| Abstract: | The study of the geochemistry of trace elements in magmatic and ultrametamorphic rocks permits the characterization of geochemical types of granitoids and basites that differ in their mode of formation, major chemical, trace-element and mineral composition, and potential as indicators of ore bodies.For 12 geochemical types of granites the ratio F(Li + Rb)/(Sr + Ba) is characteristic and may change by more than 3 orders of magnitude.For 8 geochemical types of basites the most informative ratio is K(Sr + Ba)/(Ni + Cr), the value of which may change within 5 orders of magnitude.The geochemical typification of magmatites and ultrametamorphites permits consideration of the geochemical particulars of the formation of the Precambrian rocks of the continental Earth's crust.The formation of the continental crust, hydrosphere and atmosphere (3.8−1.6 Ga), and the formation of the granitic cores of Archean cratons, apparently resulted from a process of ultrametamorphism and granitization of a basic protocrust and proceeded under the influence of intratelluric diffusions, originating as a result of the degassing of the upper mantle.During the initial stages of the process (formation of plagio-migmatites and enderbites from the rocks of the protocrust) a marked loss of Mg, Ca, Fe and a series of trace elements (Ni, Au, etc.) took place. Later during the formation of K-feldspar migmatites, charnockites and true ultrametamorphic granites, a marked increase of K, Rb, Ba and a series of other trace elements took place. During the final stage, marked by an intensive influx of F, (rapakivi) granite and prefault rare-metal metasomatites were formed.During the formation of greenstone belts, the second important feature of Archean cratons, a process of penetration, differentiation, crystallization and degassing of silicate melts of ultrabasic and basic composition, resulting from the selective melting of parts of the upper mantle, predominated. Later sedimentary-metamorphic strata also formed in these belts.The peculiarities of the processes of the formation of the Precambrian continental crust predetermined the metallogenic characteristics of this important and long stage of the Earth's development, characterized by the formation of enormous commercial concentrations of Fe, Ni, Au and a series of other elements. |
