Comparison of metamorphic evolution and tectonic position of metasediments of the Austevoll islands with rocks from the Samnanger Complex (Major Bergen Arc, Norwegian Caledonides)
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Metasedimentary rocks represent a substantial component of the Bergen Arcs System and they are mostly confined to the Minor and Major Bergen Arcs. The arcs represents imbricate structures composed of island arc lithologies, prism sediments and gneissic continental substrate thrust, one upon another during the Caledonian Orogeny. In the westernmost Austevoll, surrounded by the massive Sunnhordland Batholith, islands consisting of metasediments occurs. Due to their location, their tectonic position is somewhat enigmatic with respect to the nearby nappes and they have only been vaguely correlated with the metasediments of the Major Bergen Arc. In order to gain insight in the metamorphic evolution and the tectonic position of the metasediment in Austevoll, a petrological study on garnet-bearing mica-schists from the island of Møkster has been carried out, complemented by thermodynamic modelling in order to constrain the pressure and temperature conditions at which the prograde assemblage of the rock formed. The same methods have been utilized on garnet-bearing micaschists collected from the Samnanger Complex of the Major Bergen Arc to compare them. The petrological study suggests that the metasediments all share common textures with respect to prograde metamorphism. The metasediments from Møkster experienced the highest metamorphic grade resulting in a mineral assemblage, containing staurolite, while the metasediments from the Samnanger Complex only contain garnet-bearing assemblages at the metamorphic peak. The PT conditions for the prograde assemblages could be constrained via thermodynamic modelling to 605-635 <degrees>C and 7.5-8.2 kbar for the metasediment from Møkster, and to 480-565 <degrees>C and 4.7-6.6 kbar for the metasediment from the Samnanger Complex. Corresponding depths were calculated to 30-31 km and 24-25 km, respectively. By calculating a geothermal gradient based on the above estimations, a common geotherm of 20-24 <degrees>C/km was found for the metasediments which is considered a «normal» geothermal gradient, associated with burrial of sediment along an active margin. From their similar textures and metamorphism along a common thermal gradient it is inferred that the metasediments on Møkster and in the Samnanger Complex may have been part of the same tectonic unit during prograde metamorphism. Thus, the tectonic position of the metasediments on Møkster may have been similar to that of the metasediments of the Samnanger Complex in the Major Bergen Arc.