Thermobarometry and Geotectonic Significance of High-Pressure Granulites: Examples from the Moldanubian Zone of the Bohemian Massif in Lower Austria |
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| Authors: | CARSWELL D A; O'BRIEN P J |
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| Institution: | 1Department of Earth Sciences, University of Sheffield Beaumont Building, Brookhill, Sheffield S3 7HF, UK
2Bayerisches Geoinstitut, Universitt Bayreuth Postfach 101251, W-8580 Bayreuth, Germany |
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| Abstract: | Petrographic details together with mineral and whole-rock compositiondata are provided for acid-intermediate garnet granulites fromexposed granulite complexes in Lower Austria. Thermobarometricevaluation integrated with available isotopic age data indicatesthe initial equilibration of early Variscan ({small tilde}370Ma) high-pressure granulite assemblages at {small tilde}16 kbarand 1000C and their partial overprinting by retrograde assemblageswhich reflect the blocking of mineral exchange reaction equilibriaat {small tilde}6•5 kbar and 725C during subsequent Variscanuplift and cooling. These calculated P-T estimates, togetherwith general phase equilibria constraints and evidence frompreserved prograde coronitic reaction textures and garnet compositionalzoning profiles, indicate a clockwise P-T-t evolutionary pathof the type expected during crustal thickening in a major platecollision orogen and characterized by near-isothermal decompressionduring initial uplift. Geochemical characterization of the rockprotoliths as calc-alkaline igneous rocks and the high metamorphictemperatures suggest that garnet granulite formation involvedthe subduction of a magmatic arc at a continental plate margin. Reviewed evidence from granulites in the Central European Variscidesruns counter to suggestions by Bohlen (1987, 1991) that high-pressuregranulites are of little regional geotectonic significance incomparison with low- to medium-pressure granulites. The differentevolutionary P-T paths for these two important groups of granulitespoint to formation in contrasting plate settings. However, questionsare raised regarding petrogenetic models for low- to medium-pressuregranulites which have emphasized the importance of magmatic,rather than tectonic, crustal thickening and the recognitionof stabilization along deduced anti-clockwise P-T-t paths characterizedby post-peak near-isobaric cooling. It is suggested here that the reality of stabilization of atleast some low- to medium-pressure granulites in a collisionaltectonic regime may have been concealed either because lower-pressureassemblages have overprinted mineralogical evidence for an earlierhigh-pressure history at deeper crustal levels or through invaliddeduction of near-isobaric cooling trajectories as a resultof the different closure temperatures for the mineral reactionsused to monitor the equilibration temperatures and pressuresin granulites. However, the sequential underthrusting modelfavoured for the tectonometamorphic evolution of the Variscannappe pile in the Bohemian Massif renders it unlikely that alllate Variscan low- to medium-pressure granulites have experiencedthe early Variscan high-pressure metamorphism. |
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