DID WEATHERING PROMOTE CRUSTAL GROWTH SINCE LATE ARCHAEAN?

The formation of continental masses at the external part of the Earth is one of the most outstanding episodes occurred since the early stages of the formation of our planet. Today, it is broadly accepted that most of the continental masses were produced at the end of the Archaean and the beginning of the Proterozoic, within the period from 3.2 to 2.0 Ga. All of Earth's continents share the same blueprint. Quartz- and K-feldspar-rich rocks of igneous origin (granites s.l.) are the chief constituents of the upper continental crust. At greater depths continents are composed of plagioclase- and pyroxene-rich igneous cumulate rocks. Shallow and deep rocks have isotopic compositions that are intermediate between those of the mantle and those of ancient sedimentary material. The most likely explanation, confirmed by our experimental results, for the origin of these Si- and K-rich magmas is that they were formed by reaction of K-rich metasediments with basaltic melts. Together with K-rich granitoids, assimilation generated pyroxene- and plagioclase-rich solid residues that sank and became the lower continental crust. The Late Archaean cratonization event may have also marked the transition from vertical to lateral crustal growth. Formation of the thick and dense gabbronoritic lower crust erected a density barrier to further invasion of the crust by basaltic melts. The crucial link that makes the formation of this archetypically continental granodiorite–gabbronorite igneous rock association possible is an exogenous cycle of hydrous weathering and sedimentation that fractionates K from Na. This exogenous cycle may take place in a variety of environments, such as continental, fore-arc basins and oceanic trenches. The absence of liquid H2O on the surfaces of other terrestrial planets renders exogenous K/Na fractionation and the generation of continental-scale volumes of K- and Si-rich magmas impossible. We therefore suggest that the continent-like highlands of Venus are likely to be composed predominantly of tonalite-like igneous rocks, resembling the Earth's primordial Archaean continents.