Formation of low-pressure reaction textures during near-isothermal exhumation of hot orogenic crust (Bohemian Massif, Austria)

Dominik Sorger; Christoph A Hauzenberger; Fritz Finger; Manfred Linner; Etienne Skrzypek; Simon Schorn

doi:10.1111/jmg.12744

Formation of low-pressure reaction textures during near-isothermal exhumation of hot orogenic crust (Bohemian Massif, Austria)

J Metamorph Geol. 2024 Jan;42(1):3-34. doi: 10.1111/jmg.12744. Epub 2023 Sep 14.

Authors

Dominik Sorger^{1

2}, Christoph A Hauzenberger¹, Fritz Finger^{1

3}, Manfred Linner⁴, Etienne Skrzypek¹, Simon Schorn¹

Affiliations

¹ Institute of Earth Sciences, NAWI Graz Geocenter University of Graz Graz Austria.
² Geoscience Center University of Goettingen Goettingen Germany.
³ Department of Environment & Biodiversity University of Salzburg Salzburg Austria.
⁴ Competence Unit Hard Rock Geology GeoSphere Austria Vienna Austria.

Abstract

Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet-biotite-sillimanite-plagioclase-K-feldspar-quartz-granitic melt ± kyanite ± ilmenite ± rutile, recording peak metamorphic conditions of $\sim$ 0.9-1.1 GPa and $\sim$ 780-820°C estimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite-spinel and cordierite-quartz reaction textures at former garnet-sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching $\sim$ 0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite-spinel and cordierite-quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4-0.5 GPa and 750-800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP-UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at $\sim$ 340 Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of $\sim$ 335 Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.

Keywords: Bohemian Massif; Crd‐Spl symplectite; granulite exhumation; isothermal decompression; reaction textures.