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Plume-Lithosphere Interactions in the Generation of the Basalts of the Kenya Rift, East Africa |
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| Authors: | MACDONALD R; ROGERS N W; FITTON J G; BLACK S; SMITH M |
| Institution: | 1ENVIRONMENTAL SCIENCE DIVISION, IENS, LANCASTER UNIVERSITY, LANCASTER LA1 4YQ, UK 2DEPARTMENT OF EARTH SCIENCES, THE OPEN UNIVERSITY, MILTON KEYNES MK7 6AA, UK 3DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF EDINBURGH, WEST MAINS ROAD, EDINBURGH EH9 3JW, UK 4PRIS, UNIVERSITY OF READING, WHITEKNIGHTS, READING RG6 2AB, UK 5BRITISH GEOLOGICAL SURVEY, MURCHISON HOUSE, WEST MAINS ROAD, EDINBURGH EH9 3LA, UK |
| Abstract: | Major and trace element and Sr–Nd–Pb isotopic datafor mafic volcanic rocks are used to assess the number of mantleplumes contributing to the Tertiary–Holocene magmatismof the Kenya Rift Valley, current estimates of which vary fromnone to three. Rocks ranging in composition from nepheliniteto hypersthene-normative basalt have been sampled from threelithospheric zones: the Tanzanian craton, the craton marginreworked during the late Proterozoic, and the Mozambique mobilebelt. The magmas are interpreted as the products of variabledegrees of partial melting within the spinel–garnet peridotitetransition zone. Trace element and isotopic compositions fromall three zones are broadly similar to those of oceanic islandbasalts, but there is considerable compositional variation,which is related to a strong overprint from the lithosphereon plume-derived melts. Sr and Nd isotopic ratios provide theonly clear distinction between magmatic rocks from the threelithospheric domains. Within each setting, mafic magmatism hastended to become less silica undersaturated with time, and atany one locality magmatism has migrated towards the centre ofthe rift. Magmas may have formed as a result of the infiltrationof plume-derived melts into the base of the lithosphere. Theextent of interaction of inferred plume melts with the lithospherehas not varied systematically in time or space. The plume componentappears to be similar to the source of oceanic island basalts. KEY WORDS: Kenya Rift Valley; mantle plumes; geochemistry; metasomatism |
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