while the “grey” facies likely resulted from hydrothermal alteration of the white carbonatites. We provide evidence that the annular structures include ultrapotassic alkaline syenites, probably derived from the same primary carbonated/silicated melt as the carbonatites, after partial melting of a K-rich lithospheric mantle source. The overlying silica breccias retain sandstone/quartzite-like stratifications, whereas their trace element signature is analogous to those of the carbonatites, suggesting that they were subjected to intense hydrothermal activity during carbonatite magmas emplacement. The iron-oxide rocks are chemically comparable to worldwide references of carbonatite-related laterite profiles, indicating a similar origin. All subsolidus (hydrothermal and weathering) alteration stages were accompanied by significant REE-Nb concentration resulting in high-grade deposits of monazite, synchysite, apatite and pyrochlore. Like other peri-Atlantic alkaline occurrences, the investigated Cretaceous alkaline‑carbonatite occurrences are likely related to the Atlantic Ocean opening and reactivation of pre-existing Pan-African tectonic structures.