CRUSTAL EVOLUTION IN THE EARLY PRECAMBRIAN

Recently published models present highly conflicting views of continental evolution. At one extreme, it is envisaged that continental crust was fully developed by ∼ 4000 Myr ago and quantitatively recycled through the mantle throughout geological time without net change of crustal mass (continental steady-state hypothesis). A fundamentally contrasted view is that the earliest continental crust (∼ 3700 Myr) occurred in small, localised patches, and that once juvenile continental crust is cratonised it cannot be returned to the mantle in bulk. This corresponds to the concept of irreversible chemical differentiation of the mantle with accompanying growth of continental crust through geological time (continental growth hypothesis). In between these extremes, it has been suggested that some relatively minor continental constituents may be selectively recycled into the mantle. This does not involve wholesale recycling of continental crust, and still allows continental growth. Whilst isotopic and trace element modelling imposes important constraints it is not absolutely definitive in deciding between contrasted evolutionary models. Additional criteria for continental evolution must be sought from assorted geological, geophysical, geochemical and geochronological data. As one example, it is important to determine the age and regional extent of exposed early Precambrian continental crust, and also whether exposed crust is underlain by significantly older crust at depth.