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 the granodioritic country rock is represented by well developed CL-dark zircon rims seen in two samples, dated at 1426±18 and 1415±15 Ma. The protolith age of the country rock is 1701±10 Ma from zircon cores. Two related molybdenite samples gave ages at 957±4 and 949±4 Ma respectively, representing either protracted amphibolite facies conditions or a low grade 230-320°C alteration event. The textural and mineralogical features, together with the calculated P-T conditions show that the mafic boudins at Viared were subjected to eclogite facies conditions at ∼50 km depth. The evidence for high-P metamorphism at Viared, together with other known occurrences at Ullared, Skene and Kedum, shows that a significant part of the Eastern Segment was subjected to eclogite conditions.