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 δ13CPDB = -5·8 to 1·8ø, δ18OSMOW = 6·9-23·5ø, initial 87Sr/86Sr = 0·7031-0·7076, 143Nd/144Nd = 0·5125-0·5129 and 206Pb/204Pb =18·29-19·89. Calciocarbonatites intruding Jurassic limestones have the highest δ13C and δ18O values and the most radiogenic initial 87Sr/86Sr, but least radiogenic 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb isotope ratios, and are interpreted to have interacted with the limestones (crustal components).The magnesioand silicocarbonatites have Sr, Nd and Pb isotope ratios that are nearly identical to those of low-87Sr/86Sr calciocarbonatites.The isotope signature of the high-Sr, low-87Sr/86Sr calciocarbonatites with mantle-type O and C isotopic compositions indicates the presence of HIMUand EMI-type components in the mantle source of theTamazert 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 theTamazert 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. © The Author 2010. Published by Oxford University Press. All rights reserved.