Походження мегаструктур Українського щита з точки зору плюм-концепції

Abstract

Internal tectonic structure of the Ukrainian Shield is currently seen as an association of megablocks (megastructures) divided by abyssal fractures. Within this region, six megablocks (megastructures) are differentiated: Volynski, Dnistrovo-Buzki, Rosynsko-Tikytski, Ingulski, Middle Prydniprovian and Pryazovian, the last of which is sub-divided by some researchers into two separate megablocks: Western Pryazovian and Eastern Pryazovian.Volynska, Ingulska, Middle Prydniprovian and Western Pryazovian megastructures of the Ukrainian Shield can be considered as such that formed as a result of abyssal magmatic plumes. The geological-structural position of these megastructures with obvious signs of influence of abyssal processes on their formation is in favor of this assumption. The structural-geological body of these megastructures is determined by granite-gneiss domes with the development (in some structures) in their central parts with large granitoid batholites of complex internal structure and a wide range of material composition, surrounded by synclinal- and graben-like trough structures composed of igneous-sedimentary formations metamorphized under greensсhist and amphibolite facies conditions of regional metamorphism. The process of establishment of megastructures of the Ukrainian Shield, influenced by mantle plumes, didn’t involve horizontal moves. The probable initiator of the following mantle plume was catastrophic sinking of a significant mass of cooled previous plume back into the mantle. This explains the time sequence of formation of megablocks of the Shield. Megastructures appeared as a result of complex multi-staged process that was similar to all of them, while the difference in age among rock complexes and certain difference in their structural position are not the criteria of different tectonic processes of the megastructure development. Conditionally, four stages of formation of megastructure resulting from the development of abyssal plume can be pointed out: the first stage is the rising of mantle magmatic plume from the depth; the second stage, intensive metamorphism of granite and basalt layers, partial melting with the formation of the above mentioned granite-gneiss domes and formation of compensation marginal troughs around them; the third stage is the formation of synclinorium around the domes, manifestations of intensive metamorphism of igneous-sedimentary rock masses, their ultramorphism with the formation of migmatites; the fourth stage is the establishing of the trough structures laid at the earlier stages with the formation of greenstone belts. At the final stage was an intensive bedding of multi-stage intrusives of two-feldspar granites.