The Cenozoic magmatism of East Africa: Part III – Rifting of the craton

Abstract

Magmatic activity in the Tanzania Craton-influenced regions of the East African Rift System (EARS) exhibits clear evidence of derivation from a variety of metasomatic sources. These metasomatic sources, which may span a range of ages, have likely been preserved within the lithospheric mantle. The influence of these metasomes on the composition of the rift magmas represents the single greatest heterogeneity in the composition of erupted lavas within the EARS. The earliest widespread magmatic event in the craton-influenced regions of the EARS south of Lake Turkana is the Samburu Phase (ca. 20–17 Ma), which manifests as alkaline volcanics derived from the lithospheric mantle. This phase represents the initial destabilization of the continental lithosphere as a precursor of rift development. The Flood Phonolite Phase (ca. 16–12 Ma) was most pronounced in regions south of those impacted by the prior Samburu Phase, and likely represent the continued southward expansion of lithospheric destabilization. The Mid-Miocene Resurgence Phase (ca. 12–9 Ma) is poorly represented in the Southern Kenya Rift, though magmatic activity is recorded in the Western Branch of the EARS at Kivu during this time. In contrast, the Early Rift Development Phase (ca. 9–4 Ma) is particularly well-represented in the Kenya Rift. Large-scale magmatic events include: commencement of activity in the Northern Tanzania Divergence (NTD) and Mt. Kenya, Rungwe Volcanic Province, and the Southern Kenya Rift. The Stratoid Phase (ca. 4 Ma – 0.5 Ma) commences with significant magmatic events at ca. 3 to 2.5 Ma within the Southern Kenya Rift and NTD. This phase manifests as basalts and trachytes that flood the newly formed graben in the Southern Kenya Rift. This phase also correlates with the main phase of magmatism in the NTD, the commencement of volcanism at Virunga, and the intermediate shield phase in the Rungwe Volcanic Province. The Axial Phase (0.5 Ma to present) is defined by narrow axial grabens in the Kenya Rift with central volcanoes and interposing basaltic cinder cones. In the less mature regions of the EARS, alkaline volcanoes dominate. Three distinct pulses of magmatism and extension initiated in Turkana commencing at ca. 23 Ma, 12 Ma, and 4 Ma; the consistent temporal lag in magmatic activity in the Southern EARS following each pulse reveals the southward expansion of these extensional events. Lavas at any given location within the rift exhibit a consistent temporal sequence characterized by a shift from Type II lavas (lithospheric mantle derived) to Type IV lavas (hybrid lithospheric mantle – sub-lithosphere derived) to Type III lavas (sub-lithosphere derived), requiring a progressive change in the reservoirs contributing to rift magmatism from lithospheric to sub-lithospheric. When examined in aggregate, these data paint a picture of progressive rift development and attendant lithospheric thinning, mediated by pulses of extension originating in Turkana.