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Deformation-induced resetting of Rb/Sr and (super 40) Ar/ (super 39) Ar mineral systems in a low-grade, polymetamorphic terrane (Eastern Alps, Austria)

Wolfgang Mueller, R. David Dallmeyer, Franz Neubauer and Martin Thoeni
Deformation-induced resetting of Rb/Sr and (super 40) Ar/ (super 39) Ar mineral systems in a low-grade, polymetamorphic terrane (Eastern Alps, Austria)
Journal of the Geological Society of London (March 1999) 156, Part 2: 261-278

Abstract

A paradox in dating metamorphic events in low-grade, polymetamorphic terranes is exemplified by the eastern Lower Austroalpine nappes of the Eastern Alps. Here, the last metamorphic event, best recorded in post-Variscan cover rocks, is dated as Late Cretaceous in age (c. 80-85 Ma) by white mica Rb/Sr and (super 40) Ar/ (super 39) Ar systems. Within the underlying polymetamorphic basement, (super 40) Ar/ (super 39) Ar and Rb/Sr ages of phengitic white mica record only Early and/or Late Variscan ages (375-270 Ma), indicating that the Alpine greenschist facies metamorphic overprint virtually caused no rejuvenation of Variscan mineral ages. Based on these results, the timing of a penetrative, ductile, top-to-WNW simple shear deformation recorded within both basement and cover rocks was contradictory. Deformation within the post-Variscan cover rocks had to be Alpine in age, whereas phengite (super 40) Ar/ (super 39) Ar ages from basement mylonites yield Variscan ages. To date this deformation directly, we isolated different mineral size fractions (63-30 and 30-10 mu m) from highly strained shearbands within the Wechsel basement nappe. A resulting Rb/Sr errorchron pointed to an Upper Cretaceous age (c. 85 Ma) for the deformation, consistent with the timing of Alpine metamorphism in the cover rocks. Coarser-grained white mica (150-300 mu m) from similar basement mylonites do not reflect any Alpine overprint of either K/Ar and/or Rb/Sr systems. It follows that dynamic re-and/or neocrystallization induced by ductile deformation within the shearbands was the dominant process by which the Rb/Sr system locally virtually re-equilibrated. This is valid even for overprinting metamorphic conditions below the temperatures required for Ar diffusional loss in phengitic white mica (c. <350 degrees C). The data suggest that mineral ages that date low-grade mylonitization (e.g., white mica (super 40) Ar/ (super 39) Ar) should be considered with caution.


ISSN: 0016-7649
EISSN: 2041-479X
Coden: JGSLAS
Serial Title: Journal of the Geological Society of London
Serial Volume: 156, Part 2
Title: Deformation-induced resetting of Rb/Sr and (super 40) Ar/ (super 39) Ar mineral systems in a low-grade, polymetamorphic terrane (Eastern Alps, Austria)
Affiliation: University of Vienna, Institute of Geology, Vienna, Austria
Pages: 261-278
Published: 199903
Text Language: English
Publisher: Geological Society of London, London, United Kingdom
References: 42
Accession Number: 1999-030541
Categories: Igneous and metamorphic petrologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 10 tables, geol. sketch map
N47°00'00" - N48°00'00", E16°15'00" - E17°30'00"
Secondary Affiliation: University of Georgia, USA, United StatesUniversity of Salzburg, AUT, Austria
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 199911
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