Petrogenesis of the late Paleoproterozoic Gleibat Lafhouda dolomite carbonatite (West African Craton Margin, Moroccan Sahara) and its relevance to the onset of fragmentation of the Columbia supercontinent

The Gleibat Lafhouda dolomite carbonatites of the Moroccan Sahara occur as three separate cone-shaped plugs intruding an autochthonous succession of Archean supracrustal basement rocks. Geochemically, the Gleibat Lafhouda dolomite carbonatites are characterized by a compositional range of 11.3-27.1 wt% MgO, 3.1-29.7 wt % CaO, 3.5-38.0 wt% FeOtot and < 0.1-7.5 wt% SiO2, and enrichment in large-ion lithophile elements (LILE), particularly Sr (2173-11,310 ppm), Ba (174-4537 ppm), U (0.1-296 ppm) and light REEs (LREEs) (131-1295 ppm), but not in the heavy REE (HREEs) and high-field strength elements (HFSE) such as Ti, Zr, and Hf. Nb and Ta show, however, much higher concentrations ranging from 0.5 ppm to 1.0 wt%, and < 0.0 to 199 ppm, respectively, which set them apart from naturally occurring carbonatites and the experimentally derived carbonated melts. The combined stable (delta C-13(V-PDB )= -2.5 to -6.6 parts per thousand, delta O-18(V)-SMOW = 6.0 to 20.7 parts per thousand) and radiogenic( 87)Sr/Sr-86(in), (0.7032-0.7046), Nd-143/Nd-144(in), (0.5105-0.5106) or epsilon Nd(t) (+ 3 to +6), and (206)pb/(204)pb (19.06-49.05), Pb-207/Pb-204 (15.90-18.87), and Pb-208/Pb-204 (37.87-38.50) isotope compositions are consistent with low degree partial melting, at convecting upper mantle conditions, of a predominantly depleted mantle source in a rift-related environment. Based on these geochemical features, it is suggested that the Gleibat Lafhouda dolomite carbonatites represent the earliest manifestation of rifting processes related to the fragmentation of the Columbia supercontinent at 1.85 Ga. Accordingly, we propose that these carbonatitic rocks represent the initial Mg-rich melt in the mantle plume head that derived from decompressional adiabatic melting of a depleted mantle source.