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Geothermobarometry of Al2SiO5-bearing metapelites in the western Austroalpine Ötztal-basement

Geothermobarometrie in Al2SiO5 führenden Metapeliten des westlichen Ötztal Kristallins

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Summary

The dominant amphibolite-facies Variscan event in the Austroalpine Ötztal basement can best be studied in the northwestern part of the Ötztal block. Further to the southeast it is overprinted by Alpine metamorphism. Metapelites with the assemblage garnet-staurolite-kyanite-sillimanite±andalusite-biotite-muscovite-plagioclase were used to reconstruct pressure and temperature conditions with exchange thermometry, net transfer equilibria and multi-equilibrium methods. Assuming kyanite as equilibrium Al2SiO5 polymorph, conditions of 570–640°C and 5.8–7.5kbar are derived using garnet rim compositions. Typical nonequilibrium textures are (1) continuous chemical zoning of garnets, (2) inclusions of kyanite and fibrolite in andalusite porphyroblasts and (3) the spectacular replacement of garnet by fibrolite and biotite. The latter two textures were used to decipher the retrograde part of the P-T path. Application of the differential thermodynamics approach (Gibbs method) indicates prograde garnet growth during pressure release. Addition of CaO to the KFASH-system allows the garnet breakdown within the staurolite stability field and its quantification, using the quartz-garnet-aluminosilicate-muscovite geothermobarometer, revealed temperatures of 530–630°C and 3.5–5.7kbar. Andalusite formation is thought to constrain the final stage of the P-T path. Textural and chemical data clearly indicate a continuous pre-Alpine metamorphic evolution.

Zusammenfassung

Die dominierende variszische Metamorphose im ostalpinen Ötztal Kristallin kann aufgrund der Zunahme der alpidischen Überprägung nach Südosten am besten im nordwestlichen Teil des Ötztal Kristallins untersucht werden. Die Metamorphosebedingungen wurden in den Metapeliten anhand der Paragenese Granat-Staurolith-Kyanit-Sillimanit±Andalusit-Biotit-Muskovit-Plagioklas mit Hilfe von Kationenaustauschthermometrie, kontinuierlichen Mineralreaktionen und der Berechnung aller formulierbaren Mineral-Gleichgewichte durchgeführt. Mit der Annahme von Kyanit als Teil der Gleichgewichtsparagenese wurden Temperaturen von 570–640°C und Drucke von 5.8–7.5kbar für die Granatränder ermittelt. Folgende Ungleichgewichtsphänomene wurden zur Belegung des Metamorphosepfades herangezogen: (1) Kontinuierlicher Granatzonarbau (2) Andalusitporphyroblasten mit Kyanit-und Fibrolitheinschlüssen und (3) die Reaktion von Granat zu Biotit und Fibrolith Pseudomorphosen. Die Anwendung der Gibbs-Methode lässt Rückschlüsse auf ein progrades Granatwachstum bei fallenden Drücken und steigenden Temperaturen zu. Die Erweiterung des KFASH-Systems mit CaO ermöglicht den Granatzerfall noch im Stabilitätsfeld von Staurolith und die Anwendung des Granat-Muskovit-Aluminiumsilikat-Quarz Geothermobarometers belegt den Granatzerfall bei 530–630°C und 3.5–5.7kbar. Die Andalusitkristallisation wird als letztes Stadium des Druck-Temperatur Pfades angenommen. Texturelle und chemische Untersuchungen lassen den Schluss auf eine einphasige variszische Metamorphose zu.

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  1. P. Tropper

    Present address: Department of Geological Sciences, Ann Arbor, Mi, USA

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  1. Institut für Mineralogle-Kristallographie und Petrologie, Universität Graz, Universitätsplatz 2, A-8010, Graz, Austria

    P. Tropper & G. Hoinkes

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  2. G. Hoinkes
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Tropper, P., Hoinkes, G. Geothermobarometry of Al2SiO5-bearing metapelites in the western Austroalpine Ötztal-basement. Mineralogy and Petrology 58, 145–170 (1996). https://doi.org/10.1007/BF01172094

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