Metamorphic paragenesis and P-T conditions in garnet amphibolite from the Median Segment of the Idefjorden Terrane, Lilla Edet
Abstract: The Sveconorwegian orogeny (c. 1 Ga) lead to the recrystallization of large parts of southwestern Sweden, with widespread metamorphism as well as local migmatization. The Idefjorden Terrane, which has been studied less extensively than the nearby Eastern Segment, is divided by the Göta Älv Shear Zone and the Dalsland Boundary Thrust into two main units; the Western and Median Segments. A garnet amphibolite located in the Median Segment, approximately 10 km east of the Göta Älv Shear Zone, was studied in order to get a better understanding of the P-T conditions of the area as well as the implications this has for the geological history of the Idefjorden Terrane. The metamorphic paragenesis and textures of the rock helped determine that the garnet amphibolite was close to equilibrium conditions during peak metamorphism and that the metamorphic minerals observed grew in the presence of partial melt. P-T calculations using the software Thermobarometry (GTB) showed that the core of the garnet grew under temperatures between 630 and 680°C and pressures between 9 and 10 kbar, while the temperature and pressure reached 680 to 750°C and 8.5 to 10 kbar during the growth of the garnet rim. The temperature intervals are consistent with observations suggesting that the rock underwent partial melting, and these P-T conditions correspond to upper amphibolite facies and a depth of between 31.5 and 37 km. Recent U-Pb data by Ingered (2019) of migmatites located close to the sample location gives a minimum age of migmatization of 1019±27 Ma in the Median Segment, which correlates well with the theory of the Idefjorden Terrane being buried to a depth of at least 35 km during the Agder phase (1.08-0.98 Ga; Bingen et al. 2008b; Ingered 2019). Evidence of local retrograde metamorphism, likely in the prehnite-pumpellyite facies, was also observed in the garnet amphibolite in the form of partial replacement of biotite by prehnite.
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