Phyllonite Formation and Alteration of Gneisses in Shear Zones (Gleinalmkristallin, Eastern Alps/Austria) (original) (raw)

Summary

The leucophyllite (quartz-muscovite-chlorite-phyllonites) deposit Weißkirchen is located within the Core-Complex of the “Gleinalmkristallin” which is part of the Austroalpine Crystalline Unit. Retrograde metamorphism and shearing responsible for the formation of the phyllonites took place during the Alpine (Upper Cretaceous) event. The age of the micas from the leucophyllites and from the unaltered host rocks is 79.6 ± 0.8 Ma. The positive correlation of the HREE, A1203, TiO2, Zr and Nb versus Mg show that these elements are enriched together with Mg. On the other hand SiO2, CaO, Sr and alkali elements decrease with advanced alteration. This source for the formation of the phyllonites may be of minor importance. During the alteration and the decomposition of the plagioclase Eu is removed from the system.

Oxygen isotope mineral equilibria for the host rock paragenesis (quartz-muscovitegarnet) yielded temperatures in the range from 623 to 642 °C which is interpreted as the temperature of the Variscan metamorphism. The biotite of these gneisses has evidently been reset by later metamorphic events.

The investigated phyllonites in the shear zone represent totally reequilibrated rocks under Early Alpine metamorphic conditions. Oxygen isotope analyses yielded concordant temperatures of 480°C for the formation of the peak metamorphic paragenesis muscovite, chlorite and rutile. Slightly lower temperatures (approx. 450°C) were obtained by the measured fractionations between phlogopite, plagioclase and quartz (late stage metamorphic assemblage). Calculation of the oxygen isotope composition of the coexisting fluid for Alpine peak metamorphism yielded a mean 0180-value of + 7.5‰.

Zusammenfassung

Die Bildung der Leukophyllite in Scherzonen in den Ostalpen

Als Leukophyllite werden in den Ostalpen Gesteine bezeichnet, die aus Muskovit, Chlorit und Quarz bestehen und die in Scherzonen unter retrograd-metamorphen Bedingungen entstehen. In der Lagerstätte Weißkirchen in Steiermark werden diese Gesteine abgebaut und hauptsächlich als mineralischer Füllstoff verwendet.

Die Lagerstätte liegt im Bereich der Gleinalm, die zum Ostalpinen Kristallin gehört. Die Hauptmetamorphose der Nebengesteine ist variszischen Alters und fand unter amphibolitfaziellen Bedingungen (630°C) statt. Die Leukophyllite wurden während der alpidischen Metamorphose gebildet, O-Isotopenanalysen ergaben Temperaturen von 480°C. Die K/Ar- Abkühlalter der Hellglimmer der Nebengesteine und der Leukophyllite liegen bei ca. 80 Ma.

Während der Bildung dieser Scherzonen fand eine vollständige Rekristallisation statt, in den Leukophylliten können keine Relikte des älteren Mineralbestandes beobachtet werden. “Stabile Elemente” wie z.B. die SSEE, A1203, TiO2 etc. und auch MgO wurden mit fortschreitender Alteration relativ angereichert, während SiO2, CaO und die Alkalien teilweise mobilisiert wurden. Dieser Mechanismus kann zumindest z.T. zur Erklärung der hohen MgO-Gehalte der Leukophyllite herangezogen werden.

Die O-Isotopenzusammensetzung der koexistierenden Fluide liegt bei +7,5‰ in den Leukophylliten und entspricht metamorphogenen Lösungen, die während des jeweiligen Höhepunktes der Metamorphose mit den Leukophylliten (alpidisch) bzw. mit den Gneisen des Nebengesteins (variszisch) im Gleichgewicht standen.

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Authors and Affiliations

1. Institut für Geowissenschaften, Montanuniversität Leoben, A-8700, Leoben, Austria
   W. Prochaska
2. Mineralogisch-Petrologisches Institut der Universität Bonn, Poppelsdorfer Schloß, D-W-5300, Bonn 1, Germany
   A. Bechtel
3. Abteilung für Isotopengeologie, Universität Bern, Erlachstrasse 9a, CH-3012, Bern, Switzerland
   U. Klötzli

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1. W. Prochaska
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2. A. Bechtel
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3. U. Klötzli
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Prochaska, W., Bechtel, A. & Klötzli, U. Phyllonite Formation and Alteration of Gneisses in Shear Zones (Gleinalmkristallin, Eastern Alps/Austria).Mineralogy and Petrology 45, 195–216 (1992). https://doi.org/10.1007/BF01163112

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• Received: 24 October 1991
• Accepted: 10 March 1992
• Issue Date: September 1992
• DOI: https://doi.org/10.1007/BF01163112

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