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40Ar/39Ar ages of crystallization and recrystallization of rock-forming polyhalite in Alpine rocksalt deposits

By
C. Leitner
C. Leitner
1
Department of Geography and Geology, University of Salzburg, Hellbrunner Straße 34, 5020 Salzburg, Austria
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F. Neubauer
F. Neubauer
1
Department of Geography and Geology, University of Salzburg, Hellbrunner Straße 34, 5020 Salzburg, Austria
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J. Genser
J. Genser
1
Department of Geography and Geology, University of Salzburg, Hellbrunner Straße 34, 5020 Salzburg, Austria
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S. Borojević-Šoštarić
S. Borojević-Šoštarić
2
Faculty of Mining Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6 p.p. 390 Zagreb, HR-10000, Croatia
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G. Rantitsch
G. Rantitsch
3
Department of Applied Geosciences and Geophysics, Mining University of Leoben, Peter-Tunner-Straße 5, 8700 Leoben, Austria
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Published:
January 01, 2014

Abstract

Polyhalite rocks of the evaporitic Haselgebirge Formation are emplaced within a rocksalt–mudrock tectonite. The rheologically weak series served as a major detachment level during nappe stacking of the Northern Calcareous Alps (Eastern Alps). To test the mineral polyhalite [K2Ca2Mg(SO4)4·2H2O] as a useful geochronometer for various diagenetic and deformation fabric types, 40Ar/39Ar age dating was combined with microstructural analysis. Vein infills, polyhalite intergrown with anhydrite and polyhalite within mudrock, crystallized in several stages between c. 235 and 210 Ma. Mylonites of fine-grained polyhalite rock indicate subsequent stages of tectonothermal overprint between c. 155 and 105 Ma, which is roughly consistent with previously measured feldspar and muscovite 40Ar/39Ar data from the region. Illite crystallinity points to temperatures of c. 200 °C. The peak temperature of overprint was at c. 180 °C in the Berchtesgaden mine (vitrinite reflectance, fluid inclusions) and >240 °C in the Altaussee mine (fluid inclusions). These temperatures are below the value of 255 °C, where polyhalite starts to dehydrate. Disturbed age spectra patterns result from multiphase polyhalite growth; however, single phases and completely recrystallized fabrics yield good results. As in the Alpine test case, polyhalite may characteristically serve as a geochronometer for diagenetic and very-low-grade metamorphic processes.

Supplementary material:

A table of used materials and methods and the detailed 40Ar/39Ar step-heating data for polyhalite are available at: http://www.geolsoc.org.uk/SUP18574.

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Contents

Geological Society, London, Special Publications

Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences

F. Jourdan
F. Jourdan
Curtin University, Australia
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D. F. Mark
D. F. Mark
Scottish Universities Environmental Research Centre, UK
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C. Verati
C. Verati
University of Nice, France
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Geological Society of London
Volume
378
ISBN electronic:
9781862396623
Publication date:
January 01, 2014

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