The Archean formation of the Sarmatian continental crust

Based on our new U-Pb isotope data (SHRIMP-II) for the Voronezh Crystalline Massif (VCM) and on published U-Pb data for the Archean rocks of the Ukrainian Shield (USh), we have substantiated the main stages of the Archean evolution of the Sarmatian crust, a fragment of the basement of the East European Platform. New data have been obtained for 20 indicative rocks of the Kursk block (KB) of the VCM. We compared the Eoarchean and Paleoarchean KB rocks (mafic rocks and tonalites of the Oboyan' complex), orthogneisses of the USh Dniester-Bug province (Bug granulite complex), and tonalites and mafic rocks of the USh Azov Province and have established the existence of ancient continental crust (3.75-3.60 Ga) in Sarmatia. The presence of Paleoarchean xenogenic zircons in younger intrusions indicates a wide spread of Paleoarchean rocks in the deep VCM crust section. In the Mesoarchean (3.2-3.0 Ga), eastern Sarmatia (KB and Azov and Middle Dnieper provinces) was a single granite-greenstone terrain. Two stages of felsic magmatic activity have been dated: 3.15-3.10 Ga (volcanics in the greenstone belts and tonalite-trondhjemite granites in the VCM and USh) and 3.05-3.00 Ga (volcanics and tonalite- trondhjemite granites in the USh and granites in the VCM KB). Magmatic and tectono-metamorphic processes (2.95-2.85 Ga) have been established throughout the eastern part of Sarmatia. The latest Mesoarchean endogenic activity (2.85-2.80 Ga) testifies to the tectonic differentiation of the area. Mafic and felsic magmatism, deformations, metamorphism, and ultrametamorphism under amphibolite and granulite facies conditions took place in the most part of the KB and in the Azov and Bug areas. It is shown that ultrametamorphic granites formed from an ancient (3.0-3.5 Ga) protolith. The USh Middle Dnieper province had a different tectonic position. Here, intrusion of post-tectonic granites and formation of mature sediments proceeded at 2.81-2.86 Ga. Our geochronological data show that most of the Sarmatian continental crust formed in the Mesoarchean as a result of both the intrusion of juvenile material and the reworking of the older protolith rocks. Neoarchean events (2.8-2.5 Ga) are weakly expressed in Sarmatia in contrast to Baltia, another large fragment of the East European Platform basement, where endogenic processes at 2.65-2.75 Ga were the major crust-forming geologic events.