Bulk composition and O–C isotope constraints on the petrogenesis of Richat carbonatites (Mauritania)

Abstract

In the northwestern Mauritanian Taoudeni Basin, the famous Richat structure is the only occurrence known of carbonatite. Despite several studies on this unique geological feature, the mineralogy and petrogenesis of the Richat dome carbonatites are underdocumented.Major, trace element whole rocks and carbon-oxygen isotope data are reported for Cretaceous carbonatites from Richat dome in Mauritania. More than 45 carbonatite dykes have been mapped, they are oriented N°15-N°80. Based on color, four groups have been identified, dark, gray, red breccia, and brown carbonatites. They exhibit different mineralogical and texture characteristics. The studied carbonatites are mainly composed of dolomite, calcite, and ankerite, with lesser amounts of apatite and quartz. Accessory minerals such as pyrochlore, spinel, pyrite, chalcopyrite, magnetite, hematite, baryte, phlogopite, and zircon are also present. Other REE-bearing minerals (britholite, monazite, and bastnaesite) were revealed by X-ray diffraction.Three types of carbonatites are identified: magnesiocarbonatites (dominants), ferrocarbonatites and calciocarbonatite. The presence of spinel and pseudomorphed olivine, as well as high contents of Ni (380 ppm), Co (34 ppm), and Cr (840 ppm) in the dark and brown carbonatites, suggests that these rocks are the result of the low-degree partial melting of a spinel-bearing peridotite. Furthermore, the Zr/Hf (48–80) and Nb/Ta (26–48) ratios show significantly higher values than those observed in the primitive mantle, they are interpreted as formed by partial melting of the metasomatized mantle. The high values of δ18O in addition to the presence of baryte, ankerite, and quartz in the Richat carbonatites suggest that they are the result of secondary processes involving a low-temperature fluid-rock interaction.The carbonatites from the Richat dome are enriched in LREE (La = 258–1070 ppm) and LILE as Sr, Ba, Th, U, and depleted in HREE (Yb = 2–14 ppm) and HFSE such as Zr, Hf, Ti. These characteristics are like those of Cretaceous peri-atlantic alkaline carbonatites such as Barra do Itapirapua in Brazil, Ondurakorume and Kalkfeld in Namibian, lupangola in Angola, and Twihinate in Morocco. Reactivation of a deep pan-African NNE-SSW lineament would have been a fundamental factor in the formation of the Richat carbonatites.