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

The volume and composition of volcanic rocks associated with the Gregory rift are interpreted in the light of inversions performed on the REE concentrations of the most magnesian basalts. When the estimated volume of salic rock (˜88 000 km3) is converted into basalt (˜792 000 km3) the total volume of basaltic melt generated over the last 30 My is at least 924 000 km3, corresponding to an average rate of melt production of ˜0⋅03 km3/yr and an average melt thickness of between 7 and 26 km everywhere beneath the rift. The mean compositions of the basaltic magmas erupted within the rift and on the rift flanks during the Upper Oligocene and Miocene, the Pliocene, and the Quaternary are taken to be representative of the average compositions of melts produced by fractional melting in the asthenospheric mantle. When the REE concentrations of the observed average compositions are inverted they suggest that much of the melt was produced in the depth and temperature range of the transition from garnet to spinel peridotite. For a mantle potential temperature of ˜1500°C the top of the melting region predicted from the inversions is at ˜70 km beneath the rift axis and ˜80 km beneath the rift flanks. Within the rift zone the predicted thickness of melt increases from the Upper Oligocene and Miocene to the Pliocene and is always greater than that predicted for the rift flanks, and the timeaveraged thickness of melt predicted is 0/5 km. To generate the observed volume of melt the asthenospheric mantle must continually upwell through the melting region (extending from 70 to 150 km) with a vertical velocity of between 40 and 140 mm/yr. The results suggest that, volumetrically and compositionally, magmatic activity associated with the Gregory rift is quantitatively consistent with a model of a mantle plume upwelling beneath thinned continental lithosphere. Predictions made by such a model are in broad agreement with geophysical observations.