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ABSTRACT

A vast amount of analytical material (more than 8,000 analyses) on the chemical composition of the main rock types occurring in the crystalline shields and sedimentary covers of the Russian and North American platforms has been compared, taking into account the abundance of these rocks. The comparison covers structures as a whole and separately—their sections representing very large stratigraphical complexes (A. Pt1_2, Pt3. Pz. Mz-Cz). Based on generalization of these data, the more important development stages in the composition of the shields and platforms were traced for the last 3.5 billion years and the more general tendencies in their geochemical history were outlined. An almost complete uniformity of the “granite” shell's average composition, a striking similarity of the average composition of the sedimentary cover and similar ways of its development have been proved for different continents. At the same time a discrepancy was established in the average compositions of the rocks of the shields and derivative platform sediments, which by their composition are nearer to basic than to acid rocks. This fact obviously indicates a wider spread of the basic rocks during the earlier stages of the development of the Earth's crust. The more general tendencies in the evolution of the rock composition of shield and platform sediments have been retraced. They outlined a causal relationship, with the development of granitization processes of the primary, more basic material, with the passing of denudation products through numerous cycles of weathering and sedimentation and with the biosphere growth and life development which determined the change of the atmospheric and oceanic compositions as well as the oxidation-reduction conditions on the surface of the Earth and in the formation of sedimentary series.