Geochemical Stratigraphy of a Continental Flood Basalt - the Peleketch section of the Turkana Depression (East African Rift System)

The Peleketch Section is a 300-400 meter thick series of stacked basaltic lava flows located within the Turkana Depression in northwestern Kenya. Continental flood basalts, like those preserved at Peleketch, represent an important precursor to the development of the East African Rift System. However, little attention has been given to the sequences in northwestern Kenya in comparison to detailed studies of the NW Ethiopian plateau. An important tool in examining these lavas is the analysis of the bulk composition of the flows to the north. Here we present X-ray fluorescence data on major elements from selected Peleketch lavas. We compare these data to a previously constructed petrology-based stratigraphy of the flows to create a unified model of magma chamber evolution. The geochemical data provides insight on crystallization conditions that were based upon petrographic observations. Initial observations show that the sequence can be divided into three sections: a lower section with compositional variability, a middle section showing geochemical consistency, and an upper section exhibiting more mafic compositions with high TiO2 (wt. %), with a silicic break between the middle and upper sections. These changes in geochemistry can be indicative of temporal breaks within the sequence, which in turn could represent different magmatic differentiation processes occurring in the magma chamber. These visible differences in the data are used to interpret what specific processes occurred in the magmatic system. The chemical heterogeneity of the first pulse of magma suggests low magmatic flux as the system initiates, whereas the geochemical consistency of the second pulse suggests a steady state of frequent recharge (high flux) that occurs with fractional crystallization. The silicic break suggests that a decrease in magma recharge occurred in the system before a final, resurgent pulse of mafic magma. These initial geochemical observations are in agreement with the textural changes observed in the stratigraphy. The variability in magmatism in the Peleketch section is broadly consistent with continental flood basalt sequence as observed elsewhere in the EARS e.g., the NW Ethiopian Plateau.