EARLY CENOZOIC MAGMATISM IN EAST AFRICA: EXPANDING THE EOCENE AMARO AND GAMO BASALTS

Magamatic activity in the East African Large Igneous Province (LIP) is characterized by pulsed eruptive events. Early magmatism in this region is separated into two phases, an older Eocene phase and a younger Oligocene phase, of which the Oligocene flood basalts have attracted considerably more attention. The Eocene phase of magmatic activity in East Africa was centered in southern Ethiopia, with a duration of ca. 10Ma (45-35Ma). This event produced 150,000 km³ of lava, representing about 20% of the entire LIP volume. Despite the importance of this event, it has attracted limited geochemical attention. Prior studies divided the magmatism in the province into a lower Amaro and upper Gamo lava series; however, these studies were of limited geographic coverage, and consisted of three sections only located along the margin of the Main Ethiopian Rift. Here we expand the geographic distribution of geochemical data of the older magmatic event. We present geochemical data for the Eocene basalts of the Omo region ~150 km west in order to better understand their relationship to the contemporaneous volcanics of the Amaro and Gamo basalts. Major and trace element data for the Eocene basalts were obtained from a suite of samples collected during a joint expedition of the Ethiopian Government and the Canadian Geological Survey that mapped Eocene lavas in southern Ethiopia. In order to accommodate the small volume of remaining samples, data were collected by laser ablation ICP-MS as a surface scan raster. Compositions for each sample are hereby described as an average of each line within the raster. We find that the composition of the Omo Eocene basalts are identical to the Gamo basalts exposed in the east along the rift margin. These data suggest that the Gamo basalts are more widespread than previously known. We explore the potential modes of origin for the Gamo basalts and compare these models with those of the younger Oligocene volcanism. We then compare the pulsed magmatic events of east Africa with those of the Keweenaw LIP in N. America to examine the impact of heterogenous flux from mantle thermochemical anomalies.