The Twihinate carbonatite (Moroccan Sahara): Evidence for compositional mantle heterogeneity and northwestern African plate geodynamics

The Cretaceous Twihinate carbonatite in the Moroccan Sahara is a ∼ 5 km diameter ring-shaped intrusion made of an inner core preserving sparse occurrences of medium- to coarse-grained calcite carbonatite encircled by a ring of vuggy siliceous breccia. The Twihinate carbonatite is enriched in large ion lithophile elements (Cs, Rb, Ba, U and Th) and light rare earth elements (LREE), but shows negative anomalies in high field strength elements (particularly Ta, Zr, Hf and Ti). Stable and radiogenic isotope ratios vary in the range of δ¹³C_v-PDB = −10.5 to −1.6‰, δ¹⁸O_V-SMOW = 6.4–28.3‰, initial ⁸⁷Sr/⁸⁶Sr = 0.7034–0.7043 (εSr_i between −14.5 and − 1.8), ¹⁴³Nd/¹⁴⁴Nd = 0.51282–0.51283 (εNd_i between 2.8 and 3.6), ²⁰⁶Pb/²⁰⁴Pb_i = 19.52–23.78, ²⁰⁷Pb/²⁰⁴Pb_i = 15.56–15.69 and ²⁰⁸Pb/²⁰⁴Pb_i = 38.69–39.02). Altogether, these isotopic compositions reflect compositional mantle heterogeneity, and are interpreted to reflect partial melting of heterogenous mantle sources with a potential eclogite component in an intraplate, rift-controlled tectonic setting. From a geodynamic perspective, the time span ascribed to age emplacement of Twihinate carbonatite shortly follows the Upper Jurassic hyper-extension event which ultimately resulted in mantle exhumation and subsequent onset of drifting in the Central Atlantic Ocean and Maghrebian Tethys.