The evolution of illite to muscovite: mineralogical and isotopic data from the Glarus Alps,Switzerland |
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| Authors: | J C Hunziker M Frey N Clauer R D Dallmeyer H Friedrichsen W Flehmig K Hochstrasser P Roggwiler H Schwander |
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| Institution: | (1) Abteilung für Isotopengeologie, Erlachstrasse 9a, CH-3012 Bern, Switzerland;(2) Mineralogisch-Petrographisches Institut, Bernoullistrasse 30, CH-4056 Basel, Switzerland;(3) Centre de Sédimentologie et Géochimie de la Surface, 1 rue Blessig, F-67084 Strasbourg Cedex, France;(4) Department of Geology, University of Georgia, 30602 Athens, Georgia, USA;(5) Mineralogisch-Petrographisches Institut, Wilhelmstrasse 56, D-7400 Tübingen, Federal Republic of Germany;(6) Sediementpetrographisches Institut, Goldschmidt-Strasse 1, D-3400 Göttingen, Federal Republic of Germany;(7) Institut für anorganische Chemie, Freiestrasse 3, CH-3012 Bern, Switzerland;(8) Brown Boveri & Co. Research Centre, CH-5401 Baden, Switzerland |
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| Abstract: | Thirty-five illite and muscovite concentrates were extracted from Triassic and Permian claystones, shales, slates and phyllites along a cross-section from the diagenetic Alpine foreland (Tabular Jura and borehole samples beneath the Molasse Basin) to the anchi- and epimetamorphic Helvetic Zone of the Central Alps. Concentrates and thin sections were investigated by microscopic, X-ray, infrared, Mössbauer, thermal (DTA and TG), wet chemical, electron microprobe, K-Ar, Rb-Sr, 40Ar/39Ar and stable isotope methods.With increasing metamorphic grade based on illite  crystallinity data (XRD and IR) the following continuous changes are observed: (i) the 1Md 2M1 polymorph transformation is completed in the higher grade anchizone; (ii) K2O increases from 6–8 wt. % (diagenetic zone) to 8.5–10% (anchizone) to 10–11.5% (epizone), reflecting an increase in the total negative layer charge from 1.2 to 2.0; (iii) a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone/ epizone boundary; iv) a shift of an endothermic peak in differential thermal curves from 500 to 750° C; (v) K-Ar and Rb-Sr apparent ages of the fraction <2  m decrease from the diagenetic zone to the epizone, K-Ar ages being generally lower than Rb-Sr ages. The critical temperature for total Ar resetting is estimated to be 260±30° C. K-Ar and Rb-Sr ages become concordant when the anchizone/ epizone boundary is approached. The stable isotope data, on the other hand, show no change with metamorphic grade but are dependent on stratigraphic age.These results suggest that the prograde evolution from 1 Md illite to 2M1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals, however this is not a recrystallization process. This restructuration is completed approximately at the anchizone/epizone boundary. The isotopic data indicate significant diffusive loss of 40Ar and 87Sr prior to any observable lattice reorganization. The restructuration progressively introduces a consistent repartition of Ar and K in the mineral lattices and is outlined by the 40Ar/39Ar age spectra.Concordant K-Ar and Rb-Sr ages of around 35-30 Ma. with concomitant concordant 40Ar/39Ar release spectra are representative for the main phase of Alpine metamorphism (Calanda phase) in the Glarus Alps. A second age group between 25 and 20 Ma. can probably be attributed to movements along the Glarus thrust (Ruchi phase), while values down to 9 Ma., in regions with higher metamorphic conditions, suggest thermal conditions persisting at least until the middle Tortonian. |
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