What happened to the Early Precambrian granulite complexes of the Bug region (Ukrainian shield) and the Limpopo belt (South Africa) and how to stratify them? A tectonist’s view

Abstract

The author reviews two alternative approaches (stratigenic-metamorphogenic and deformation-metamorphogenic) to the geology and mapping of the Earth’s oldest crust rocks that were metamorphized in РТ-conditions of the granulite facies, on the examples of two granulite belts — the Bug area complex (Ukrainian shield) and Limpopo belt (Southern Africa).
 There was shown a fairly good correlation of the composition and metamorphism (including the dynamometamorphism) of the rock complexes of both belts and their at least three-stage origin and transformation. The granulitic metamorphism processes within both belts happened before 3,0 b.y.a., 2,6—2,8, and c. 2,0 b.y.a. The structural-metamorphic transformations of the granulitic complexes, formation of their new structural plans with the destruction of the former (archean) by the following (proterozoan) do not allow falsifiably stratifying the archean granulitic complexes within the stratigenic-metamorphogenic approach already at the facies level. For the Bug area granulitic belt it is feasible only to isolate the Dniester-Bug and Bug series. Two of the important processes of the belt development are rock deformation in the compression and slip conditions and formation within the granulitic complexes of the sub-vertically-layered medium, slip folds with sub-vertical folds’ joints and wings. In both belts these processes began in the Neoarchean. This shows that in that time the tectonic processes were already to some significant degree propelled by plate tectonics mechanisms. Both belts had in common the formation of the linear-typeshearzonesat micro- to macroscale and their superimposition onto the deformation structures of the granulitic complexes of the preceding developmental stages. On their basis there formed the internal structure of the belts’ and their interconnections with the adjacent blocks of crust. Presumably, the archean charnokitoids and TTG-granitoids form the same structural layer of the crust, given their geochemical similarity and close age, and this layer is the base for the greenstone belts. The mineral composition, РТ-metamorphism conditions and age of granulitic belt rocks based on their U-Pb, Lu-Hf, Sm-Nd isotope systems compositions andoxygen isotope composition in Zirconium grains suggest that in the eo- and paleoarchean mantle and crust there should have already been amassed large quantities of medium-acidic matter which gave rise to the TTG complexes.