Petrogenesis and Tectonic Implications of the Tertiary Choke Shield Basalt and Continental Flood Basalt from the Central Ethiopian Plateau

Abstract

The voluminous Choke Shield basalts and flood basalts are distributed in the central Ethiopian Plateau. They are tholeiitic in composition and have OIB-like geochemical features. The ca. 23 Ma Choke Shield basalts have SiO₂ (47.1 wt.%–59.6 wt.%,), MgO (1.01 wt.%–7.8 wt.%,), Na₂O + K₂O (2.7 wt.%–8.4 wt.%), and display right inclined REE patterns ((La/Yb)_N = 21.4–24.2) with enrichment of Nb, Ta, Zr, Hf and Pb in the primitive mantle-normalized trace element diagrams. They show low initial ⁸⁷Sr/⁸⁶Sr ratios (0.703 47–0.703 77) and high ε _Nd(t) values (+4.4 to +5.0). In comparison, the 24 Ma high-Ti (HT1) flood basalts have SiO₂ (38.9 wt.%–50.8 wt.%), MgO (3.9 wt.%–11.4 wt.%), Na₂O + K₂O (1.6 wt.%–5.8 wt.%), and display right inclined REE patterns ((La/Yb)_N = 24–130.3) with enrichment of Nb, Ta, Zr, Hf, and Pb. They also show low initial ⁸⁷Sr/⁸⁶Sr ratios (0.703 30–0.704 44) and high ε _Nd(t) values (+2.2 to +5.3). Both types of basalts were contaminated by minor crustal materials and underwent fractional crystallization of clinopyroxene, plagioclase, olivine, and minor Fe-Ti oxide. The Choke Shield basalts were generated by 1%–5% melting of garnet-spinel to phlogopite-bearing spinel lherzolite in a shallow zone of the mantle plume, while the flood basalts were formed by <20% melting of amphibole-bearing garnet to garnet-spinel lherzolite in a deeper zone of the same mantle plume. The mantle source beneath the central Ethiopian Plateau was significantly heterogeneous during the Tertiary. It was characterized by EMI and EMII end-members that were formed by the metasomatism of the different components.

Keywords

basalt / shield basalt / continental flood basalt / Ethiopian Plateau / mantle plume

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