THE ENIGMATIC ORIGIN OF THE GIANT SEMI-CIRCULAR LAKE VICTORIA DYKE SWARM

Aeromagnetic data on the NW and SW sides of Lake Victoria (East Africa) clearly define a 100 km-wide and remarkably semi-circular structure across Uganda and Tanzania, with an outer diameter of 500 km. These anomalies are evidently caused by relatively fresh dolerites, belonging to the Lake Victoria Dyke Swarm (LVDS). Two Sm-Nd ages of ~1.37 Ga relate this arcuate swarm to an inner region, across Rwanda and Burundi, of parallel striking Mesoproterozoic sediments that host coeval, Ni-rich (ultra-)mafic layered intrusions, sills, as well as granitoids; i.e., a large igneous province in the Greater Congo Craton. Compositional signatures of the surrounding LVDS are compatible with a single evolved magma chamber source, initially derived from a partially melted sub-continental lithospheric mantle, whereas enriched signatures of the inner intrusions are indicative of mantle plume involvement (Mäkitie et al., 2014, J. Afr. Earth Sci. 97, 273-296).

Other giant circumferential swarms on Earth, as well as Venus and Mars, have tentatively been related to sub-lithospheric mantle plumes (Ernst et al., 1998, 29^th Lunar & Plan. Sci. Conf., Abstract 1021), but it is uncertain how such settings translate into the observed surface swarm patterns. Modelling of geophysical profiles across the LVDS indicates that its dykes dip moderately inwards and thus resemble giant cone sheets (Ruotoistenmäki, 2014, J. Afr. Earth Sci. 93, 23-41). However, simple enlargement of a classical cone sheet swarm crowning a mildly oblate sub-volcanic magma chamber requires an unrealistically large magma volume and depth. A highly oblate and shallow crustal magma chamber needs to have an equally wide and circumferential margin as the dyke swarm, where central caldera subsidence may squeeze magmas into a tube-like periphery. As an alternative to stress fields induced by crustal magma chambers, a down-warped crustal flexure allows the lateral injections of sills from an oblate, yet reasonably small magma chamber to bend upward into a more distal giant ‘cone’ sheet swarm within the encircling flexural ridge. This latter model may also be superimposed on the crest of a wider dome-like flexure that allows the lateral injection of giant radiating dykes, so far not observed together with the LVDS but often in conjunction with other circumferential swarms.