Magmatism in the Gregory Rift, East Africa: Evidence for Melt Generation by a Plume

The volume and composition of volcanic rocks associated withthe Gregory rift are interpreted in the light of inversionsperformed on the REE concentrations of the most magnesian basalts.When the estimated volume of salic rock ({small tilde}88 000km³) is converted into basalt ({small tilde}792 000 km³) thetotal volume of basaltic melt generated over the last 30 Myis at least 924 000 km³, corresponding to an average rate ofmelt production of {small tilde}0•03 km³/yr and an averagemelt thickness of between 7 and 26 km everywhere beneath therift. The mean compositions of the basaltic magmas erupted withinthe rift and on the rift flanks during the Upper Oligocene andMiocene, the Pliocene, and the Quaternary are taken to be representativeof the average compositions of melts produced by fractionalmelting in the asthenospheric mantle. When the REE concentrationsof the observed average compositions are inverted they suggestthat much of the melt was produced in the depth and temperaturerange of the transition from garnet to spinel peridotite. Fora mantle potential temperature of {small tilde}1500C the topof the melting region predicted from the inversions is at {smalltilde}70 km beneath the rift axis and {small tilde}80 km beneaththe rift flanks. Within the rift zone the predicted thicknessof melt increases from the Upper Oligocene and Miocene to thePliocene and is always greater than that predicted for the riftflanks, and the timeaveraged thickness of melt predicted is0/5 km. To generate the observed volume of melt the asthenosphericmantle must continually upwell through the melting region (extendingfrom 70 to 150 km) with a vertical velocity of between 40 and140 mm/yr. The results suggest that, volumetrically and compositionally,magmatic activity associated with the Gregory rift is quantitativelyconsistent with a model of a mantle plume upwelling beneaththinned continental lithosphere. Predictions made by such amodel are in broad agreement with geophysical observations. ^* Present address/reprint requests to: B.P. Exploration, 4/5 Long Walk, Stockley Park, Uxbridge UB11 1BP, UK