THE MID-PROTEROZOIC RAPAKIVI GRANITES WITHIN THE CONTEXT OF THE COLUMBIA SUPERCONTINENT

Mid-Proterozoic rapakivi granites and associated suite (anorthosite, mangerite and charnockite) escape the global rules that control usual granitic magma formation. They differ from common magmatism by about forty points. Rapakivi granites develop without a global orogenic context in the supercontinent Columbia. Intrusions extend from the East European shield to western US, through Fennoscandia, Greenland and Labrador. Other occurrences in the Amazonian shield, Australia or South China, are less documented. They show no definite trend in age or chemistry that would explain large scale (mantle plume) effects. A mantle upwelling of material is contradictory with the re-assembly of a supercontinent as it occurred before this magmatic episode (1.9-1.8 Ga). A model is suggested that takes into account the supercontinent re-assembly, determining a downwelling flow in the mantle, hence anchoring the continent above it. Contrasting with material flow, heat is still delivered to the base of the continental lithosphere, and is focused toward the juvenile suture zone. The base of the crust must reach 1200-1300°C before producing anorthosite magmas. Under such high temperature gradient lasting over a long time, magmas are transferred toward the upper crust, giving the thin (5km) and square shape (100 km) the intrusions presently have. Heat delivery is essentially conductive, leading to the specific time spans for intrusions. The presence of the supercontinent, immobile over a descending cell (poloidal mode of convection) develops a tangential force (toroidal mode of convection) that partly splits the continent through strike-slip deformation due to plate rotation. It occurs progressively between 1.57 and 1.3 Ga, starting from Fennoscandia, and then passing to Amazonia, western US and Labrador in clockwise sense. The associated rotation induces sinistral shear manifested by small-scale shear zones and the orientation of late magmatic facies in each province. The Proterozoic magmatism seems unique since it requires a supercontinent with a zone of juvenile crust surrounded by older cratons. The actual Moho still presents remnants of this process, showing bumpy undulations of 22 km in amplitude on a distance of 200 km.