Pressure-temperature estimation in orthopyroxene-garnet bearing granulite facies rocks, Bamble Sector, Norway

Pressures and temperatures are estimated for the high grade portion (Region D) of the Bamble granulite facies terrane using barometers based on the as semblages almandine-grossular-ferrosilite-anorthite-quartz and pyrope-gross ular-enstatite-anorthite-quartz and a garnet-orthopyroxene Fe2+-Mg K-D exch ange thermometer based on the experimental data of Lee and Ganguly (1988). Both barometers utilize the same thermochemical data base (Berman, 1988, 19 90) and the same garnet (Berman, 1990) and orthopyroxene mixing models (Woo d and Banno, 1973; Sack and Ghiorso, 1989) which are internally consistent both with each other and the Berman (1988, 1990) data base. Thus any disagr eement between the two barometers is dependent only on the mixing models ch osen. When Fe2+-Mg mixing in orthopyroxene and garnet is taken to be ideal, pressures from the two barometers are found to be in poor agreement with P Alm-Ps (P-avg = 9.2 +/- 1.0 kb (1 sigma)) consistently greater than PPyr-En (P-avg = 7.5 +/- 1.2 kb (1 sigma)) by 1.4 to 2.4 kb per sample. If the non -ideal orthopyroxene mixing model of Sack and Ghiorso (1989) is used and th eir internally consistent value for mixing on the Fe2+ Mg join in garnet is incorporated in the Berman (1990) garnet mixing model, PAlm-Fs and PPyr-En are now in good agreement with the mean pressures for PAlm-Fs and PPyr-En at 7.1 +/- 0.9 kb (1 sigma) and 7.0 +/- 1.1 kb (1 sigma) respectively. Indi vidual pressures per sample are generally within +/- 1 kb of each other whi ch is within the error range of either barometer. The mean temperature is f ound to be 793 +/- 58 degrees C (1 sigma) which is in good agreement with a mean temperature of 795 degrees C obtained from titaniferous magnetite-ilm enite thermometry (Harlov, 1992) as well as the stability fields of sillima nite and Mg-rich cordierite.