The Orekhov–Pavlograd Zone, Ukrainian Shield: Milestones of its evolutionary history and constraints for tectonic models

Abstract

The Orekhov–Pavlograd Zone (OPZ), which is located in the Ukrainian Shield, represents the reworked western margin of the Archaean Azov Province and separates it from the Archaean Middle Dnieper Province. In the Vasilkovka area, which is located in the northern OPZ, Archaean tonalites and amphibolites and Palaeoproterozoic quartzites, high-Al paragneisses, and mafic granulites were studied. Structural observations allow the OPZ to be interpreted as a steep transpressional ductile shear zone. The tonalites and amphibolites were dated at 3.50 and 2.88Ga, respectively (SIMS zircon ages hereafter). The metasedimentary rocks originated from mature sediments from Archaean continental sources and were deposited in a shallow intracratonic basin. The basaltic protoliths of the mafic granulites were produced by a mantle-plume-related source in a quiescent intraplate setting. Several metamorphic events were recognised (3.5, 3.4, 3.3, 2.85 and 2.1–2.0Ga). The main metamorphic reworking occurred in the Palaeoproterozoic; its onset was dated at 2.08–2.07Ga, and it ended at 2.00–2.01Ga. The earliest Palaeoproterozoic metamorphism occurred at P=ca. 8.5kbar and T=ca. 600°C, then at P=5–7kbar and the peak metamorphic temperature T=830°C, and the final stage was at T=ca. 600°C and P=3.3–4kbar (based on the TWEEQU and THERIAK-DOMINO methods), which suggests a clockwise P–T–t loop. In small shear zones, the Archaean tonalites were converted to garnet–biotite gneisses that inherited the tonalitic geochemistry and Palaeoarchaean magmatic zircons. Subduction-related igneous rocks and accretionary prism sediments are absent in both the Vasilkovka area and the entire AP, which makes subduction-related models questionable. An alternative tectonic model for the OPZ could be an intracratonic (intraplate) ductile shear zone that developed due to far-field stresses due to the tectonic juxtaposition of the Sarmatia and Volgo-Uralia continental blocks, which were parts of the East European Craton. Regional metamorphism and the coeval emplacement of mantle-derived igneous rocks and crust-derived granites are suggested to have been produced by magmatic underplating at ca. 2.02–2.10Ga. This model needs to be developed further.