REGIONAL METAMORPHISM AND STRATIGRAPHY OF THE BASEMENT OF THE UKRAINIAN SHIELD

Abstract

The stratigraphic dismembrement of the Lower Precambrian has always been inextricably linked with the study of metamorphism. The degree of metamorphism of the complexes was even used for some time as an indication of their relative age. This sign was not confirmed by isotope dating and was no longer taken into account in the stratigraphic dismembrement of the basement of the shields, including the preparation of official stratigraphic schemes of the Precambrian of the Ukrainian Shield. The authors of the article believe that the degree of metamorphism can still be used in developing the stratigraphy of the Ukrainian Shield. The possibility of such use of metamorphism is considered in a series of publications. The first article describes the stratigraphy and metamorphism of all megablocks of the Ukrainian Shield. It is shown that older stratigenic complexes in each of the megablocks are characterized by higher temperature 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 large successive stages of the geological development of the Ukrainian Shield in the Early Precambrian and can serve as the basis for compiling a regional stratigraphic scheme on a historical-geological basis. A version of the regional stratigraphic scheme of the Ukrainian Shield on a historical-geological basis, compiled at the level of complexes, proposed in the second article of the series. In this final article of the cycle, the conditions and duration of lithogenesis and accompanying metamorphism of various sequentially formed stratigenic complexes of the Lower Precambrian are considered. The idea is put forward that the initial pre-metamorphic composition of the complexes and their metamorphism are determined by the temperature state of the upper shells of the Earth and their directed thermal evolution at the early stages of geological history. The existence of hydrospheric, early thermohydrospheric, late thermohydrospheric and normohydrospheric global stages of lithogenesis, lasting from 300 to 700 Ma, and continuous metamorphism from 3.8 to 2.0–1.9 Ga are assumed. During this time, a single metamorphic (paleotemperature) zoning of the basement of the Ukrainian Shield was formed, in which all stratigenic complexes of different ages participate.