Petrogenesis of the Eocene Tamazert Continental Carbonatites (Central High Atlas, Morocco): Implications for a Common Source for the Tamazert and Canary and Cape Verde Island Carbonatites

Abstract

The Tamazert Eocene alkaline complex of the Central High Atlas Range of Morocco hosts the largest outcropping occurrences of carbonatites in northern Africa. The complex consists of carbonatites and undersaturated ultramafic to syenitic alkaline to peralkaline silicate rocks. Mineralogically and geochemically the Tamazert carbonatites are classified as calciocarbonatites, magnesiocarbonatites and silicocarbonatites. They are enriched in light rare earth elements and large ion lithophile elements (Cs, Rb, Ba, U, Th), but depleted in high field strength elements (particularly, Ti, Nb and Ta). Stable and radiogenic isotope ratios vary in the range of delta C-13(PDB) = -5 center dot 8 to 1 center dot 8 parts per thousand, delta O-18(SMOW) = 6 center dot 9-23 center dot 5 parts per thousand, initial Sr-87/Sr-86 = 0 center dot 7031-0 center dot 7076, Nd-143/Nd-144 = 0 center dot 5125-0 center dot 5129 and Pb-206/Pb-204 = 18 center dot 29-19 center dot 89. Calciocarbonatites intruding Jurassic limestones have the highest delta C-13 and delta O-18 values and the most radiogenic initial Sr-87/Sr-86, but least radiogenic Nd-143/Nd-144, Pb-206/Pb-204 and Pb-208/Pb-204 isotope ratios, and are interpreted to have interacted with the limestones (crustal components). The magnesio- and silicocarbonatites have Sr, Nd and Pb isotope ratios that are nearly identical to those of low-Sr-87/Sr-86 calciocarbonatites. The isotope signature of the high-Sr, low-Sr-87/Sr-86 calciocarbonatites with mantle-type O and C isotopic compositions indicates the presence of HIMU- and EMI-type components in the mantle source of the Tamazert carbonatites, similar to what has been proposed for the Cape Verde and Canary Islands. The close similarity in carbonatite composition between the Cape Verde and Canary Islands and Tamazert suggests a common sublithospheric source for these carbonatites. We therefore propose that the Tamazert carbonatites originated through melting of Canary plume material that may have flowed through a sub-lithospheric corridor extending from the Atlantic near the Canary Islands to the Middle Atlas, formed by the delamination of the subcontinental lithosphere in response to Africa-Europe collision at c. 42 Ma. Seismic tomography data suggest that the common source may be within the lower mantle at depths > 1000 km.