Journal
CHEMICAL GEOLOGY
Volume 594, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.chemgeo.2022.120764
Keywords
Moroccan Sahara; Dolomite carbonatite; Mineral chemistry; C-O-Sr-Nd-Pb isotopes; Mantle plume; Columbia supercontinent breakup
Categories
Funding
- European Union [BE01-KA107-016242]
- APR AlkaCarboLipsWac grant
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The Gleibat Lafhouda dolomite carbonatites of the Moroccan Sahara are three separate cone-shaped plugs that intrude an Archean supracrustal basement rocks. These carbonatites have specific geochemical characteristics and may represent the earliest manifestation of the fragmentation of the Columbia supercontinent at 1.85 Ga.
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.
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