EVOLUTION OF THE MINERAL COMPOSITION OF PELITIC AND PSAMMITIC ROCKS IN THE GEOLOGIC HISTORY OF CONTINENTS

Abstract

The evolution of the average composition of shales and sandstones during the geologic history of continents (from the Early Archean to Mesozoic-Cenozoic) was evaluated using trends of changes in main mineral phases in connection with parallel changes in the average mineral composition of igneous rocks (komatiites, basalts, andesites, felsic extrusive rocks, and granites) on the basis of a great body of our own and published materials. The normative mineral composition of sedimentary rocks were calculated by the MINLITH program. The comparison of trends for sedimentary and igneous rocks showed that they changed sympathetically. The abundance of magnesium-bearing mineral phases decreased and those of potassium-bearing minerals and, in part, quartz increased with time (from the Early Archean to the Phanerozoic). Successive changes in the average abundance of major minerals of shales and graywackes correlated with changes in the mineral composition of the main types of igneous rocks were related primarily to variations in the proportions and composition (in accordance with the change with time in the prevailing tectonic setting of rock formation) of the basic, silicic, and sedimentary rocks that made up erosion areas. Based on the distribution of igneous and sedimentary rocks and their mineral composition for various stages of the history of continents, the average content of major mineral phases in the rock complex of erosion areas were calculated for greenstone belts and the upper continental crust as a whole. Regular variations during geologic time in the average mineral composition of pelitic rocks correspond in most cases to the evolutionary trends of the composition of greenstone belt provenances. Changes in graywackes are more subtle and are usually correlated with changes in the mineralogy of the entire upper continental crust dominated by the granite-gneiss complex. It is supposed that the following factors can explain the different sources of pelitic and psammitic rocks: (a) owing to the difference in the resistance to weathering, pelitic rocks were formed mainly at the expense of the rocks of greenstone belts, whereas psammitic rocks, at the expense of granite-gneiss complexes, and (b) because of the different destruction rates of greenstone belts and gneiss complexes, their contributions to the accumulation of Early Archean pelites were different from those of modern clays. Ancient clays experienced, therefore, a stronger influence of komatiites and basalts in comparison to that expected from the modern proportions of rocks in the Early Archean complexes.