The floor contact of the eastern compartment of the Bushveld Complex has been mapped in detail for 130 km. Field relations, petrographical and geochemical data have enabled 3 important marginal border groups and a possible parent to the lower zone to be demarcated. These represent the 4 main magma influxes which formed the malic phase of the Bushveld Complex and are, in chronological order: 1, a contaminated magnesian basalt (B1) which gave rise to a saucer-shaped laminated marginal zone at the base of the Complex with diverse related sills; 2, a harrisitic peridotite representing quenched lower zone magma; 3, a border group to the more evolved, probably basaltic, critical zone magma which was intruded in two phases—an early quenched norite facies (N) being followed by a chilled bulk critical zone facies (B2) which forms a skin to the whole critical zone. Finally, 4, a border group to the main and upper zone (B3) which represents an early magnesian pulse of this last immense volume of tholeiitic liquid. There is also evidence for minor influxes of magma within the critical zone and main zone.

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