Tracing the plume material in the Arabian-Nubian Shield

Abstract

The formation of the Arabian-Nubian Shield (ANS) during the late Proterozoic Pan-African orogeny was associated with the rise of a plume head to the shallow mantle, leading to the production of an enriched “plume mantle” and oceanic plateaus. The “plume mantle” was later transformed by the subduction mechanism into continental crust and lithospheric mantle. In this paper, I summarize the geological and geochemical evidence for the presence of the plume material in the magmatic inventory of the ANS. The isotope composition of the “plume mantle” ( ε Nd=+5±1 and 87 Sr / 86 Sr =0.7027±3 ) is defined by the values of early Pan-African oceanic plateau basalts and ophiolitic magmas . Similar isotope ratios are displayed by early calc-alkaline volcanics and by calc-alkaline batholithic granites that were produced in the late stages of the Pan-African orogeny. These granites are used here as samplers of the pre-existing mafic crust that accumulated over 200 Ma of subduction activity. The “plume-mantle” composition is also revealed by alkali granites and Phanerozoic alkali basalts , which were derived from the lithospheric mantle. All these magmas indicate that substantial parts of the ANS were composed of the enriched plume-related material. Cenozoic alkali basalts from other segments of the Gondwana (e.g. Ahaggar, North Africa and Ross Sea Rift, Antarctica) exhibit the ANS “plume-mantle” isotope composition, suggesting a large distribution of the Pan-African plume activity.