Timing and Sequence of Endogenous Events in the Bug Granulite–Gneiss Domain of the Ukrainian Shield Based on the Study of a Composite Tectono-Magmatic Breccia

In the Bug granulite-gneiss domain of the Ukrainian shield, a composite tectono-magmatic breccia including enderbites and various mafic rocks was studied in the gneiss-enderbite complex. Based on the assessment of the thermodynamic conditions of rock and mineral formation and the analysis of the U–Pb and Lu–Hf isotope systems of zircon, the endogenous history of the gneiss–enderbite complex is deciphered. The predominant rocks are enderbites with numerous enclaves of two-pyroxene and pyroxene–amphibole crystalline schists that were deformed under granulite facies metamorphism. The oldest zircons have a concordant U–Pb age of 3.6–3.7 Ga, which is close to the age of the protolith of the Bug enderbites. The zircons are characterized by a heterogeneous structure, negative ɛHf(t) values from –1 to –38, and a variation in 176Hf/177Hf(0) isotope ratios from 0.28035 to 0.28095, which is consistent with their different ages and origins. Based on the age, geochemistry, and isotope composition of zircon from composite breccia rocks, following geological events from Archean to Proterozoic are distinguished: (1) 3.67–3.60 Ga stage of magmatic crystallization of early mineral associations; (2) 3.0‒2.8 Ga stage of granulite metamorphism and partial melting (2.9 Ga) of the enderbites with the preservation of melt mineral phases in zircons, but with a strong disturbance of its U–Pb isotope system; (3) 2.0–1.9 Ga stage of Proterozoic granulite metamorphism with the reorganization of cation and isotope systems of rock-forming and accessory minerals. The obtained data are consistent with the model of polychronous evolution of the continental crust of the Ukrainian Shield in the Paleo- and Mesoarchean with its strong reworking in the Paleoproterozoic.