Geochemical and thermodynamic controls in the formation of mineral assemblages from the metamorphic aureole of the Bushveld Complex in the Potgietersrus area

Abstract

The metamorphic aureole of the Bushveld Complex in the Potgietersrus area was created in a two-stage metamorphic event. In the first stage which was related to the intrusion of the lower zone magma, maximum temperatures of 750°C were attained at 1 ,5 kbar pressure. Pressure estimates are based on the rare cordierite +olivine+ orthopyroxene ± spinel ± quartz assemblage. At lower temperatures the cordierite + chloritoid assemblage also appears to have been stable. Equilibrium temperatures and pressures during the second stage, which was brought about by the intrusion of more voluminous gabbroic liquids that gave rise to the upper critical, main and upper zones were considerably higher at 900°C and 4 to 5 kbar pressure. Geothermobarometry is based on the garnet+ cordierite + orthopyroxene + biotite; clinopyroxene + plagioclase + quartz and cordierite + spinel ± sillimanite ± corundum ± orthopyroxene ± olivine assemblages. The temperature dependence of the Mg-Fe exchange reaction between cordierite and spinel was used as the basis of a geothermometer calibrated against published data for natural mineral assemblages. High alumina (44 wt. percent), low silica (36 wt. percent) and low alkali pelites from the Timeball Hill Formation also originated during this stage through the removal of about 65 percent granitic melt from nonnal pelites. The maximum pressures calculated for the second stage of the metamorphic event are about 2 kbar higher than the lithostatic pressure that could have been exercised by the presently exposed succession of layered- and roof-rocks. This difference can be accounted for by a direc~ed stress contribution, which there is evidence for in the form of a large fold in the floor rocks, and by assuming that the lithostatic pressure during the second stage of the metamorphic event was about 1 ,0 kbar higher than the value suggested by the current field relations, brought about by a superincumbent load of layered rocks.