Schists and amphibolites of the kleinelendtal (ankogel-hochalm-gruppe/hohe Tauern, Austria)/new insights on the Variscan basement in the eastern Tauern Window

Author(s): Linda Lerchbaumer, Urs Kloetzli, Gerhard Pestal
Abstract: In the Kleinelendtal/Ankogel-Hochalm-Group (eastern Tauern Window) pre-Mesozoic units are well preserved. These units comprise Variscan orthogneisses (the Zentralgneise) and their host rocks, the "Pre-Zentralgneis-Complexes" (formerly "Altes Dach"). The "Pre-Zentralgneis-Complexes" comprise the "Zwischenelendschiefer" (biotite-schists, garnet-micaschists and sericite-schists) and amphibolites. Their contacts with the Zentralgneise are mostly concealed by debris- and rock flows or overprinted by shear zones and faults. Preserved primary contacts are rare. Characteristic of the biotite-schists are biotite-blasts of golden colour due to weathering. Besides this biotite, quartz, plagioclase, potassium feldspar, white mica, and chlorite also calcite, garnet, magnetite/ilmenite, titanite, and zircon are present. Its whole rock analysis shows an intermediate chemism. The calculated initial ⁸⁷Sr/ ⁸⁶Sr _{(360 Ma)}-value of 0.70676 ± 0.000004 and an εNd _{(360 Ma CHUR)} of -1.8 indicate that considerable amounts of juvenile crust/mantle material have been added to the sedimentary protolith. Detrital zircons from the biotite-schists yield an U/Pb - LA-MC-ICP-MS maximum age of sedimentation of 360 ± 13 Ma for the host sediment, confirming that the Zwischenelendschiefer do not date back to pre-Variscan times or even earlier. This suggests a close relationship with the units of the central Tauern Window, i.e. the so-called Biotitporphyroblastenschiefer of the Granatspitz-Group. Garnet-micaschists crop out adjacent to the biotite-schists. Major- and minor constituents are white mica, quartz, chlorite, and garnet; accessory minerals magnetite/ilmenite and titanite. The appearance of sericite-schists is limited to a few outcrops. They are leucocratic crumbly rocks that are made up of sericite, quartz, and biotite. The amphibolites show metamorphic banding and fine-grained mineral contents of hornblende, epidote-group minerals, and plagioclase, furthermore titanite, rutile, and magnetite/ilmenite. Main- and trace elements shows a basaltic chemism. The initial ⁸⁷Sr/ ⁸⁶Sr( _{360 Ma)} -ratio is 0.70525 ± 0.000004 and the _£Nd( _{360 Ma CHUR)} is +5.3. Thus the amphibolites are interpreted to be meta-magmatic. Furthermore also the amphibolites of the Kleinelendtal are geochemically similar to the Basisamphibolit of the central Tauern Window which again proposes a genetic relationship between these units. All lithologies of the Pre-Zentralgneis-Complexes have been overprinted in multiple phases; the latest thermal event was a retrograde greenschist-facies metamorphism which is evident by new growth of chlorites, epidote-group minerals and albite. Due to the Late Devonian maximum sedimentation age of the biotite-schists there is no evidence for the existence of pre-Variscan lithologic units within the Kleinelendtal, contrary to any former interpretations of the regional geology. Thus a position within the Variscan orogenic cycle is proposed for the genesis of some parts of the wall rocks of the Zentralgneise. This is in line with the dating-results of the Basisamphibolit and Biotitporphyroblastenschiefer in the central Tauern Window.
Organisation(s): Department of Lithospheric Research
External organisation(s): Universität Bayreuth, Geologische Bundesanstalt
Journal: Austrian Journal of Earth Sciences
Volume: 103
Pages: 138-152
No. of pages: 15
ISSN: 2072-7151
Publication date: 12-2010
Peer reviewed: Yes
Austrian Fields of Science 2012: 105120 Petrology, 105127 Geochronology
Keywords
ASJC Scopus subject areas: Geology, Stratigraphy, Palaeontology
Portal url: https://ucrisportal.univie.ac.at/en/publications/fe9591a0-abf3-4230-a099-e022a1fbd155

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