FACIES MODEL FOR INTERVAL BETWEEN UG2 AND MERENSKY REEF, WESTERN BUSHVELD COMPLEX, SOUTH-AFRICA

A lithological and geochemical study of the interval between the UG2 c hromitite and the Merensky Reef in the Upper Critical Zone of the Bush veld Complex, South Africa, revealed significant lateral variation ove r 170 km of strike in the Western Bushveld. Comprising ca 30 m at the Union Section, the interval reaches a total thickness of 220 m near Br its. Concurrently, the cumulates become more evolved and leucocratic w ith distance from the Union Section towards both the northeast (Amande lbult) and the southeast (Impala, Rustenburg and Brits areas). It is i nferred that the reduced thickness of the interval is mainly the resul t of the non-deposition of felsic cumulates in the proximity of a puta tive feeder zone near the Union Section. The scale of non-deposition w as governed by a thermal regime that was related to the heat flux from the feeder. A persistent heat flux delayed crystallization of success ive phases in those parts of the chamber which were proximal to the fe eder. New influxes were therefore deposited on a footwall of varying t hickness and lithology in response to different degrees of crystalliza tion and accumulation along strike. The development of cyclic units om itite-harzburgite-pyroxenite-norite-anorthosite) was interrupted at di fferent stages in different parts of the chamber.