A top to bottom stratigraphic investigation of two transects through the Ethiopian flood basalt province
Thursday, 18 December 2014
The most striking feature of the Ethiopian-Arabian large igneous province is the thick cap of flood basalts that dominate the northwest and southeast Ethiopian Plateaus. These flood basalts are interpreted to reflect the interaction of the Afar plume head with the continental lithosphere, and have been geochemically divided into three groups: low- Ti (LT) which is spatially restricted to the western half of the NW Ethiopian plateau, furthest from the hypothesized center of the Afar plume head; and two High-Ti groups (HT1/2), which occur on the eastern half of the plateau, closest to the plume head. While the source of HT1/2 basalts have been geochemically linked to the Afar plume with contributions from the depleted mantle and African lithosphere, the origin of the roughly contemporaneous LT basalts remains unclear. Here we present preliminary stratigraphy of two densely sampled, parallel E-W transects along escarpments that dissect the LT basalts, providing a flow-by-flow glimpse at magma evolution. Our study includes 326 samples collected between 542-2175 masl, incorporating the entire flood basalt event from the Mesozoic basement to Miocene capping lavas. Flows are extensive and flat lying, varying from aphyric to olivine- and/or plagioclase-phyric with two distinct horizons dominated by plagioclase megacrysts. We observe correlations between flows in the north and south transect (~70 km distance) based on contact elevation, flow thickness, modal mineralogy and paleosols. Our initial inspection of petrography and whole-rock geochemistry of the northern transect has identified changes in modal mineralogy, phenocryst abundance (0-30%) with rare ophitic textures, and MgO (4-10 wt. %) that occur on 50-200 m intervals, and are suggestive of magma storage, differentiation and potentially recharge events. Magma-crystal disequilibrium is often observed in the form of zoning, sieve texture, and glomercrysts (plagioclase or clinopyroxene) and further illustrates the subtle, but complex nature of flood basalt magma reservoirs through time. This work adds petrographic detail of individual flows in concert with geochemical data that can be used for correlation in future studies of Ethiopian flood basalts.