A NEW STRATIGRAPHY FOR THE MAIN ZONE OF THE BUSHVELD COMPLEX, IN THE RUSTENBURG AREA

The Main Zone of the western lobe of the Bushveld Complex is easily an d logically subdivided into five subzones which can be recognized in t he field. Although not as dramatically layered as the Lower, Critical, and Upper Zones of the Complex, the Main Zone also displays significa nt layering. This paper presents a new stratigraphy which, although ba sed on previous schemes, recognizes the significance of layered packag es and has as its main concept for subdivision, the variation in relat ive abundance of different pyroxenes throughout the Main Zone. Subzone A is a noritic lithology which is well documented and has at the top an obvious transition to gabbronorite in subzone B. Subzone B is chara cterized by the occurrence of glassy euhedral crystals of primary orth opyroxene, together with minor clinopyroxene, in a homogeneous gabbron orite. This is separated from subzone C by the Zebra Unit, a layered p ackage which is approximately 80 m thick. In subzone C, inverted pigeo nite is present within relatively homogeneous gabbronorite. The C subz one is extensively exposed since the majority of the positive topograp hic relief in the area is of this rock type. Towards the top of this s ubzone is the Hexrivier Unit, comprising 80 m or so of rhythmically la yered gabbronorites. Above this marker unit, the gabbronorite grades o ver approximately 25 m vertically into a rhythmically layered microgab bronorite which is approximately 75 m thick. This microgabbronorite ma rks the transition from subzone C to D. Subzone D is characterized by the reappearance of primary orthopyroxene. At the base, euhedral cryst als of primary orthopyroxene up to 1 cm in size characterize a porphyr itic gabbronorite facies which is interlayered with a gabbronorite fac ies containing needle-like clinopyroxene crystals in a matrix of plagi oclase feldspar. This facies at the base of subzone D is at least 25 m thick. Subzone E is mineralogically similar to subzone C and is chara cterized by tie presence of inverted pigeonite within the microgabbron orite and by the increasing occurrence of accessory magnetite. The tex tural and mineralogical difference between the five subzones is distin ctive and is recognizable both in the field and in thin section. This five-fold subdivision is not confined to the western Bushveld and can also be recognized in the lithologies of the eastern Bushveld. Thus, t his new stratigraphy is widespread, continuous, and easily correlated in the field.