U-Pb zircon and Sm-Nd model ages of high-grade Moldanubian metasediments,Bohemian Massif,Czechoslovakia |
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Authors: | A Kröner I Wendt T C Liew W Compston W Todt J Fiala V Vankova J Vanek |
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Institution: | (1) Institut für Geowissenschaften, Universität Mainz, Postfach 3980, D-6500 Mainz, Federal Republic of Germany;(2) Max-Planck-Institut für Chemie, Postfach 3060, D-6500 Mainz, Federal Republic of Germany;(3) Research School of Earth Sciences, Australian National University, G.P.O. Box 4, 2601 Canberra, ACT, Australia;(4) Institute of Geology and Geotechnics, Czechoslovak Academy of Sciences, V Holesovickách 41, 18209 Praha 8, CSSR;(5) Geophysical Institute, Czechoslovak Academy of Sciences, Bocni II, 14131 Praha 4, CSSR |
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Abstract: | We report single grain U-Pb ion-microprobe as well as conventional bulk size fraction ages for zircons from 3 metasediment samples of the Moldanubian Complex, Bohemian Massif, one of the largest crystalline complexes of the Hercynian foldbelt in Europe. These are complemented by whole-rock Sm-Nd model ages. The metasediments are of upper amphibolite to granulite grade and come from the Bory Massif in Moravia, NW of Brno (sample AA-1) and from the Varied Group (AA-2) and Monotonous Group (AA-3) in the Ceske Budejovice region of SW Bohemia.Ion-microprobe data for detrital zircons yielded 207Pb/ 206Pb ages between ca. 1750 Ma and 2680 Ma and reflect chronologically heterogeneous source terrains. One grain in sample AA-1 of the Bory granulite massif may be as old as 2684±14 Ma, and this constitutes the oldest reported zircon age for the Hercynian belt of central Europe. The single grain data are much less discordant than previously published conventional U-Pb analyses from bulk zircon samples and suggest a significant early Proterozoic crust-formation event between 2 and 2.2 Ga ago. The size fraction data are compatible with the single grain ages and give a fairly precise definition of the Hercynian event between 347 and 367 Ma ago while their upper Concordia intercept ages between 1700 and 2050 Ma represent the mean of the respective grain populations and are probably geohronologically meaningless. The Nd whole-rock model ages between 1.7 and 3.0 Ga confirm mid-Proterozoic to Archaean source terrains for the dated metasediments. |
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