Magmatic stratigraphy in the Bushveld Northern Lobe: continuous geophysical and mineralogical data from the 2950 m Bellevue drillcore

Abstract

We present a large database of geophysical, petrological and mineralogical measurements for the ~3000 m Bellevue borehole through the entire Upper Zone (UZ) and about half of the Main Zone (MZ) of the Northern Lobe of the Bushveld Complex, South Africa. Magnetic susceptibilty readings were taken every 2 cm (n = 109,360) and densities were measured on average every 1.7 m (n = 2252). Petrographic data and microprobe analyses (n = 14,160) were obtained for plagioclase, mafic silicates, Fe-Ti oxides, amphibole and biotite in 502 samples throughout the entire sequence of layered mafic cumulates. The Bellevue UZ, as marked by the first appearance of cumulus magnetite, is ~1190 m thick (corrected for mean dip of 17.5°), which is less than UZ thicknesses in the Eastern and Western Lobes. A prominent 4 m thick pyroxenite horizon occurs ~390 m below the UZ-MZ boundary, but we show on the basis of mineralogy that this horizon cannot be correlated with the well known Pyroxenite Marker (PM) of the Eastern and Western Bushveld Complex. If the PM is indeed absent in the Northern Lobe, then a substantial portion, perhaps 500 m of the uppermost MZ may be missing; possible causes include non-deposition ( e.g. due to syn-magmatic upwarping or diapirism) or removal (e.g. due to emplacement of UZ magmas). The Bellevue drillcore penetrated only about half of the MZ (total dip-corrected thickness ~1270 m), and the lowermost ~200 m contains unusual olivine-bearing (or troctolitic) horizons that are atypical of MZ rocks elsewhere in the Bushveld Complex. These troctolites have mineral compositions as primitive as those of the upper Critical Zone (CZ), suggesting that they might represent a sliver of CZ rocks dismembered by intrusion of MZ magmas. Alternatively, they may represent an intrusive sill of syn- or post-Bushveld age, or merely a mineralogically unusual horizon in otherwise typical MZ lithologies. Mineral compositions show broad normal fractionation upwards, with plagioclase An78→21, opx (and inverted pigeonite) En 80→26, cpx Mg 86→27, olivine Fo 78→74 (in the lowermost troctolitic horizon) and Fo 59→06 (in the UZ olivine ferrodiorites). There are, however, numerous prominent reversals and discontinuities in mineral compositions, some of which are likely related to magma additions. The extensive dataset of mineral compositions allows the estimation of a new fractionation trend for the Bushveld Complex. On an En-An diagram, the Bushveld trend is shifted toward more An-rich plagioclase at equivalent Mg# of coexisting pyroxenes relative to those for Kiglapait or Skaergaard. This is attributed to the relative paucity in Bushveld of augite, which has a high fractionating power for Ca/Na in evolving liquids. Magnetic susceptibility data clearly reveal the presence of the UZ-MZ boundary. MZ cumulates have susceptibilty values <0.05 SI units, and generally <0.02 SI units. Above the UZ-MZ boundary, susceptibilty varies enormously, from anorthosites (<0.1 SI units) to magnetitites (to almost 5 SI units), and there is excellent correlation between susceptibility and lithology, in many cases to a resolution of <5 to 10 cm. Anorthositic rocks, especially in the MZ, commonly show higher susceptibilty than surrounding polyphase cumulates, due to intercumulus and/or dust-like inclusions of magnetite. Density data reveal surprising cyclicity in the Main Zone on the scale of 50 to 200 m, with progressively increasing density upwards in individual layered units, reflecting gradual increase in modal colour index from 0 to 10% to 50 to 60%. In some cases the upward density increases are correlated with broad reversals in chemical fractionation trends (e.g. upward increases in Mg# of pyroxenes), arguing against simple fractionation. We suggest that such layers may represent blending zones in which dense liquids and/or crystals from new magma additions drain downwards into the existing cumulate pile. MZ cumulates, therefore, may have been constructed by successive, compositionally different magmatic influxes, implying the existence of a sub-Bushveld magmatic staging chamber.