Two Expressions of the Transition from Mafic Cumulates to Granitoids in the Bushveld Complex, South Africa: Examples from the Western and Eastern Limbs

In the Bushveld Complex of South Africa, numerous granitic and granophyric rocks termed the Lebowa Granite and Rashoop Granophyre Suites, respectively, overlie the layered ultramafic-mafic rocks of the Rustenburg Layered Suite. Despite their close spatial and temporal association, the granites and granophyres are often interpreted as being unrelated to the Rustenburg Layered Suite. This paper describes the transition from the uppermost Rustenburg Layered Suite into overlying granite and granophyre at three locations in the western (the Bierkraal drill core) and eastern (Diepkloof farm and Stoflberg town) limbs of the Bushveld Complex. In the western limb, a similar to 60 m thick transition zone bridges the petrological gap between the overlying Nebo Granite (Lebowa Granite Suite) and Upper Zone (Rustenburg Layered Suite). Across the transition zone, the composition of olivine changes from Fo(6) to Fo(1), clinopyroxene from Mg#(25) to Mg#(2) and plagioclase from An(45) to An(16). At Stoflberg in the eastern limb, the Upper Zone dioritic cumulates grade into the overlying monzonitic Roof Zone. Across the Roof Zone, plagioclase compositions change from An(42) to An(4), clinopyroxene from Mg#(30) to Mg#(11) and olivine from Fo(9) to Fo(5). Based on geochemistry and petrography, we correlate the lowermost stratigraphy at the farm Diepkloof with the Stoflberg Roof Zone. At Diepkloof, the Roof Zone grades into the overlying Stavoren Granophyre (Rashoop Granophyre Suite). All units are indistinguishable in terms of bulk rock Nd (epsilon(Nd) = -6.4 to -5.4) and Hf (epsilon(Hf) -9.2 to -6.6) isotopes (corrected to 2055 Ma). Similarly, the Upper Main Zone, Upper Zone and transition zone are indistinguishable in terms of bulk rock Sr isotopes ((Sr-87/Sr-86)(2055 )- 0.7071 to 0.7076, except one transition zone outlier at 0.7058). We submit that the transition zone represents the fossil record of bulk and/or diffusional mixing between coexisting Upper Zone and Nebo Granite magmas. We test this hypothesis by combining field, petrographic and geochemical observations with forward modelling using the Rhyolite-MELTS algorithm. Our work on the Bierkraal core (western limb) shows that at least a portion of the granitic magma was emplaced before the residual liquid of the Upper Zone had solidified. At Stoflberg and Diepkloof in the eastern limb (where the granite is absent), the Roof Zone underwent uninterrupted fractional crystallization. (C) 2020 Elsevier B.V. All rights reserved.