Petrogenesis of the Adigrat basalts from the Oligocene Ethiopian flood basalt province in Tigrai, Northern Ethiopia

Abstract

The Adigrat basalts are exposed along a prominent E-W extending ridge between Adigrat and Shire at the northern edge of the Ethiopian continental flood basalt province. They form up to twenty three individual lava flows with a total thickness of ∼ 1200 m. Though this region exposes a nearly complete succession of the trap series basalts, it is a data scarce region. In this paper, we present major and trace element geochemistry of basalts sampled systematically from the bottom to the top of these succession of basaltic lava flows. The lava flows form two distinct formations: (i) the stratigraphically older lower basalts, forming ∼1000 m thick package of inclined to gently dipping, massive to stratified, quartz-amygdaloidal, aphanitic to plagioclase-clinopyroxene-phyric basalt flows, occasionally intercalated with scoriaceous basalts and basaltic agglomerates towards the top; and (ii) the stratigraphically younger upper basalts, forming ∼200 m thick horizontally layered and columnarly jointed aphyric to rarely plagioclase-clinopyroxene- or olivine-phyric massive basalts. The lower basalts are dominantly tholeiitic with higher MgO contents, low incompatible element and light REE concentrations and generally flat middle to heavy REE patterns with low (La/Yb)_N < 4, low Nb/La, and Sr > 100 ppm, suggesting a mixed source of depleted (N-MORB) and metasomatized, enriched (E-MORB) mantle components with higher degree of partial melting at shallow depth. The upper basalts are mostly tholeiitic but culminate at the top in alkaline rocks with particularly low MgO and CaO contents. The upper basalts have relatively higher contents of TiO₂ and FeO, and show more fractionation of the light REEs with higher (La/Yb)_N > 5 and Nb/La, and generally enriched light REE and depleted heavy REE, suggesting their origin from a mixing of OIB-type melts of the upwelling Afar plume and metasomatized, enriched (E-MORB) mantle component at depth within the garnet stability field. There was a general magmatic evolution from low-Ti tholeiitic basalts to high-Ti tholeiitic and alkaline basalts in the northern edge of the Ethiopian flood basalt province.