Peculiarities of the present structure of the Bug granulite complex developed within the Middle Bug area and its partitioning into series and suits

Abstract

The Gaivoron-Zavalie site and the Golovanevsk suture zone (GSZ) of the Bug mining region (BMR) in the Middle Bug region have been a «testing ground» for studying the structure and stratigraphy of the Early Precambrian complexes of the USh for many years. In the article, using their example, the Early Precambrian part of the Correlation Chronostratigraphic Scheme of the USh (CCS USh) and the issues arising around its discussion and the need for changes have been considered here. The main problem is to find out series, formations and rock strata in the CCS of USh, which is quite debatable. It has been shown that some rock strata (leucogranulite formation and Zelenolevadov rock strata, a significant part of the Koshar-Alexandrov suite) refer to the formation-genetic series of quartz-feldspar metasomatites, in particular, to the structural-formation association of metasomatites of zones of regional faults and they are not stratigenic. According to isotope-geochronological data, the bereznin rock strata is not Archean and therefore does not belong to the Dniester-Bug series. The layering, banding and gneissovity of the enderbite-gneiss complex of the Gaivoron-Zavalie section are associated with its intersection of a dense network of fault zones and therefore this area refers to the area of development of highly dispersed metamorphic tectonites, rather than the stratigene rock strata, which some researchers try to call the «Salkovska Formation». The association of the iron-ore strata of the Bug mining region into the Pavlov rock strata of the Archean Dniester-Bug series is not correct, because the present overlapped section of such rock strata has been studied in the Moldovian structure in the GSZ structure, and it refers to the Bug series. However, the genesis of iron-ore rocks is disputable, so the authors consider it undesirable to include them in the CCS of the Ukrainian shield. The genesis of graphitic gneisses and quartzites of the Khaschevato-Zavaliev suite is also disputable, which leads to the conclusion that it should be excluded from the CCS of the Ukrainian shield. With the exclusion of the Zelenolevadov, Bereznin, and Pavlov strata from the Dniester-Bug Series, only the strata of hyperstene and dipyroxene plagiogneisses and crystalline schists, called as Tyvrovka, remains in it. Its stratotype becomes the stratotype of the entire series, so the stratum “Tyvrov rock strata” can also be removed from the series. The authors conclude that the division of the Bug granulite complex into suites and rock strata is impractible. When considering the division of the Bug granulite complex into series, it is shown that the concept “series” as set forth in the Stratigraphic Code of Ukraine, although it does not take into account the features of formation of the Archean series, however does not contradict them. Therefore, separation of the Dniester-Bug and Bug series of different composition and age is quite natural. Contrary to the opinions of some researchers on the continuous process of accumulation of sedimentary-volcanogenic rocks of the Middle Bug area, it is shown that the anomalies in the age distribution of clastogenic zircons from the rocks of the Gaivoron-Zavalie and GSZ correspond to the global one and reflect a worldwide process of crustal formation, in which relatively quiet tectonic regimes alternated with periods of tectonic activation. During such periods, both magmatic and metasomatic processes and sedimentary-volcanogenic processes were activated. Materials of the study of the hafnium isotope composition from the zircons of the Dniester-Bug and Bug series rocks indicate a possible difference in the age of their formation of 600—700 Ma. The questions of the origin and rearrangement of the Bug granulite complex developed in the study area are also considered here.