Magmatism of the Kenya Rift Valley: a review

Abstract

Tertiary±Recent magmatism in the Kenya Rift Valley was initiated c. 35 Ma, in the northern part of Kenya. Initiation of magmatism then migrated southwards, reaching northern Tanzania by 5±8 Ma. This progression was accompanied by a change in the nature of the lithosphere, from rocks of the Panafrican Mozambique mobile belt through reworked craton margin to rigid, Archaean craton. Magma volumes and the geochemistry of ma®c volcanic rocks indicate that magmatism has resulted from the interaction with the lithosphere of melts and/or¯uids from one or more mantle plumes. Whilst the plume(s) may have been characterised by an ocean island basalttype component, the chemical signature of this component has everywhere been heavily overprinted by heterogeneous lithospheric mantle. Primary ma®c melts have fractionated over a wide range of crustal pressures to generate suites resulting in trachytic (silica-saturated and -undersaturated) and phonolitic residua. Various Neogene trachytic and phonolitic¯ood sequences may alternatively have resulted from volatile-induced partial melting of underplated ma®c rocks. High-level partial melting has generated peralkaline rhyolites in the south±central rift. Kenyan magmatism may, at some future stage, show an increasing plume signature, perhaps associated ultimately with continental break-up. KEY WORDS: Plume±lithosphere interactionsThe Red Sea and the Gulf of Aden are two arms of a postulated triple junction centred on the Afar region (Fig. 1). Oceanic crust began to form under these arms c. 10 Ma. The third arm, the East African Rift System (EARS), extends southwards through Ethiopia, Kenya and Tanzania into Mozambique. Whilst lithospheric extension is occurring beneath the EARS, the rift has not developed to the stage of ocean crust formation. The EARS has come to be seen, therefore, as a prime location in which to investigate the early stages of continental break-up.This review focuses on the magmatism of the Kenya Rift section, used here to indicate that section of the EARS extending from northern Kenya to northern Tanzania. The primary aim is to demonstrate the complex polybaric evolutionary history of the magmatism, starting with the postulated plume-induced generation of primary magmas by variable degrees of partial melting of heterogeneous, metasomatised lithosphere, through to generation of salic magmas by assimilation±crystal fractionation processes over a range of crustal pressures and by partial melting of crustal protoliths, also over a range of pressures.

Mount Kenya volcanic activity and the Late Cenozoic landscape reorganisation in the upper Tana fluvial system

“…This observation indicates that initiation of the Mt Kenya volcanism and phonolitic eruptions had already started in the Miocene but the main volcanic body as it is today was not yet in existence. The age of the Gachoka phonolite is too young to relate this phonolite to the Miocene flood phonolites that lasted from 16 to 8 Ma (Macdonald, 2003).…”

The Lava sequence of the East African Rift escarpment in the Oldoinyo Lengai – Lake Natron sector, Tanzania

“…5 in Macdonald, 2003). Our data lie in the range, which points to low degrees of melting in the order of a few percent.…”

“…Differing in many ways from the basanites, the preceding alkali olivine basalt of the Waterfall Sequence has a low Mg# of 57 and is least enriched in trace elements. The Ce/Y and Zr/Nb relationship in the approach of Hardarson and Fitton (1991) and Macdonald (2003) could suggest a higher degree of melting at shallower depths than for the basanite sequence.…”

“…Macdonald et al (2001Macdonald et al ( , 2003, Furman et al (2004), Keller et al (2006) to assess the possible asthenosphere -lithosphere interaction during melt generation in the EAR. Zr/Hf of the Waterfall sequence is 39, very close to the OIB value of McDonough and Sun (1995) and similar to many continental alkali basalts.…”

Edifice growth and collapse of the Pliocene Mt. Kenya: Evidence of large scale debris avalanches on a high altitude glaciated volcano

“…Kenya has been published already in the 60's by Baker (1967). Since then, the main geological research emphasis has shifted towards the Kenya Rift valley (Gregory Rift System) and its closer surroundings (Smith, 1994;MacDonald, 2003).…”