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Evolution of the Archaean Aulian Gneiss Complex, Middle Dnieper gneiss-greenstone terrain, Ukrainian Shield: SHRIMP UPb zircon evidence

Authors
Journal
Precambrian Research
0301-9268
Publisher
Elsevier
Publication Date
Volume
78
Identifiers
DOI: 10.1016/0301-9268(95)00069-0
Disciplines
  • Earth Science

Abstract

Abstract The Aulian Gneiss Complex (AGC) forms the main part of the Middle Dnieper gneiss-greenstone terrain (Ukrainian Shield) as domal blocks between greenstone belts. The Complex consists of three main units. The oldest consists of interlayered tonalitic gneisses, amphibolites and ultramafic schists and are grouped into the supracrustal Aulian Series. The second unit comprises coarse-grained tonalitic and trondhjemitic gneisses which formed as small intrusive bodies within Aulian Series rocks. The third unit contains diverse migmatites and gneissic granitoids collectively named the Dniepropetrovsk granitoids and forming the main component of the AGC. SHRIMP UPb geochronology has been undertaken on these units of the AGC. Weighted mean 207Pb/ 206Pb (2 σ) ages for different zircon types in each sample are as follows. (1) Aulian Series: tonalitic gneisses 3196 ± 13 and 3079 ± 9 Ma, coarse-grained amphibolite (metagabbro) of tholeiitic composition 3181 ± 5 Ma, fine-grained melanoamphibolite (metavolcanic) of basalt komatiitic composition 3078 ± 17 Ma. (2) Small intrusions into the Aulian Series: trondhjemitic gneiss 3122 ± 2 and 3081 ± 5 Ma. (3) Dniepropetrovsk granitoids: tonalite 3074 ± 7 Ma, trondhjemite ∼ 3070 Ma. These results allow the following synthesis for the AGC. 3200-3180 Ma ago, supracrustal dacite-basalt (komatiite) sequence was formed and then 3120 Ma ago it was intruded by trondhjemites and tonalites. At ∼ 3080 Ma metamorphism of amphibolite facies and perhaps local in situ partial melting caused growth of new zircon in the older lithologies and was accompanied by intrusion of younger of tonalites and trondhjemites. These UPb zircon results have not substantiated a previous interpretation of SmNd whole-rock isotopic data that very early Archaean (∼ 3900 Ma) mafic components are present in the Aulian Series of AGC.

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