The main stages of the Ukrainian Shield evolution and plate tectonics

The results of the U-Pb-Hf-O isotope study of zircon from (meta)igneous rocks sampled in all domains of the Ukrainian Shield allow recognition of the four main stages of continental crust formation:

1. The Eoarchean stage (ca. 4000-3600 Ma). Rocks of this stage occur in the Dniester-Bouh and Azov domains. In the former, they are represented by heavily metamorphosed enderbites and mafic schists reaching an age of 3.8 Ga. In contrast, tonalites with an age of 3.67 Ga were identified in the Azov Domain. The oldest zircon reaching an age of 3970 Ma was found in the Mesoarchean metadacite in the Azov Domain. The Eoarchean rocks are rare, but their presence indicates that crust-forming processes have started already in the Eoarchean, or even in Hadean, time.

2. The second major event took place between c. 3.2 and 2.7 Ma. Rocks, formed during this age interval, compose around half of the Ukrainian Shield. Considering the long duration of this event, it may have consisted of several separate episodes. The whole set of rock associations typical for the Archean continental crust, including TTG series, greenstone belts and sedimentary basins, has been formed. Hafnium isotope composition in zircon reveals the juvenile nature of this event. Some remobilization of the older crust is also recorded from several samples.

3. Nearly half of the rock assemblages were dated at ca. 2.15-1.90 Ga. In contrast to the Archean events that resulted in the formation of apparently more or less equant terranes, the Paleoproterozoic events led to the formation of orogenic belts. These belts comprise metamorphosed in amphibolite or epidote-amphibolite facies supercrustal sequences, and abundant granitic intrusions. According to the existing models, the formation of the orogenic belts was related to the assembly of Baltica as a part of the Columbia/Nuna supercontinent. Hafnium-in-zircon and whole-rock Nd isotopes indicate the predominantly juvenile nature of these rocks, with some contamination by the Archean crust.

4. The last major stage of the Ukrainian Shield evolution was linked to the formation of the Prutivka-Novohol large igneous province, which between 1.8 and 1.72 Ga affected the whole Shield. It resulted in the emplacement of numerous mafic dykes and layered massifs, alkaline intrusions, and huge anorthosite-mangerite-charnockite-granite complexes. All igneous rocks formed during this stage reveal signs of crustal contamination, although input of moderately depleted mantle material is also evident.

Obtained isotope and geochronological data demonstrate that the growth of the continental crust in the Ukrainian Shield was episodic. The mechanisms of the crustal growth were different at different times. During both Archean events, the main mechanism was mafic underplating with further remelting and generation of TTG series, whereas greenstone belts represent the results of mantle plume activity. In the Paleoproterozoic, the main mechanism of crustal growth was the subduction of the oceanic lithosphere that led to the formation of volcanic arcs. Mantle plumes remained an important mechanism of the input of mantle-derived material into the continental crust.