Polymetamorphism of the Variscan basement of the Moldanubian Black Forest (Germany) documented in zircon and garnet minerals from gneisses

High-grade metamorphic Variscan basement is exposed in the Moldanubian zone of the Black Forest (BF), being the internal zone of the European Variscan belt. Zircon grains from K-rich felsic orthogneisses and an anatectic paragneiss in the Moldanubian Black Forest demonstrate a multi-stage crystalliz...

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Bibliographic Details
Published in:Acta geochimica 2002-04, Vol.21 (2), p.107-119
Main Author: Fukum, Chen
Format: Article
Language:English
Subjects:
Basements
Carboniferous
Crystallization
Devonian
Forests
Garnet
Garnets
Geochemistry
Geochronometry
Isotopes
Leaching
Lead
Metamorphic rocks
Metamorphism
Minerals
Palaeozoic
Paleozoic
Radiometric dating
Zircon
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Summary:High-grade metamorphic Variscan basement is exposed in the Moldanubian zone of the Black Forest (BF), being the internal zone of the European Variscan belt. Zircon grains from K-rich felsic orthogneisses and an anatectic paragneiss in the Moldanubian Black Forest demonstrate a multi-stage crystallization at 600 Ma, 480 Ma, 400-380 Ma, and 350 Ma. The last three stages of crystallization probably represent metamorphic overprint during pre-Variscan and Variscan metamorphism. Using stepwise leaching procedures, garnet minerals from felsic orthogneisses as well as paragneisses in the Moldanubian Black Forest yielded Early Carboniferous Sm-Nd ages (330-340 Ma), which are consistent with the well-constrained Variscan HT metamorphic event, and Early Palaeozoic (480 Ma) to Devonian (400-370 Ma) Pb-Pb ages. The coincidence of growth time for zircon and garnet minerals at Early Palaeozoic is interpreted as dating a metamorphic event. These garnet data demonstrate that the Moldanubian BF basement underwent at least two metamorphic events during the Early Palaeozoic and Early Carboniferous. During the Variscan HT metamorphism, the Sm-Nd system of garnet was disturbed, but not the U-Pb system, implying the peak metamorphic temperature was lower than 800°C.[PUBLICATION ABSTRACT]
ISSN:1000-9426
2096-0956
1993-0364
2365-7499
DOI:10.1007/BF02873768