Geochemical constraints on the evolution of the early continental crust

Abstract

The most important process affecting both major and trace-element concentrations in the mantle and crust is melting producing silicate liquids which then migrate. Another process whose effects are becoming more apparent is the transport of elements by CO 2 - and H 2 O-rich fluids. Due to the relatively small amounts of fluids involved they have but little effect on the major-element abundances but may severely affect minor- and trace-element abundances in their source and the material through which they travel. The Archaean crust was a density filter which reduced the possibility of komatiite or high FeO melts with relative densities greater than about 3.0 from reaching the surface. Those melts retained in the lower crust or at the crust-mantle boundary would have enhanced the possibility of melting in the lower crust. The high FeO melts may have included the Archaean equivalents of alkali basalt whose derivatives may form an important component in the Archaean crust. The occurrence of ultramafic to basic to alkaline magmas in some Archaean greenstone belts is an assemblage most typical of modern ocean-island suites in continental environments. The rock types in the assemblage were modified by conditions of higher heat production during the Archaean and thus greater extents of melting and melting at greater depths. If modern ocean-island suites are associated with mantle plumes, which even now may be an important way to transport heat upward from the deeper mantle, it is suggested that during the Archaean mantle plumes were an important factor in the evolution of the continental crust. It appears that the Archaean continental crust was of comparable thickness to that of the present based on geobarometeric data. If the freeboard concept applied then, this would suggest that plate tectonics was also an active process during the Archaean. If so, it is probably no more realistic to assume that all Archaean greenstone belts had a similar tectonic setting than to assume that all modern occurrences of basic rocks have a common tectonic setting.