Abstract

Mafic rocks of the Bushveld Complex at the southeastern end of the western limb, intersected in bore core from the Cullinan Diamond Mine, are described. A 260 m thick ultramafic body of orthopyroxene and chromite cumulate rocks, with mg# – 100*Mg/(Mg+Fe) – values from 77 to 84 and 0.25 to 0.5% Cr2O3 in the pyroxene, is considered to have affinity to the Critical Zone. Such an interpretation considerably extends the eastern limit of Critical Zone rocks of the western limb of the Bushveld Complex. The whole-rock composition of the lower, chilled basal contact of this body has 10% MgO and 500 ppm Cr, and is comparable to magmas considered parental to the Bushveld Complex. Due to intrusion of a younger sill, the upper contact is not preserved in the bore core. The cumulate rocks have higher interstitial component, inferred from incompatible trace element abundances (Zr, Ti and K), than normal Critical Zone rocks, interpreted to be a result of more rapid cooling due to proximity to the basal contact. The near-constancy of mg# in the pyroxene in the entire succession suggests that large volumes of magma flowed through this conduit, with only the liquidus phases of orthopyroxene and chromite being precipitated.

Five generations of sills, intruded into the underlying metasedimentary rocks, are identified. The oldest is tholeiitic, and was metamorphosed prior to the emplacement of the Bushveld Complex. The second equates to the magma proposed as being parental to the Bushveld Complex (2060 Ma). The third represents the products of differentiation of that magma. The fourth is syenitic, and related to the Pienaars River Alkaline Complex (1430–1300 Ma). The fifth is tholeiitic (1150 Ma), and cuts the Cullinan kimberlite.

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