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Late Cenozoic xenoliths as a guide to the chemical – isotopic composition and thermal state of the upper mantle under northeast Africa

¹ GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany
² Technische Universität Berlin, Fachgebiet Petrologie – ACK 9, Ackerstr. 71–76, 13355 Berlin, Germany

^* Corresponding author, e-mail: lucassen{at}gfz-potsdam.de

Spinel-lherzolite xenoliths from the upper lithospheric mantle from young, 1.5 Ma intra-plate magmatism of the Bayuda volcanic field (Sudan), ca. 500 km west of the Red Sea, cover a broad range from fertile to depleted lherzolite. The trace element patterns are rather uniformly depleted with respect to the primitive mantle with only few samples showing enrichment of incompatible trace elements. Mineral chemistry and fabrics indicate equilibrium in the rocks at temperatures between ~800 and 960 °C in most samples (range between 760 and 1190 °C) as determined from ortho- and clinopyroxene compositions. Sr, Nd, and Pb isotope compositions of clinopyroxene separates vary between values of old depleted mantle (e.g. ⁸⁷Sr/⁸⁶Sr ~ 0.70178; ¹⁴³Nd/¹⁴⁴Nd ~ 0.51349; ²⁰⁶Pb/²⁰⁴Pb ~ 16.14; ²⁰⁷Pb/²⁰⁴Pb ~ 15.22; ²⁰⁸Pb/²⁰⁴Pb ~ 35.85) and mantle showing strong influence of metasomatism (e.g. ⁸⁷Sr/⁸⁶Sr ~ 0.70315; ¹⁴³Nd/¹⁴⁴Nd ~ 0.51279; ²⁰⁶Pb/²⁰⁴Pb ~ 19.66; ²⁰⁷Pb/²⁰⁴Pb ~ 15.60; ²⁰⁸Pb/²⁰⁴Pb ~ 39.67). The old depleted mantle formed before the Pan-African cycle. The formation of the metasomatized lithospheric mantle beneath the Arabian Plate and NE-Africa is explained as the result of the Pan-African orogeny. The depleted trace element patterns are at odds with the isotope composition in many samples and indicate a relatively young melt extraction from the upper lithospheric mantle, likely in the Cenozoic. Therefore, the thermal state of the upper mantle lithosphere indicated by the xenoliths is likely Cenozoic.

Key-words: lherzolite xenoliths, Sr, Nd, Pb isotopes, lithospheric mantle evolution, intra-plate magmatism, northeast African lithosphere.