Primitive Archaean element distribution patterns: Chemical evidence and geotectonic significance

Abstract

Major and trace element data from low-grade isochemically metamorphosed Archaean igneous suites in Transvaal, Rhodesia, Western Australia and Canada show consistent variations from those of corresponding younger igneous suites. The metabasalts of Archaean basalt ultrabasic-basic assemblages conform to recent oceanic tholeiites with respect to K, Na/K, Sr, Zr and Fe 3+/Fe 2+, but have considerably lower Al and Ti, and tend to have higher Mn, Ni, Cr, Co, Rb and Fe/Fe+Mg (total iron as FeO) than recent equivalents. Metabasalts of Archaean calc-alkaline volcanic series are more differentiated, and have K, Ba, Sr, Zr and Fe/Fe+Mg values higher than those of metabasalts of ultrabasic-basic assemblages and recent oceanic tholeiites. The evidence suggests a derivation of the early basalts through partial melting of the mantle at depths shallower than 15 km. Possibly, the Archaean mantle had higher levels of siderophile and certain transition elements than the present mantle; this may have been related to a lesser degree of development of the core. The earliest Archaean granites have high Na/K, Ni and Cr levels, and were probably derived from partial melting of the oceanic crust. Granitic cycles showing a progressive decrease of the Na/K and K/Rb ratios reflect an increase in the degree of differentiation and a thickening crust. The pattern and evolution of Archaean volcanic-sedimentary troughs were controlled by early nuclei of sodic granites and granodiorites emplaced in the oceanic crust; these greenstone belts are therefore not analogous to Alpine or island-arc systems. The stabilization of these belts through deformation and metamorphism, resulted in the aggregation of the early granitic nuclei into shields; subsequently high-grade belts developed along shield-ocean transition zones.