REGIONAL METAMORPHISM AND STRATIGRAPHY OF THE BASEMENT OF UKRAINIAN SHIELD

Abstract

Stratigraphic complexes of the Lower Precambrian are everywhere metamorphosed. That is why the stratigraphic dismemberment of the Lower Precambrian has always been inextricably linked with the study of metamorphism. For some time, metamorphism was even used as a sign of the relative age of stratigenic metamorphic complexes according to the principle – “the higher the degree of metamorphism, the older the complex”. The high-temperature complexes of granulite and amphibolite facies belonged to the Archean, while the less metamorphosed complexes belonged to the Proterozoic. But already at the beginning of the widespread use of methods of isotope dating of metamorphic shield complexes, the most ancient age determinations were obtained from weakly metamorphosed complexes. After that, the degree of metamorphism was no longer taken into account in the stratigraphic dismemberment of the shield basement. The degree of metamorphism of the complexes has not been taken into account for a long ago when compiling official stratigraphic schemes of the Precambrian of the Ukrainian Shield, in which isotope dating plays a leading role. This led, according to many geologists, to distortions in the schemes of the real stratigraphy of the basement of the 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 in which this is the first article. The article presents the characteristics of stratigraphy and metamorphism of all megablocks of the Ukrainian Shield. In the structure of the megablocks of the Ukrainian Shield, as well as all the shields of ancient platforms, the following stratigenic metamorphic complexes of the Lower Precambrian have been established, differing in the type and degree of metamorphism: a) granulite-gneiss, b) amphibolite-gneiss, c) greenstone (metavolcanogenic), d) ferruginoussiliceous- shale (metavolcanogenic-chemogenic-terrigenous), e) gneiss-shale (metacarbonate-terrigenous). It has been established that in all megablocks granulite-gneiss complexes are represented and are the most ancient, and all younger complexes are always less metamorphosed than the previous ones. Thus, the relationship between the degree of metamorphism and relative age in individual megablocks is preserved, but this feature cannot be extended to the entire shield. Coeval complexes of neighboring megablocks may have different degrees of metamorphism, and younger complexes of one of the megablocks are sometimes more intensely metamorphosed than older ones in the neighboring megablock.