Evolution, growth and stabilization of the Precambrian lithosphere

Abstract

Modern plate tectonic processes do not appear to be very efficient in continent-building since they have contributed little to the total volume of the continental lithosphere over the last 900 Ma. The much higher crust-formation rate in the Archaean cannot be ascribed to faster plate motion or frequent collision of many small plates during that time but appears to be related to extensive melting in the upper mantle and underplating of dense magmas that could not penetrate the overlying continental crust. These magmas provided a reservoir for most of the typical Archaean magmatic rock associations found in greenstone-grainite-gneiss terrains. It is argued that the predominantly biomodal volcanic suites of Archaean greenstone belts and the genetically related tonalite-trondhjemite-granite plutons did not originate through growth on intra-oceanic island arcs or in Andean-type arcs and marginal basins but in intracontinental rift settings. Magmatic underplating led to non-linear growth and differentiation of Archaean continental crust and is also seen as the prime cause for early lithospheric thickening and stabilization, allowing large intracontinental basins to form since 3.5 Ga ago. The end-Archaean global crust-forming event generated the first large cratons whose subsequent stability may be the result of subcrustal lithospheric thickening through melt extraction and depletion, underplating and carbonic granulite metamorphism in the lower crust. Magmatic underplating continued to remain an important process in the generation of volcanic sequences in the Lower Proterozoic intracontinental basins although there appears to be first evidence for local horizontal crustal accretion in magmatic arcs at about 2 Ga ago. Most of the widespread Proterozoic mobile belts lack typical Wilson-cycle signatures and are interpreted by a model involving extensive crustal thinning, subcrustal lithosphere delamination and crust restacking during basin closure and orogeny. New and fertile, but initially mechanically weak lithosphere is accreted to the crust from below during this process. By the end of the Precambrian horizontal accretion of juvenile continental crust became widespread, but vertical accretion may still be important in Phanerozoic crust-forming events. It is concluded that vertical rather than horizontal growth of lithosphere has dominated global evolution during the first 3.5 Ga of earth's history and that this mechanism also accounts for the observed magmatic rock assemblages as well as for lithospheric stabilization and cratonization of continental crust.