Plume related alkaline magmatism in central Africa—the Meidob Hills (W Sudan)

Abstract

We investigated in a combined petrological, geochemical and isotope study the Miocene to Holocene volcanic activity in the Meidob Hills Volcanic Field, the north-eastern part of the Darfur Dome Volcanic Province in western Sudan. The Darfur Dome is related to a domal uplift, associated with a negative Bouguer anomaly, and crustal shearing along the Central African Fault Zone. Magmas of the Meidob Hills evolved from basanites to phonolites by fractionation of olivine, clinopyroxene, magnetite, and in a late stage by plagioclase–anorthoclase and apatite. Significant contamination by crustal rocks is observed in basaltic to benmoreitic–trachytic (the latter mostly pyroclastic) rocks. Isotope data demonstrate the presence of a HIMU-like component, which is most pronounced in 7 Ma old basanites ( 87 Sr/ 86 Sr=0.70306, 206 Pb/ 204 Pb=20.075, 207 Pb/ 204 Pb=15.690, 208 Pb/ 204 Pb=39.785 ). The basanites are interpreted as a mixture between hypothetical sources of depleted mantle and HIMU with small amounts of some other sources. 3 He/ 4 He -isotope ratios in olivine from mafic rocks are much lower than the primitive He mantle source (PHEM) and range between 6.6 and 9.2 R a, corrected for atmospheric contamination, measured values between 5.4 and 7.5 R a. Trace element ratios of Ba, Nb, La and Zr show a wide range and indicate source heterogeneities, possibly a subcontinental lithospheric source. The Darfur Dome provides an example for an isolated intracontinental mantle plume with a low He—high HIMU-type composition. Its igneous activity is not related to graben structures, and we see no connection to the Cameroon Line, the Tibesti and the volcanism of the Red Sea and the Afar plume.