Metamorphosed pelitic rocks were studied in the contact aureole around the Bushveld mafic layered suite, northeastern Transvaal, South Africa. The vertical variation along the cross section at the Penge area indicates that the metamorphic grade increases from chloritoid-chlorite through andalusite-cordierite or andalusite-staurolite-garnet to cordierite-orthopyroxene-sillimanite zones with approaching the contact. Except for the chloritoid-chlorite zone, biotite is ubiquitous in low- to high-grade pelitic hornfelses. Ti content in biotite coexisting with ilmenite also increases toward the contact. Using available geothermometry, metamorphic temperatures vary gradually from 750±50°C at the contact to 420±20°C at Penge. Metamorphic pressures at the Penge area are estimated to be 2.1±0.4 kbar at 5.4km and 2.4± 0.9 kbar at 8.9km apart from the contact. The lateral variation of mineral assemblages in the Lower Shale Unit of the Timeball Hill Formation, Pretoria Group, shows that the metamorphic grade increases from east (chloritoid-chlorite at Penge) to west (andalusite-staurolite± garnet at Malipsdrift). This variation reflects the reduction of the distance from the contact between the mafic layered suite and Pretoria Group, because the intrusion plane cuts gradually into the lower horizons of the Group from east to west.
The metamorphic conditions along the cross section at Penge suggests that the contact metamorphism recorded the gentle geothermal gradient (80±20°C/km) which is several times smaller than that due to the Duluth Complex, northwestern Minnesota, U.S.A.