The oldest crust in the Ukrainian Shield – Eoarchaean U–Pb ages and Hf–Nd constraints from enderbites and metasediments

Abstract

The oldest crust in the Ukrainian Shield occurs in the Podolian and Azov domains, which both include Eoarchaean components. U-Pb age data for Dniestr-Bug enderbites, Podolian Domain, indicate that these are c. 3.75 Ga old, and Lu-Hf isotope data indicate extraction from chondritic to mildly isotopically depleted sources with 1 Hf up to c. +2. Nd model ages support their Eoarchaean age, while model ages for Dniestr-Bug metasedimentary gneisses indicate that these also include younger crustal material. Most of the Hf-age data for metasedimentary zircon from the Soroki greenstone belt, Azov Domain, reflects Eoarchaean primary crustal sources with chondritic to mildly depleted Hf isotope signatures at 3.75 Ga. A minor portion is derived from Mesoarchaean crust with a depleted 1 Hf signature of c. +4 at 3

The zircon archive of continent formation through time

“…Boekhout et al 2013;Claesson et al 2014;Kleinhanns et al 2013;Hiess et al 2014;Lancaster et al 2014;Petersson et al 2013), zircon is commonly used to characterize a wide variety of geological scenarios, from individual igneous units to whole terranes and entire orogens. Detrital zircons from sediments form the oldest known crustal materials on Earth (4.37 Ga; Wilde et al 2001;Valley et al 2014), much older than the oldest crust itself; estimated at 3.92 Ga to 4.03 Ga (Bowring & Williams 1999).…”

Can oxygen isotopes in magmatic zircon be modified by metamorphism? A case study from the Eoarchean Dniester-Bug Series, Ukrainian Shield

“…Glevassky and Glevasska, 2002;Kalyaev, 1976 reflecting the strong polyphase metamorphic overprint. This is also reflected in internal zircon 107 textures, with common overgrowths on cores of varying appearance (Claesson et al, 2015).…”

Continent formation through time

“…The generation of Archaean granite-greenstone crust is more controversial, but there is growing consensus that both plate tectonics and mantle plumes played a role in the formation of this type of crust (e.g. Polat et al 1998;Percival 2007;Van Kranendonk et al 2007a, b, 2014Van Kranendonk 2010;Smith et al 2012;Bédard et al 2013). Higher mantle heat and more radiogenic-bearing granitic rocks created overall more mafic and less rigid crust, characterized by thick volcanic successions dominated by basaltic and komatiitic lavas, with subordinate felsic volcanic rocks and lesser volumes of sedimentary rocks compared with post-Archaean crust.…”

“…The balance of these orogens, and thus crustal growth rate, is shown to be controlled by the supercontinent cycle. Claesson et al (2014) present an isotopic study of Archaean domains exposed in the Ukrainian Shield to understand the crustal evolution of this region. Both the Podolian and Azov domains of this shield contain Eoarchaean (c. 3.75 Ga) components, dated by U -Pb and isotopic model ages.…”

“…Claesson et al 2014;Lancaster et al 2014;Nutman et al 2013). This has important implications for crustal growth models through time, with increasing evidence for very rapid, early growth of continental crust, followed by recycling.…”