Alkaline magmatism and uplift of continental crust

Abstract

Doming associated with alkaline magmatism can be developed in East Africa and elsewhere on three quite different scales; lithospheric, crustal and sub-volcanic. Each is considered to be the result of separate mantle and crustal processes, and require distinction. Lithospheric domes, e.g. the East African Dome, are seen as extensive areas of continental crust, often 1000 km across, which have been raised by a few kilometres, and which could be the result of a phase change deep in the upper mantle, such as the α → β olivine inversion at 400 km depth. Crustal domes, e.g. the Kenya and Rhine domes, are less extensive, although still often as much as 500 km across, but with similar elevation and arising from the formation of a ‘pillow’ some 100–200 km across, of anomalously low velocity (7·3 km/sec) upper mantle at a depth of about 30–50 km, and usually associated with alkaline basaltic volcanism. Sub-volcanic domes are very restricted in extent, commonly only 2–10 km across, and usually occur under carbonatitic and nephelinitic volcanic structures. They are attributed to the metasomatic introduction of alkalis from these hyperalkaline intrusions into the surrounding basement rocks, before or during the growth of the volcanic structure.