REGIONAL METAMORPHISM AND STRATIGRAPHY OF THE BASEMENT OF THE UKRAINIAN SHIELD

Abstract

Stratigraphic complexes of the Lower Precambrian are everywhere metamorphosed. Therefore, the stratigraphic subdivision of the Lower Precambrian has always been inextricably linked with the study of metamorphism. For some time, the degree of metamorphism of the complexes was even used as an indication of their relative age. With the beginning of the use of isotopic dating, this sign was not confirmed, after which the degree of metamorphism was no longer taken into account in the stratigraphic dismemberment of the shields basement. The degree of metamorphism of the complexes was no longer taken into account for a long time when compiling official stratigraphic schemes of the Precambrian of the Ukrainian Shield, which, in the opinion of many geologists, led to distortions of the real stratigraphy of the basement of this region. The authors of the article believe that the degree of metamorphism can still be used in the development of the stratigraphy of the Ukrainian Shield and, above all, in the stratigraphic study of individual megablocks. The possibility of such use of metamorphism is considered in the cycle of publications. This is the second article in the cycle. The first article describes the stratigraphy and metamorphism of all megablocks of the Ukrainian Shield. A number of conclusions have been made about the regularities of the manifestation of metamorphism. It is shown that older stratigenic complexes in each of the megablocks are characterized by higher temperature metamorphism. This pattern provides a basis for establishing the relative stratigraphic sequence of complexes within individual megablocks based on the degree of their metamorphism. At the same time, the distinctive features of the composition and metamorphism of the stratigenic complexes, according to the authors, are a reflection of the successive stages of the geological development of the Ukrainian Shield in the Early Precambrian and can serve as the basis for compiling of the regional stratigraphic scheme on a historical and geological basis. In this second article of the cycle, modern approaches to the geochronological periodization of the Precambrian are considered: a) geochronometric, adopted for the International “The Geological Time Scale” and b) historical-geological, which is the basis of the “General Stratigraphic Scale of the Lower Precambrian of Russia”. The current “Correlation Chronostratigraphic Scheme of the Early Precambrian of the Ukrainian Shield” is based on the geochronometric approach, which the authors, like many other researchers, consider unacceptable for practical use. The article proposes a variant of the regional stratigraphic scheme of the Ukrainian Shield on a historicalgeological basis, compiled at the level of complexes, which in the final version of the scheme can be divided into series and suites.