Early Archean crust of the Middle Dnepr and Azov domains, Ukrainian Shield--evidence from ages of detrital zircons in Mesoarchean greenstone belts

Abstract

To clarify the early evolution of the Ukrainian Shield we have dated detrital zircons, separated from metasedimentary rocks of Mesoarchean greenstone belts from the Middle Dnepr and Azov domains (Ukrainian Shield). For the Middle Dnepr domain, the main population of analyzed zircons from three samples of metasediments from the upper Belozerka series of the Verkhovtsevo and Belozerka greenstone belts is in the age range 3.0 to 3.1 Ga, with some crystals as old as 3.2 to 3.3 Ga. The results indicate that the sediment sources were dominated by volcano-plutonic rocks of the belts themselves, with some contribution from basement rocks of the Auli series. For the Azov domain, zircons from four samples of mica schists from the Soroki greenstone belt were analyzed, focusing mainly on cores identified by CL and inner parts of crystals. The data show a group of ages in the range 3.5 to 3.6 Ga. Several zircon cores are older than 3.7 Ga. Overgrowths clearly identified by CL are about 3.2 to 3.0 Ga old. Trace element compositions of the studied metasedimentary rocks (fractionated REE patterns, low HREE contents and negative Nb anomalies) reflect contributions from TTG magmatic series and related volcanic rocks to the sediments, and input from ultramafic-mafic sources is identified using Cr/V and Ni/Co ratios. Sm-Nd model ages of 3.1 to 3.3 and 3.3 to 3.4 Ga for the Middle Dnepr and Azov domains, respectively, and ε~Nd~(T) for the analyzed samples, are in a good agreement with the zircon data. Our new results indicate that Paleoarchean crust in the Azov domain was more widely distributed than previously recognized.