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Stefan Claesson, Swedish Museum of Natural History (Sweden)
Elena Bibikova, Vernadsky Institute of Geochemistry and Analytical Chemistry (Russian Federation)
Svetlana Bogdanova, Lund University (Sweden)
Leonid Shumlyanskyy, Institute of Geochemistry, Mineralogy and Ore Formation (Ukraine)
Vladimir (?) Skobelev , Institute of Geochemistry, Mineralogy and Ore Formation (Ukraine)
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The Volyn Domain, northwestern Ukrainian Shield, is traditionally interpreted to be mainly Archaean. The age of this crustal segment is important for our understanding of Precambrian evolution in NE Europe. The inferred Archaean age is based on various lines of evidence, including a U-Pb zircon age of 2.43 Ga (Verhoglyad and Skobelev 1995) for a metadacitic dyke cross-cutting surrounding paragneisses belonging to the Teterev metasediments, which are considered to represent the oldest identified crust in the domain.
U-Pb microanalytical data for zircon from various rocks in the Novograd-Volynskyy area, including the dacite, were briefly reported by Claesson et al. (2006). Obtained ages were invariably Palaeoproterozoic, in the range 2.0-2.2 Ga. Furthermore, various Volyn Domain rocks have given Palaeoproterozoic Nd model ages. Here we present new U-Pb zircon and Nd whole rock results, which provide more detail to these earlier data.
Samples for this study come from a restricted area near Novograd-Volynskyy where the relations between different analysed rocks are well understood. Zircon from a migmatitic metagreywacke gneiss give a distinctly bimodal age distribution where the majority of analysed crystals cluster around a well defined age of 2.10 Ga, and three spots in two crystals are 3.0 Ga. The crystals are interpreted as clastic, providing a maximum deposition age for the sediments. This is corroborated by a Nd model age of ca. 2 Ga for a metavolcanic amphibolite interpreted to be syn-deposited. Zircon from migmatite leucosome give similar Palaeoproterozoic ages, averaging 2.08 Ga. The bulk of the analysed zircon from two dacite samples also give similar ages. Three analyses in two crystals are ca. 2.9 Ga, indicating the presence of Archaean zircon xenocrysts in the dacite.
These results demonstrate a sequence of events including initial formation of zircon bearing rocks, erosion and deposition of sediments followed by tectonic deformation and migmatisation, and intrusion of dacite into the newly formed crust, all in a restricted time interval between ca. 2.1 and 2.0 Ga. The uniform age of detrital zircon suggests a juvenile source region, such as an active plate margin environment. This supports a recently proposed scenario (Claesson et al. 2006) where the Volyn Domain is part of a larger region of Palaeoproterozoic accretionary crust, which continues further to the east in the Ukrainian Shield and can be followed to the northwest into Fennoscandia.
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