CROSSING THE GAP: LINKING THE EOCENE AND OLIGOCENE EAST AFRICAN RIFT VOLCANICS

The Ethiopian Large Igneous Province (LIP) was produced by the interaction of thermo-chemical plume(s) and extensional plate-tectonic processes. Geochemical and petrological studies have revealed two pulses of magmatism, the first in southern Ethiopia during the Eocene, and the second in northwestern Ethiopia during the Oligocene. The Makonnen basalts in S. Ethiopia, which exhibit a stratigraphic thickness of 700 m, are temporally bound by Eocene and Oligocene volcanics. The spatial separation between these events has complicated efforts to understand the relationship between the pulses. Despite the important link that these lavas represent, the relationship between the Makonnen, Eocene, and Oligocene events remains unknown. Here we present a new geochemical dataset from the Makonnen type sections, using sample fragments collected by a joint expedition of the Ethiopian Government and the Canadian Geological Survey in the 1970s. Due to the available sizes of individual samples, they were prepared using a novel technique whereby both major and trace element data were collected by laser ablation-ICPMS. A 100 wt. % sum of oxides was assumed and used as the internal standard. Samples were mounted, polished, and then ablated in a raster pattern. Data quality control procedures included evaluation of Z-scores and examination of the ten individual surface scans performed for each sample. Preliminary results demonstrate that the Makonnen basalts differ from the Oligocene magma types. Instead, these basalts closely resemble those of the Eocene Gamo lava pulse. The implications of this observation are that the source of the Eocene basaltic pulse is more extensive both spatially and temporally. We discuss these results in the context of the dual pulse plume model and the northward movement of the African plate.