Challenges in high-pressure granulite metamorphism in the era of pseudosections: reaction textures, compositional zoning and tectonic interpretation with examples from the Bohemian Massif |
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| Authors: | P J O'BRIEN |
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| Institution: | Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, D-14476 Golm, Germany () |
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| Abstract: | High‐pressure granulites are an important record of geodynamic processes in overthickened or subducted continental crust. Orthopyroxene‐free assemblages in granitic (ternary feldspar(s) + quartz + garnet + kyanite + rutile), intermediate (ternary feldspar(s) + quartz + garnet + clinopyroxene ± kyanite ±rutile ± titanite) and basic (garnet + clinopyroxene + plagioclase ± quartz + rutile) compositions indicate formation conditions at mantle depths. Clinopyroxene compositions in Variscan high‐pressure granulites are unusual in that they include omphacite (in plagioclase‐bearing rocks thus not eclogite) and Al‐rich diopside (i.e. indicating high Ca‐Tschermak content), with both yielding temperatures above 900 °C. Problems such as compositional zoning, multiple generations of key phases in reaction domains and unmixing of high‐temperature solid‐solution phases during cooling (ternary feldspars, omphacite) clearly indicate disequilibrium and require very careful interpretation as to which phases and compositions possibly represent a former equilibrium association. Pressure–temperature (P–T) determination by the pseudosection method, although allowing prediction of mineral assemblages, compositions and molar proportions for a fixed bulk composition for modelled P–T conditions, still requires reliable activity–composition information for the key phases feldspar and clinopyroxene as well as an interpretation of former equilibrium compositions in the investigated samples, i.e. the same restrictions applying to conventional thermobarometry. The interpretations of some recently determined pseudosections for the composition of Variscan clinopyroxene‐bearing high‐pressure granulites contradict numerous published P–T paths. However, quantitative information from thermobarometry or pseudosections must be integrated with key petrographic observations. In the case of the Variscan example, it is argued that petrographic observations and published P–T paths are consistent with mineral assemblages predicted in pseudosections and support existing tectonometamorphic models. |
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| Keywords: | clinopyroxene-bearing granulite disequilibrium high-pressure granulite mesoperthite reaction texture |
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