PETROCHEMICAL TRENDS AND EVOLUTION OF THE ARCHEAN CONTINENTAL-CRUST

Alkalinity/lime ratio geochemical trends for the evolutionary sequence of Precambrian metamorphic and magmatic series as well as for schield average chemical compositions (Canadian, Ukrainian, Anabar and Aldan schields) were presented. The trends are characteristic of continental crust successive formation stages. Dissimilarity of Precambrian trend s with that of contemporary crust forming volcanogenic series of icela nd arcs and continental margins was demonstrated. The Precambrian tren ds are characterised by increased alkalinity gradients, the latter are much above that of differentiated mantle tholeiitic and calc-alkaline magmas. Thus the increased alkalinity of Archean continental crust se ries could be associated not with the mantle source evolution but with successive rise of intracrustal sedimentary/metamorphic differentiati on and granitisation. Earlier stages of the Archean continental crust evolution could be presented by Aldan and Anabar schield petrology whe reas the later stages (AR - PR) were exemplified by Canadian and Ukrai nian schields.