Detrital glaucophane in graywackes of the Rhenohercynian Harz mountains and the geodynamic implications |
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Authors: | M Ganssloser T Theye H Wachendorf |
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Institution: | (1) Institut für Geowissenschaften der TU, Postfach 3329, D-38023 Braunschweig, Germany |
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Abstract: | Detrital blue amphibole was found for the first time in two samples of the Famennian section of the South Harz-Selke Graywacke and the Tanne Graywacke (Middle Visean) of the Harz Mountains. Microprobe analyses reveal that the blue amphiboles represent glaucophane with Fe3+/(Fe3++AlVI)=0.22 molar ratio. The minimum pressure required for the formation of glaucophane of this composition is estimated to be approximately 8 kbar. The source area of the detrital glaucophane is assumed to be located between the Northern Phyllite Zone and the Mid-German Crystalline Rise, in areas which have been downfaulted (?subducted) during the Variscan orogeny. The age of blueschist-facies metamorphism in the source area must be of pre-Upper Devonian age. This metamorphic event is significantly older than the Lower Carboniferous high-pressure/low-temperature metamorphism documented in parts of the northern Phyllite zone. Hence, the convergent tectonics connected with blueschist-facies metamorphism is not restricted to the Lower Carboniferous, but can be traced back at least to the early Upper Devonian. These data are in accordance with a southerly directed underplating (?subduction) at the northern margin of the Saxothuringian zone active during at least from early Upper Devonian to Lower Carboniferous. |
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Keywords: | Harz Mountains Northern Phyllite Zone Mid-German Crystalline Rise Saxothuringian zone Detrital glaucophane Blueschist-facies metamorphism |
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