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Deciphering the petrogenesis of deeply buried granites: whole-rock geochemical constraints on the origin of largely undepleted felsic granulites from the Moldanubian Zone of the Bohemian Massif

Published online by Cambridge University Press:  26 July 2007

Vojtěch Janoušek
Affiliation:
Institut für Mineralogie, Universität Salzburg, Hellbrunnerstraße 34, A-5020 Salzburg, Austria. e-mail: [email protected]
Fritz Finger
Affiliation:
Institut für Mineralogie, Universität Salzburg, Hellbrunnerstraße 34, A-5020 Salzburg, Austria. e-mail: [email protected]
Malcolm Roberts
Affiliation:
Institut für Mineralogie, Universität Salzburg, Hellbrunnerstraße 34, A-5020 Salzburg, Austria. e-mail: [email protected]
Jiří Frýda
Affiliation:
Czech Geological Survey, Klárov 3/131, 118 21 Prague 1, Czech Republic.
Christian Pin
Affiliation:
Département de Géologie, CNRS, Université Blaise Pascal, 5, rue Kessler, F-63038 Clermont-Ferrand, France.
David Dolejš
Affiliation:
Department of Earth and Planetary Sciences, McGill University, 3450 rue University, Montreal, Quebec, Canada H3A 2A7.

Abstract

The prominent felsic granulites in the southern part of the Bohemian Massif (Gföhl Unit, Moldanubian Zone), with the Variscan (∼340 Ma) high-pressure and high-temperature assemblage garnet+quartz+hypersolvus feldspar ± kyanite, correspond geochemically to slightly peraluminous, fractionated granitic rocks. Compared to the average upper crust and most granites, the U, Th and Cs concentrations are strongly depleted, probably because of the fluid and/or slight melt loss during the high-grade metamorphism (900–1050°C, 1·5–2·0 GPa). However, the rest of the trace-element contents and variation trends, such as decreasing Sr, Ba, Eu, LREE and Zr with increasing SiO2 and Rb, can be explained by fractional crystallisation of a granitic magma. Low Zr and LREE contents yield ∼750°C zircon and monazite saturation temperatures and suggest relatively low-temperature crystallisation. The granulites contain radiogenic Sr (87Sr/86Sr340 = 0·7106–0·7706) and unradiogenic Nd ( = − 4·2 to − 7·5), indicating derivation from an old crustal source. The whole-rock Rb–Sr isotopic system preserves the memory of an earlier, probably Ordovician, isotopic equilibrium.

Contrary to previous studies, the bulk of felsic Moldanubian granulites do not appear to represent separated, syn-metamorphic Variscan HP–HT melts. Instead, they are interpreted as metamorphosed (partly anatectic) equivalents of older, probably high-level granites subducted to continental roots during the Variscan collision. Protolith formation may have occurred within an Early Palaeozoic rift setting, which is documented throughout the Variscan Zone in Europe.

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 2004

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