Pressure–temperature evolution of a kyanite–garnet pelitic gneiss from Åreskutan: evidence of ultra-high-pressure metamorphism of the Seve Nappe Complex, west-central Jämtland, Swedish Caledonides

Abstract

Abstract New evidence is presented for ultra-high-pressure metamorphism of kyanite–garnet pelitic gneiss in the Åreskutan Nappe of the Seve Nappe Complex, in the central part of the Scandinavian Caledonides. Modelled phase equilibria for a peak pressure assemblage garnet+phengite+kyanite+quartz (coesite) in the NCKFMMnASH system record pressure and temperature conditions of c. 26–32 kbar at 700–720 °C, possibly up to ultra-high-pressure conditions. Subsequent decompression, simultaneous with an increase of temperature to c. 800–820 °C, led to partial melting largely owing to the dehydration and breakdown of phengite. Based on existing isotope age data, we conclude that the Middle Seve Nappe in central Jämtland experienced deep subduction in the late(st) Ordovician, prior to decompression and partial melting of the pelitic protoliths during Early Silurian extrusion, giving way in the Mid to Late Silurian to thrusting on to the Baltoscandian platform. Nappe emplacement probably continued into and through the Early Devonian.