“Wet” low angle subduction: a possible mechanism below the Tanzania craton 2 Ga ago

Abstract

The hypothesis that much of the lithosphere of the Archaean Tanzania Craton was hydrated, by the dehydration of a buoyant subduction 2 Ga ago is presented in this study. Buoyant subduction is a potential mechanism for thermal erosion and metasomatism of extensive regions of the cold overlying continental lithosphere. This hypothesis could explain why the Tanzania Craton forms an undeformed island within the intensely deformed mobile belts. Furthermore, it would explain the formation of the eclogite and lherzolite bearing kimberlites within the Tanzania Craton far away from the trench. A buoyant, slow subduction is required because this would provide sufficient cooling from the overlying cratonic lithosphere and therefore the dipping slab could retain hydrous minerals such as antigorite in hydrated aureoles in peridotites. To test this hypothesis, the release of water during prograde metamorphism of a flat-subducting plate was modeled. It is shown that water can be transported ∼800 km laterally, inboard of the trench, which is close to the north-south extension of the Archaean Tanzania Craton.