Tracing the HIMU component within Pan-African lithosphere beneath northeast Africa: Evidence from Late Cretaceous Natash alkaline volcanics, Egypt

Abstract

International audienceA large late Cretaceous (~ 90 Ma) volcanic field (the Natash volcanic province) crops out in southeast Egypt at the northwestern boundary of the Arabian-Nubian shield. The lavas are mainly of alkaline affinity and exhibit a continuous compositional range from alkali olivine basalt (AOB) to trachyte and rhyolite. All basaltic lavas in the province record various extents of fractional crystallization of olivine, clinopyroxene, plagioclase and spinel. The basaltic lavas show variations in Sr-Nd-Pb-Hf isotopic ratios [(87Sr/86Sr)i = 0.7030–0.70286; (143Nd/144Nd)i = 0.512653–0.512761; (206Pb/204Pb)i = 19.28–19.94; (177Hf-176Hf)i = 0.28274–0.28285], that correlate markedly with the major and trace element ratios and abundances. Assimilation of crustal material cannot explain these correlations, and we invoke instead melting of a multicomponent mantle source. We infer the existence of High-μ (HIMU), Enriched mantle type-I (EM-I) and Depleted mantle (DM) domains in the melting source, with a predominant contribution from the HIMU-type. We suggests further that the basaltic lavas originate from low degrees of partial melting (F < 5%) at moderate potential temperatures (TP) 1391–1425 °C and pressures of 2.0–2.6 GPa. The melting pressure estimations imply that melting entirely occurred within lithospheric mantle, most likely in the presence of residual amphibole as presence negative K-anomalies in the primitive mantle-normalized patterns of the fractionation-corrected melts. The presence of amphibole within the lithosphere is a strong evidence that the lithospheric mantle underwent metasomatic enrichment prior to melting in Late Cretaceous. This metasomatic event affected on the Pb isotopic compositions of the Natash volcanics by adding Th and U to the melting source. Time-integrated calculations to remove the decoupling between 206Pb and 207Pb isotopes that most probably resulted from the metasomatic event indicate a tentative link between the metasomatism occurring in the Pan-African lithospheric mantle and the formation of juvenile crust during the Pan-African Orogeny. A two stage evolution model is therefore proposed for volcanism in the Natash area: fluxing of the lithosphere by hydrous fluids during Pan-African Orogeny forming a hybrid lithospheric mantle that in Late Cretaceous underwent thermal erosion and melting in response to upwelling asthenosphere, possibly at the onset of the extensional fracturing preceded the doming of the Afro-Arabian Shield

Bimodal zircon ages from Natash volcanics (southeast Egypt) and the link between eruption mechanisms and Late Cretaceous tectonics

“…Available petrographic and geochemical information on more mafic terms of Natash volcanics reveal that none of them is primary melt. All basaltic lavas record various extents of fractional crystallization of olivine, clinopyroxene, plagioclase and spinel, whereas chemical and isotopic overprint caused by assimilation of crust-derived materials seems negligible (Mohamed 2001;Abu El-Rus et al 2018. Mohamed (2001 suggested that the dominant source component was the asthenospheric, garnet-bearing mantle, favouring an active model wherein melting, melt segregation and rifting were caused by an uprising plume head.…”

“…This hybrid mantle source underwent thermal erosion and melting in response to upwelling asthenosphere at the onset of the extensional fracturing that preceded the doming of the Afro-Arabian Shield. Whatever the melt source and the proposed geodynamic model, most interpretations point to a crucial role of tectonics in melt generation and emplacement, according to the structural evidence that volcanics are arranged along the NW-SE extensional fault system of the region (Madani 2012;Abu El-Rus et al 2018), whereas plugs and domes preferentially occur at intersections of the main fault trends of the area (El Gammal et al 2013). Noticeably, the low magma fluxes recorded at Wadi Natash suggest the existence of multiple short-lived chambers, rather than an individual long-lived chamber, at moderate crustal levels (<28 km) beneath the volcanic field (Perry et al 1990, Mohamed 2001.…”

“…This complex fault system would represent a set of continental transforms developed in response to a major episode of Late Precambrian extensional continental crust formation in northernmost Afro-Arabia (Stern 1985). The Komombo, Nuqra and Kharit rift basins of Early to Late Cretaceous age are part of this geodynamic scenario and appear to be grabens or half-grabens (Madani 2000(Madani , 2012El Gammal et al 2013;Mo'men et al 2017) that formed at the onset of the extensional fracturing that preceded and accompanied the doming of the Afro-Arabian Shield (El Rus et al 2018). Noticeably, morphotectonics and geophysical investigations reveal the presence of a deep basin below the east end of Wadi Natash (El Gammal et al 2013); moreover, they show that the deepest basement surface in South Eastern Desert lies at about 4700 m b.s.l.…”

Hidden but Ubiquitous: The Pre-Rift Continental Mantle in the Red Sea Region

“…Alkaline volcanism developed in Israel since 200 Ma and continued until the Miocene (Lang and Steinitz, 1989), with the oldest volcanic activity being probably related to the emplacement of a Jurassic plume (Atlit-1 and Haifa-1 drill-holes, Khon et al, 1993). A Late Cretaceous volcanic field (∼90 Ma) crops out in south-eastern Egypt (Natash volcanic province) at the same latitude of the Uwayrid lava field (Figure 1), where alkalibasalt lavas evolve locally towards trachytic and rhyolitic compositions (Abu el-Rus et al, 2018). The presence of lava fields with alkaline affinity during the Mesozoic suggests that the continental lithosphere of East Africa and Arabia was initially subject to diffuse extension with deformation by stretching and faulting (Rooney et al, 2011).…”

“…On the other hand, basalts from central and northern Arabia contain a distinct enriched mantle component with higher Pb and lower Sr ratios, and with Nd and Hf isotope contents similar to those of the Afar basalts (Bertrand et al, 2003;Shaw et al, 2003;Moufti et al, 2012;Altherr et al, 2019). The most extreme expression of this enriched component has been found in some lavas of the East African Rift (Rooney et al, 2014;Abu El-Rus et al, 2018), where their high Pb and extremely low Nd and Hf isotopic ratios point toward the "HIMU" (high μ 238 U/ 204 Pb) endmember, as defined for oceanic basalts (Zindler and Hart, 1986;Stracke et al, 2003). In the Nubian Plate, this component is thought to be located in the lithospheric mantle, and originated by an old metasomatic event that affected the north-eastern African lithospheric mantle during the Neoproterozoic Pan-African orogeny (Stein et al, 1997).…”