Proterozoic tectonics are considered to reflect the changes from the small mantle convection cells of the Archaean to the giant cells of the present. Approach to the 'modern' regime is considered to be evident about one billion years ago. Metamorphic patterns from Brazil provide evidence for a substantial reduction in thermal gradients over the Proterozoic period. It is suggested that reduction of radioactive heat production near the Archaean-Proterozoic boundary led to thickening of stable granitoid crust and sweeping together of a very extensive granitoid veneer into super continents. Fundamental to the view presented, is the concept of large scale crust mantle mixing caused by subduction, crustal basement uplift, and weathering processes.