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Geochemical and Sr-Nd isotope ratios in Cenozoic basalts from Eritrea: Evidence for temporal evolution from low-Ti tholeiitic to high-Ti alkaline basalts in Afro-Arabian continental flood basalt province

Abstract:

Mid-Tertiary continental flood basalts covering large part of Eritrea, Ethiopia and Yemen comprise an igneous province linked to a mantle plume still active beneath Afar. In Eritrea, Oligocene-Miocene basalts on the Central and Southern Highlands unconformably overlie and partially cover a lateritised Palaeozoic-Mesozoic sedimentary strata or a basement of Neoproterozoic island-arc rocks. The Oligocene-Miocene basalts, respectively from Menguda and Ona/Durko sections, have been analysed for major and trace elements and Sr-Nd isotopic compositions; such data are the first for the Eritrean flood basalts. The older products from Menguda section are fine to medium-grained, olivine-phyric low-Ti tholeiitic basalts. They are characterised by relatively high MgO, compatible element contents and high CaO/Al2O3 ratios, and by low Fe, P, K and incompatible element contents. REE profiles are flat at near ten times chondritic values. No intercalated silicic volcanics are found. The younger basalts from Ona and Durko sections contain interbedded pyroclastic rocks up to 60 m total thickness. In contrast to the Menguda basalts, these younger lavas are high-Ti transitional to alkaline basalts. The temporal sequence in Eritrea is therefore from low-Ti tholeiitic basalts to high-Ti alkaline basalts. Furthermore, the younger basalts have relatively high Fe, P, K and incompatible element contents and low CaO/Al2O 3 ratios. 87Sr/86Sr and 143Nd/ 144Nd ratios in Menguda and Ona/Durko basalts show similar and restricted ranges, respectively 0.70331-0.70423 and 0.51280-0.51288 (εNd = +3.2 to +4.7), and 0.70341-0.70434 and 0.51284-0.51288 (εNd = +3.9 to +4.8). Correlation of Sr isotope ratios with MgO used as index of fractionation indicates that the rocks underwent crustal contamination. Moreover, the earlier magmas (low-Ti tholeiitic basalts) were derived from a higher degree of partial melting of an incorporated depleted component in the Afar plume. Whereas, the later magmas (high-Ti alkaline basalts) were derived from a lower degree of melting of the plume itself.