Chronological and geochemical constraints on the pre-variscan tectonic history of the Erzgebirge,Saxothuringian Zone |
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| Institution: | 1. Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, 56126 Pisa, Italy;2. Department of Earth Science, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway;3. Norwegian Polar Institute, Fram Centre, N-9296 Tromsø, Norway;1. Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic;2. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;3. Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic;4. Department of Geosciences, University of Mainz, D-55099 Mainz, Germany;5. Beijing SHRIMP Centre, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, 100037 Beijing, China;6. Institute of Paleontology and Geology, Mongolian Academy of Sciences, P.O. Box 45/650,Ulaanbaatar 15160, Mongolia;7. School of Geology and Mining Engineering, MUST, P.O. Box 654, Ulaanbaatar, Mongolia;8. Department of Earth Science, University of Bergen, Allégaten 41, 5007 Bergen, Norway;1. Czech Geological Survey, 11821 Praha 1, Czech Republic;2. Université de Strasbourg, CNRS, Institut de Physique du Globe de Strasbourg, UMR 7516, 67084 Strasbourg, France;3. Institute of Geophysics ASCR, v.v.i., Bo?ní II/1401, Prague 4 14131, Czech Republic;4. Department of Earth Science, University of California, Santa Barbara, CA 93106, USA;1. Geodynamics and Geomaterials Research Division, Institute of Geography and Geology, University of Würzburg, Am Hubland, 97074 Würzburg, Germany;2. KIT Karlsruher Institut für Technologie, Angewandte Geowissenschaften - Mineralogie und Petrologie, Adenauerring 20b, Geb. 50.40, 76131 Karlsruhe, Germany;3. Institute for Applied Geosciences, Ackerstrasse 76, 13355 Berlin, Germany;4. GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany;5. Institute for Geosciences, University of Frankfurt, Altenhöferallee 1, 60438 Frankfurt/Main, Germany;6. Geo-Center of Northern Bavaria, University of Erlangen, Schlossgarten 5, 91054 Erlangen, Germany |
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| Abstract: | A combined U–Pb zircon geochronological and whole-rock isotopic and geochemical study has been carried out on high-grade orthogneiss, meta-basite, and meta-sediments from the Erzgebirge. The results indicate multiple pulses of Ediacaran–Ordovician magmatism in a transitional volcanic-arc to rift-basin setting. Orthogneiss from high-pressure nappes exhibit a step-like pattern of inherited zircon ages and emplacement ages of 500–475 Ma. In contrast, granite gneiss from the medium-pressure core of the Erzgebirge is characterised by three pulses of magmatism in the Early Cambrian, Late Cambrian, and Early Ordovician. A trend of decreasing Th/U ratios in zircon is observed to c.500 Ma, after which significant increases in the trend and variability of the data is inferred to mark the transition from arc-related to rift-related magmatism. Sediments deposited in the Early Cambrian have continental island arc affinity. Major detrital peaks in the Ediacaran and subordinate Tonian, Palaeoproterozoic, and Neoarchaean data are consistent with an Avalonian-Cadomian Arc and West African Craton derivation. The Early Cambrian sediments were locally reworked by a thermal event in the Ordovician resulting in leucocratic banding and recorded in Ordovician zircon rims characterised by systematically lower Th/U ratios. Ptygmatically folded leucocratic bands containing Ordovician zircon rims, associated with low Th/U ratios, are further observed in the granite gneiss core of the Erzgebirge. Variscan ages are rare, except in a fine-grained high-pressure micaschist, which contains exclusively small, structure-less, zircon with a weighted mean age of 350 ± 2 Ma. These data, along with a re-evaluation of previously published data, have been interpreted as the product of flattening subduction during the Early Cambrian; followed by the opening of slab windows in the Late Cambrian; and finally delamination in the Early Ordovician. Delamination of the orphaned slab led to asthenospheric upwellings triggering extension, bimodal magmatic pulses, recycling of fertile crust, high-temperature metamorphism, and cratonisation of relatively young crust. |
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| Keywords: | U–Pb zircon geochronology Geochemistry European variscan belt Bohemian massif Flat-slab subduction |
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