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Retrograde evolution of eclogites: evidences from microstructures and 40Ar/39Ar white mica dates, Münchberg Massif, northern Bavaria

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Abstract

Phengites from eclogites and pegmatites (3T, 2M1, coarse-grained and recrystallized) of the Münchberg Massif (Weissenstein and Oberkotzau) have been dated by the 40Ar/39Ar method. 3T-micas from the eclogites yielded plateau and isochron ages of 365±7 Ma. 2M1-micas show disturbed degassing spectra. Micas from pegmatites show a slight excess Ar component, with an isochron age of 353 to 351±3 Ma. An age component of approximately 300 Ma was also detected. In combination with age values from the literature, the cooling history of the Münchberg Massif from eclogite-facies conditions (390 Ma) to cooling below 350°C (350 Ma) is documented. The age component of 300 Ma is attributed to a low-grade stage of mineral growth accompanied by a transitional ductile-brittle deformation. The petrological effects include formation of pumpellyite-prehnite-facies minerals, frequently precipitated in microcraks and cleavage planes of earlier formed minerals. This stage has to be seen in conjunction with the intrusions of the Fichtelgebirge granite.

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Authors and Affiliations

  1. Institut für Mineralogie FR Geochemie, FU Berlin, Boltzmannstrasse 18-20, W-1000, Berlin 33, Germany

    K. Hammerschmidt

  2. Institut für Angewandte Geophysik, Petrologie, und Lagerstättenforschung, TU Berlin, Strasse des 17. Juni 135, W-1000, Berlin 12, Germany

    G. Franz

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  1. K. Hammerschmidt
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  2. G. Franz
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Hammerschmidt, K., Franz, G. Retrograde evolution of eclogites: evidences from microstructures and 40Ar/39Ar white mica dates, Münchberg Massif, northern Bavaria. Contrib Mineral Petrol 111, 113–125 (1992). https://doi.org/10.1007/BF00296582

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