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Solid Earth An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/se-2018-18
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
12 Mar 2018
Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Solid Earth (SE) and is expected to appear here in due course.
Ca-rich garnets and associated symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria
Konstantin Petrakakis1, Nathalie Schuster-Bourgin2, Gerlinde Habler2, and Rainer Abart2 1University of Vienna, Department of Geodynamics and Sedimentology
2University of Vienna, Department of Lithospheric Research
Abstract. Mafic peraluminous granulites associated with the mantle-derived peridotites of the Dunkelsteiner Wald provide evidence of the tectono-metamortphic evolution of rocks in the Gföhl Nappe System, Austria. They contain the primary assemblage garnet + Al-rich-clinopyroxene + kyanite. Large Ca-and Mg-rich garnets are embedded in a granoblastic matrix of Al-rich-clinopyroxene, Ca-rich-plagioclase and minor hornblende. They have been partially replaced by different, locally controlled symplectites of (a) corundum + sapphirine + spinel + plagioclase formed around kyanite inclusions, (b) orthopyroxene + spinel + plagioclase ± hornblende formed at their rims and (c) clinopyroxene + orthopyroxene + spinel + plagioclase ± hornblende formed within cracks. Garnets are built up from repeatedly occurring garnet types characterized by specific component distributions. Areal extend and cross-cutting relations allowed for the derivation of the relative timing of garnet types formation. Recrystallization and compositional readjustment of the reactive garnet volume during symplectite formation has led to the development of pronounced, secondary diffusion-induced zoning profiles overprinting the different garnet types and post-dating the complex garnet compositional structure. Thermodynamic analysis showed that none of the garnet types represents a preserved equilibrium composition. Furthermore, the latest garnet types show evidence of metasomatic (Fe+Mg)-loss affecting their Ca-content. The primary assemblage is stable between 760 and 880 °C and pressures > 11 kbar. The crack-symplectites are almost isochemical to the oldest garnet type and have been formed above 730 °C and pressures between 7.5 and 5 kbar.

The studied rocks have undergone a more or less isothermal decompression from pressures above 11 kbar to ~ 6 kbar at temperatures about 800 °C. Crack- and rim-symplectites have been formed after decompression during approximately isobaric cooling under conditions of low differential stress. Due to limited availability of fluids promoting symplectite formation, the time-scale of symplectite formation calculated from secondary diffusion profiles associated with crack-symplectites is shown to be geologically very short (< 0.5 ka).

Citation: Petrakakis, K., Schuster-Bourgin, N., Habler, G., and Abart, R.: Ca-rich garnets and associated symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria, Solid Earth Discuss., https://doi.org/10.5194/se-2018-18, in review, 2018.
Konstantin Petrakakis et al.
Konstantin Petrakakis et al.

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Short summary
Aluminum-rich granulites from Austria contain large garnets with kyanite inclusions. Garnets are built up from compositionally different types. They are replaced partially by various symplectites. Thermodynamic analysis points to metasomatic alteration and rock decompression due to tectonic transport at high temperatures from deep to middle crustal levels. The various symplectites were formed after decompression during cooling within a short time interval of less than 500 years.
Aluminum-rich granulites from Austria contain large garnets with kyanite inclusions. Garnets are...
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