FLUID INCLUSIONS IN GRANULITE-FACIES METASEDIMENTS FROM THE PROTEROZOIC LOWER CRUST IN THE HISOY-TORUNGEN AREA, BAMBLE SECTOR, SOUTHERN NORWAY

Thermodynamic modelling, fluid inclusion microthermometry and Raman sp ectroscopy has been performed on metasediments from the granulite faci es Bamble Sector, southern Norway. The calculations suggest that the m etapelites were in equilibrium with a CO2-rich fluid (In mols% CO2/H2O /CH4 = 76/24/8:10(-3)) during the granulite facies (M-2) metamorphism. Local variations in fluid inclusion composition suggest a strict, loc al control on the fluid composition during the metamorphic evolution o f the metasediments; brine, brine + CO2 and CO2 are characteristic flu id inclusions in impure quartzites and semi-metapelites, metapelites a nd migmatitized metapelites, respectively. Channelized influx of CO2-b earing brine at high-grade conditions explains the observed local part ial melting. A systematic increase of XN2 in the carbonic inclusions c oupled with decreasing density, suggests increased availability of nit rogen at retrograde (M-3 to M-4) conditions. The fluid inclusion densi ties do not match granulite facies P-T conditions: The first stage of the retrograde P-T evolution involved relatively rapid uplift, which t ogether with isoclinal folding of the rocks, caused decrepitation and mobilization of the fluids along secondary fluid inclusion trails, but without any significant change in fluid composition.