LOWER AGE BOUNDARY OF THE FORMATION OF METATERRIGENOUS ROCKS OF THE VYSOKOPILLYA GREENSTONE STRUCTURE, MIDDLE!DNIEPER REGION OF THE UKRAINIAN SHIELD

Abstract

The stratigraphic relationships of meta-terrigenous rocks of the Kryvyi Rih Series belonging to the Kryvyi Rih-Kremenchuk structure with metasediments of similar composition belonging to the upper part of the greenstone belt section (the Bilozerka Series) in the Middle Dnieper Domain remain unexplored. The adjacent “strip-like” Vysokopillya, Skhidno- Hannivska and Zhovta Richka structures are of particular interest; in these structures, metasediments and ferruginoussiliceous rocks overlie metabasites of the Mesoarchean greenstone belts. The Vysokopillya greenstone belt (GB), which is adjacent to the Likhmanivka syncline of the Kryvyi Rih structure, is a steep-bedding monocline. The northern part of the belt comprises a thick homogeneous meta-tholeiite sequence (the Sura Suite). The Chertomlyk Suite is located higher in the stratigraphic section and includes meta-andesites with subordinate rhyodacites and tholeiitic basalts. They form the southern part of this structure. Subvolcanic rhyolite-dacite bodies (the Solenivka Suite) occur as crosscutting intrusions and dykes. The middle and southern parts of the Vysokopillya GB consist of a heterogeneous mass of orthogneisses and paragneisses, which developed after sedimentary (graywackes, sub-graywackes and melawackes) and volcanic (tuff sandstones, lava breccias, meta-dacites and meta-andesites with a subordinate amount of meta-tholeiites) rocks were deposited. Graywackes host thin beds of quartz-magnetite quartzites and thick (up to 100 m) beds of quartzmagnetite- cummingtonite shales with garnet. We applied LА-ICP-MS for U-Pb dating of 102 detrital zircons from biotite-quartz meta-sandstones and determined their Hf isotope compositions. Among them, four crystals have 207Pb/206Pb age over 3400 (up to 3800) Ma. A single zircon crystal was dated at 2930 Ma. The remaining 97 crystals have ages in the range of 3000-3360 Ma, with distinct peaks at 3065 (15 grains), 3185 (21 grains), 3260 (16 grains), and 3320 Ma (22 grains). Hafnium isotope composition varies over a wide range. The oldest grain dated at 3800 Ma, has eHf = –0,8, whereas the next two younger grains have positive eHf values (+1,2 at 3645 Ma, and +2,2 at 3535 Ma). Crystals dated at ca. 3320 Ma demonstrate wide variations in eHf — from +1,8 to -6,1; those dated at ca. 3260 Ma — from +4,5 to –2,5, those dated at ca. 3185 Ma — from +3,3 to –4,3, and those dated at ca. 3065 Ma — from +6,2 to –3,9. According to the obtained data, metasedimentary rocks of the Vysokopillya GB started to accumulate in the Neoarchean. The same age was established for the metamorphosed sedimentary rocks of the Skhidno-Hannivska and Krasnoyarsk-Alferivka structures, as well as for the Bilozerka Series of the Bilozerka GB. Thus, sedimentary rocks in these structures were formed during the denudation of the Mesoarchaean craton. The source rocks were predominantly represented by the TTG of the Sura complex (3.1-2.96 Ga). For the first time, a significant number (38 grains) of Paleoarchean (3.3 Ga) detrital zircons were discovered in the Middle Dnieper Domain, which indicates the presence of Paleoarchean rocks in the denudation area. Data on Hf isotope systematics indicate the possible presence of Hadean (older than 4.0 Ga) material.