Metasomatism, Melting, and Magmatism: Insights into the evolution of continental rifting in East Africa
Abstract
The role of melt in facilitating rifting is often considered in the context of a simple conceptual model - as the continental lithosphere thins, decompression melt from the upper mantle intrudes the attenuated lithosphere to form oceanic crust. However, the occurrence of magmatism at earlier stages of the rifting process points to additional complexity in lithosphere-melt interaction. Elevated mantle potential temperatures and 3He/4He throughout East Africa require a contribution from the deep mantle. A model can therefore be developed wherein deep, small-degree melts of hot mantle develop beneath an initially thick lithosphere. As rifting progresses, there is a transition to larger-degree shallow melts as the lithosphere thins and this hot mantle decompresses further. Such a model superficially addresses the common observation during rift evolution of declining magma alkalinity and fractionation of middle to heavy REE patterns. However, the contemporaneous occurrence of both these endmember magmas within the same edifice precludes the lithospheric-thinning mediated decompression melting model as sole melt generation process within the rift. Isotopic arrays formed by rift lavas show the influence of enriched metasomatic domains located within the continental lithospheric mantle. These domains, which form by the interaction of melt with the lithospheric mantle over its lifetime, may contain carbonate, phlogopite, amphibole and other unusual metasomatic phases. Melt generated from these domains will be highly alkaline, silica undersaturated, and exhibit middle to heavy REE fractionation. It is now evident that initial magmatism within a rift is dominated by melts of lithospheric domains, which eventually transition to sub-lithospheric reservoirs. However, this transition does not result in successive melt events becoming more sub-lithospheric with time. Pulses of magmatism associated with extension may result in greater sub-lithospheric contributions to magmatism, while during tectonically more-quiescent periods, lithospheric metasomes may predominate. These results show a much closer relationship between melt, magmatism, and rifting than would be anticipated in a simple decompression model and emphasize the role of the continental lithosphere in melt generation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMT046...03R
- Keywords:
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- 8105 Continental margins: divergent;
- TECTONOPHYSICS;
- 8109 Continental tectonics: extensional;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS