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Magma Generation and Break-Up Processes |
Department of Geology, University of Bristol, Bristol BS8 1RJ, UK
Except for the northwestern and southern extremities, the African plate has been anorogenic throughout the Phanerozoic, yet the interior has been subject to spectacular episodes of uplift, rifting and magmatism from Mesozoic to present. Interior Africa thus offers an excellent test of intraplate dynamics as prototypes for the initiation of continental break-up. Crucial to the test are the spatial and temporal controls on the activity. Spatially, there is unequivocal evidence that the disposition of the activity is controlled by pre-existing lithosphere structures. From Cretaceous to Recent times, alkaline magmatism (including carbonatites and kimberlites) has been the norm, typically in uplifted and rifted areas. Four major periods of activity emerge in the igneous record. The classic Early Cretaceous rifting and magmatism of central and southern Africa (135–110 Ma) is only one part of an igneous episode widely recorded across the continent. A Late Cretaceous episode (90–80 Ma), best known for kimberlite activity in the old cratons, is also widely registered elsewhere. Two major Tertiary episodes characterize the modern East African Rift zone, an initial Eocene-Oligocene (c. 40 Ma) and a Miocene-Recent episode (starting around 23 Ma), and corresponding peaks are recorded in many other areas, especially in the northern part of the continent. In most areas there is evidence for more than one episode, indicating that since the Cretaceous the same sites have been subject to repeated alkaline magmatism. Contemporaneous plate-wide activity, repetition at individual sites, and localization by existing anisotropies in the plate, are all indicative of responses to external events capable of affecting the whole plate. Such events would include continental collision, stages in Gondwana break-up, and marked changes in global plate motions. Spatial and temporal relations thus show that rifting and alkaline magmatism are permissive, and may themselves be symptoms of more fundamental processes that cause the break-up of continents. If so, pre-Cretaceous episodes of intra-plate alkaline magmatism may offer valuable time-markers, recording similar events earlier in the Earth’s history. One possible example may be the global episode of kimberlite activity recorded around 1200 Ma.
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