A Discussion on the evolution of the Precambrian crust - The granulite facies, partial melting and the Archaean crust

Abstract

As chairman of a session of the meeting I would like to add a few comments concerning some of the problems discussed. During the discussions certain important conclusions have appeared. The detailed descriptions presented of the oldest known samples of the Earth indicate that at that time the crust had already a remarkably similar structure and chemistry to that of more modern times. It has also been made clear that rocks of the granulite facies make a massive contribution to the base of the ancient crust and logically could make a large contribution to modern regions under batholith zones of mobile belts, where the crust has been thickened by tectonic events. It also seems possible that the mantle processes controlling tectonics in the Archaean, while having some features in common with the present regime, were unique. Most models of the Earth’s thermal history (Birch 1965) suggest a very hot initial stage and it is difficult to see how the general geochemical equilibrium state of the Earth could be achieved without a very large degree of melting at some period of Earth history. If a large molten fraction is assumed, it would be difficult to preclude very complex convective structures. Crust will appear when surface temperatures approach 800 °C which could be at a very early stage indeed. When liquid water appears on the surface at 100 °C or less, a crust a few kilometres thick would rapidly form. Some present models (Finale 1971) indicate that formation of a hydrosphere might be an early event and this process itself would accelerate initial cooling. It might well be expected that the earliest semi-stable crustal fragments would be granitic. Such materials would be capable of floating on basalt liquids while solid basalt would be rapidly engulfed. This crust would also concentrate radioactive species (see Heier, this volume). When an aqueous phase is present, weathering would be intense partly on account of the supply of acids from intense volcanism and also from the supply of glassy volcanic debris. Early geochemical separation processes associated with a hydrosphere would thus be intense and synchronous with thicker crust formation. The initially radioactive crust could well have been associated with thermal gradients of the order of 100 °C km-1 and could hardly be thicker than 10 km before belting would occur at the base with the occurrence of igneous overturn of the material.