Eclogites in the central part of the Sveconorwegian Eastern Segment of the Baltic Shield: Support for an extensive eclogite terrane

Abstract Mafic boudins found at Viared in the central Sveconorwegian Eastern Segment of the Baltic Shield display both mineralogical and textural features demonstrating that these rocks were once eclogites. The mafic boudins are hornblende-plagioclase amphibolite, but the interiors show evidence of retrogression from eclogite: a grid-like pattern in clinopyroxene grains containing exsolved plagioclase from former omphacite, and garnets commonly surrounded by plagioclase coronas. Geothermobarometry was carried out on two samples and the peak pressure conditions were estimated by re-integrating the original clinopyroxene (Jd23) composition, giving 15.0-16.7 kbar and temperatures of 719 to 811°C. Calculations using the existing retrograde assemblage of clinopyroxene-garnet-plagioclase-quartz give values of 10.5-12.5 kbar and 700-770°C. A 974±3 Ma Re-Os age on molybdenite and 961±26 Ma from titanite represent a minimum age for boudin formation and retrogression. Pre-Sveconorwegian regional migmatization in th...