Plume head - cratonic blocks interactions and gemstones in east Africa
Résumé
Geological and metallogenical data in east Africa have been used to study plume head-continental lithosphere interactions. Between Tanzania and Madagascar, the «Gemstone belt of east Africa» located in the Neoproterozoic Mozambique belt, includes ruby, sapphire, garnet, tourmaline and several other gemstones that require high pressures and high temperatures for their formation. Ultrahigh temperature metamorphism was also evidenced at both north and south edges of the Archean Tanzanian craton, while eclogites were found at lateral edges of the craton. Diamonds, which require more extreme P-T conditions, are found within cratonic areas. West of the craton, numerous intrusion-related tin deposits are present. Altogether, these observations which span more than 2 Ga, tend to suggest that successive mantle plume heads beneath Tanzania followed a similar history, promoting partial melting on one side of the craton and deep burying of crustal material on the other. Numerical modelling of plume head-cratonic blocks interactions was performed with a tectonically realistic formulation for the cratonic lithosphere. Topographic signatures reveal elevated plateaus above cratons while undulations are observed between cratons, as observed in east Africa. Two different types of lithospheric downwellings (the «drip-like» and the «slab-like» instabilities), show P-T paths that may explain how ultrahigh temperature metamorphism occurs. The results tend to confirm that channelized flow of plume head beneath cratons would favour crustal melting on one side and lithospheric delamination on the other. Focusing of shear stress at cratonic margins enables fluid circulation and promotes metasomatism. While plume edges escape to thinner lithosphere, material from the plume centre is stored beneath the craton until the subsequent mantle plume following a similar mantle path interacts again with the craton.