Fluid evolution during metamorphism at increasing pressure: carbonic- and nitrogen-bearing fluid inclusions in granulites from �ksfjord, north Norwegian Caledonides

Abstract

Three successive metamorphic stages M1, M2 and M3 have been distinguished in polymetamorphic granulite facies quartz-feldspathic gneisses from the Seiland Igneous Province, Caledonides of northern Norway. An early period of contact metamorphism (M1; 750–950°C, ca. 5 kbar) was followed by cooling, accompanied by strong shearing and recrystallization at intermediate-P granulite facies conditions (M2; 700–750°C, 5–6kbar). High-P granulite facies (M3; ca. 700°C, 7–8 kbar) is related to recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. On the basis of composition, fluid inclusions in cordierite, quartz and garnet can be divided into three major types: (1) CO2 inclusions; (2) mixed CO2−N2 inclusions; (3) N2 inclusions. Fluid chronology and mineral assemblages suggest that the earliest inclusions consist of pure CO2 and were trapped at the M1 contact metamorphic episode. A carbonic fluid was also present during the intermediate-P granulite facies M2 metamorphism. The CO2-rich inclusions in M2 garnet can be divided into two generations, an early lower-density and a late higher-density, with isochores crosscutting the P-T box of M2 and M3, respectively. The nitrogen-rich fluids were introduced at a late stage in the fluid evolution during the high-P M3 event. The mixed CO2−N2 inclusions, with density characteristics compatible with M3 conditions, are probably produced from intersection between pre-existing pure CO2 inclusions and N2 fluids introduced during M3. The fluid inclusion data agree with the P-T evolution established from mineral assemblages and mineral chemistry.