THE VOLCANIC STRATIGRAPHY OF AFAR – COLLABORATIONS, LEGACIES, AND NEW FRONTIERS (Invited Presentation)

The story of the volcanic stratigraphy of Afar, Ethiopia is one inextricably linked with the history of rift development and hominid evolution in East Africa. The relentless focus on establishing the temporal markers necessary to constrain the age of fossil remains has yielded an exceptional chronostratigraphic framework for Afar. Explosive, dominantly silicic volcanism has provided a critical geochronological guide in establishing the history of basin development in Afar; however, developing new insights into how these basins are linked to plate-wide extensional processes requires the incorporation of constraints from basaltic magmatism. The temporal correlation between the eruption of the Afar Stratoid Series and the accumulating sediments of the Plio-Pleistocene Hadar and Busidima Formations suggests a link between this magmatic event and a phase of extension in Afar. We present geochemical data from Afar Stratoid Series samples preserved in drill cores collected as part of the Hominin Sites and Paleolakes Drilling Project (HSPSP) in Afar. These data reveal an important missing stratigraphic control in the development of magmatism within the Afar Stratoid Series. We show a broadly coherent magmatic temporal evolution within the borehole that consistent with the development of a common crustal differentiation system. However, there exists a strong correlation between incompatible trace elements and indices of differentiation, observations inconsistent with the development of a mature buffered magmatic system, as is observed in earlier stratiform flood basalts in the region. When considered on a regional basis, the Afar Stratoid Series exhibits far more geochemical heterogeneity than might be anticipated from ‘monotonous’ stratiform basalts erupted over a short interval. These findings necessitate more detailed studies of the Afar Stratoid Series from throughout the region, with a focus on establishing the conditions of melt generation and how such magmas differentiate within the crust. More than 30 years after the first modern geochemical analyses from this area was published, it is becoming apparent that these rocks continue to deliver surprising insights on the development of the East African Rift.