Coesite in clinopyroxene in the Caledonides and its implications for geodynamics

Orthopyroxene- and kyanite-lineage eclogites are distinguished on the basis of silicate and carbonate petrography and chemistry. Different textural and chemical types of magnesite, dolomite and calcite can be satisfactorily attributed to one of the three sequential metamorphic stages of eclogite history at anhydrous eclogite facies, hydrous eclogite facies and amphibolite facies. The chronological sequence of development of magnesite, dolomite and calcite matches the carbonate sequence found experimentally in synthetic peridotite-CO 2 -H 2 O systems during decompression. Magnesite and dolomite show reasonably regular Fe/Mg distribution relationships with silicates. A new geothermobarometer based upon is provisionally calibrated. It yields T estimates for given P very similar to those given by the geothermobarometer, e.g. 759–829°C at 20 kb and 803–894°C at 40 kb. P is tentatively estimated at 30±10, 23±6 and 10 ± 3 kb respectively for the three metamorphic stages. Fluid conditions after equilibration during each stage were either low or fluid-absent, but not low . A unique quartz-magnesite-dolomite vein represents an influx of CO 2 in the P-T regime of instability of CO 2 + pyroxene. Tectonic emplacement by deep-level obduction of diverse foreign source materials remains the petrogenetic model which best fits the data for these eclogites.