The chemical composition and origin of the primeval continental crust

Abstract

Existing geological and geochemical evidence permits the judgment to be made that the primeval crust was formed through fractionation of magmatic melts from the mantle during early stages of the earth's evolution. The basalt-andesitic composition of the primeval continental lithosphere explains the formation of andesitic “proto-continents” floating above the hot basaltic bed. The consequent evolution of the proto-continents (mainly during the Precambrian time) involves, according to the scheme discussed earlier, an interaction of rocks composing the primeval continental crust with ascending flows of mantle-borne superheated aqueous solutions carrying potassium and silicon from the mantle. The development of metasomatic granitization and subsequent fusion of granitic magma from the metasomatically altered rocks are considered the main effects of such interactions which resulted, finally, in the formation of the present continental lithosphere. Average compositions of the present continental lithosphere, orthoamphibolite and andesite of the island arcs have been used to reconstitute the composition of the primeval continental crust, using the equations of geochemical balance. Only a basalt-andesitic primeval continental crust fits the requirements of the geochemical balance. The calculated model of this consists of 15,337 × 10 24 g andesites and of 5,508 × 10 24 g basalts and has the following average composition (in %): O-46.24; Si-26.22; Al-8.86; Fe-6.32; Mg-2.63; Ca-5.40; Na-2.32; K-1.04; Ti-0.63. According to the geochemical balance the rock-forming chemical elements composing the lithosphere can be divided into three groups: (1) elements which, after magmatic fractionation of the primeval crust, were not brought into the lithosphere by the mantle-borne aqueous solutions, or emigrated with the residual solutions onto the surface of the crust (Al, Mg); (2) elements which, after magmatic fractionation of the primeval crust, were additionally introduced into the continental lithosphere by aqueous solutions from the mantle (Si, K, O and, in minor quantities, Na); (3) elements of the primeval crust which were partially freed during the process of granitization and were transported in large amounts by the residual endogenous solutions onto the surface of the crust (Ca, in lesser degree Fe).