Jadeite-gneiss from the Eclogite Zone,Tauern Window,Eastern Alps,Austria: Metamorphic,geochemical and zircon record of a sedimentary protolith |
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| Institution: | 1. Institute of Geological Sciences, University of Wroclaw, 50-204 Wroclaw, Pl. Borna 9, Poland;2. John de Laeter Centre for Isotope Research, Curtin University, GPO Box U1987 Perth, Western Australia 6845, Australia;1. Universität Bonn, Steinmann-Institut, Poppelsdorfer Schloss, D-53115 Bonn, Germany;2. Georg-August-Universität, Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Goldschmidtstraße 1, D-37073 Göttingen, Germany;3. Steinmann-Institut, Universität Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany;4. Universitat Köln, Geowissenschaften, Geo-/Kosmochemie, Greinstr. 4-6, Gebäude 902, D-50939 Köln, Germany;5. Karl-Franzens Universität Graz, Institut für Erdwissenschaften, Heinrichstraße 26, A- 8010 Graz, Austria;1. Departamento de Mineralogía y Petrología, Universidad de Granada, Campus de Fuentenueva 18002 Granada, Spain;2. Department of Geology and Natural Resources, Institute of Geosciences, University of Campinas – UNICAMP, 13083-970 Campinas, SP, Brazil;3. UA Petrología Experimental, CSIC-Universidad de Huelva, Facultad de Ciencias Experimentales, 21071 Huelva, Spain;4. IGG Russian Academy of Science, Ekaterinburg, Russia |
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| Abstract: | In the Eclogite Zone of the Tauern Window, a layer of strongly retrogressed leucocratic jadeite-bearing gneiss is intercalated between mafic kyanite-eclogites. The jadeite-gneiss consists of garnet + quartz + paragonite ± phengite ± zoisite + zircon + rutile + apatite + pyrite. Kyanite, jadeite or omphacite are exclusively present as inclusions in garnet. Retrogressive hydration during exhumation led to a breakdown of matrix jadeite to form pseudomorphs of calcic amphibole + albite. Peak metamorphic conditions derived from the primary gneiss assemblage are 2.0–2.4 GPa at approximately 640 °C. Major, trace element and isotopic compositions of the jadeite-gneiss are consistent with a siliciclastic sedimentary protolith. Zircon morphology and zonation patterns reveal a complex history. The presence of fracture-truncated zircons suggests a detrital origin, whereas most internal structures and Th/U ratios are characteristic of zircons from magmatic rocks. In situ LA-ICP-MS and SHRIMP U–Pb geochronology and zircon geochemisty provide evidence of at least three magmatic events in the provenance area. These were dated at 466 ± 2 Ma, 437 ± 2 Ma and 288 ± 9 Ma. Older ages ranging from 503 to 691 Ma are preserved in the cores of some zircon grains, suggesting derivation from peri-Gondwanan sources. Surprisingly, no firm evidence of the Tertiary high-pressure metamorphic event and subsequent retrograde overprint was seen in any of the studied zircons. However, some zircons show resorbed surfaces suggesting corrosion by a superficial fluid phase undersaturated in zirconium and one extensively altered porous zircon yielded highly discordant 206Pb/238U ages in the range 325–109 Ma documenting partial recrystallization by dissolution–reprecipitation of a highly reactive grain. |
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