Metamorphic Evolution of the Middle Seve Nappe in the Snasahögarna area, Scandinavian Caledonides

Abstract: In the Snasahögarna area, situated in the central Scandinavian Caledonides, the Middle Seve Nappe Complex mainly consists of pelitic gneisses. Within these garnet and kyanite bearing gneisses the first discovery of metamorphogenic diamond in Sweden has been confirmed by micro raman spectroscopy. The garnet hosted diamond together with associated carbonate and fluid inclusions were deposited by carbon saturated fluids at a pressure exceeding 3.1 GPa. The peak mineral assemblage, Phg Mgs-Sid Grt Ky Jd Rt Coe (Dia), is constrained by microscopic observations and thermodynamic phase equilibrium modeling. Modeled garnet cores equilibrated at c. 750°C and 1.3 GPa together with phengite rims. This early exhumation stage was followed by heating reaching a peak temperature of 880-910°C at 1.05-1.18 GPa. Partial melting following the increased temperature is confirmed by anatectic segregations and microscopic melt related textures in the paragneiss. A fluid restricted environment at the late high temperature stage is inferred from mineral textures, mineral chemistry and low bulk rock loss on ignition. This is in agreement with thermodynamic models.