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 similar to 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 delta C-13(v-PDB) = -10.5 to -1.6 parts per thousand, delta O-18(V-SMOW) = 6.4-28.3 parts per thousand, initial Sr-87/Sr-86 = 0.7034-0.7043 (epsilon Sr-i between -14.5 and - 1.8), Nd-143/Nd-144 = 0.51282-0.51283 (epsilon Nd-i between 2.8 and 3.6), Pb-206/Pb-204(i) = 19.52-23.78, Pb-207/Pb-204(i) = 15.56-15.69 and Pb-208/Pb-204(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.