The Transition to Tectonic Control of Magmatism in the East African Rift System—The 20 Ma Samburu Event

Abstract The Samburu Series magmas of the East African Rift System are a key component of the Early Miocene magmatic resurgence, marking the initiation of plate‐scale volcanic events associated with extension, distinct from the prior continental flood basalt episodes. We present the first flow‐by‐flow petro‐stratigraphic description of the Samburu Series magmas, preserved in a ∼400 m section at Peleketch (Turkana, Kenya). Using new major element geochemistry, petrology, and 40 Ar/ 39 Ar geochronology we reveal three distinct pulses of magmatism: an initial phase with mafic lithologies and relatively high flux, a middle phase with evolved compositions and continuing high flux, and a final waning interval marked by an increase in alkalinity and precipitous decrease in magma flux. Mantle melting models indicate a plate thickness of ∼90 km during the Samburu Series event and elevated mantle potential temperatures due to the influence of a mantle plume. While active upwelling from the plume contributed to melt generation, extension was critical in generating the melt needed for the Samburu Series magmas at Peleketch. The 21.2 to 19.8 Ma Samburu event marks the end of major volcanism within the Lotikipi Basin, coinciding with the cessation of extension and the commencement of the thermal sag component, highlighting the critical role of plate thinning in magma generation during this period.