Structure of the Bushveld Complex from resistivity measurements

Meyer, Rudi; Beer, J. H. de

doi:10.1038/325610a0

Cited by 40 publications

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“…Lee, 1996). Known exposures of the Bushveld Complex ( Figure 1) are typically divided into 4 to 5 "Limbs" or "Lobes"; these have been interpreted as discrete magmatic bodies (Cousins, 1959;Meyer and De Beer, 1987), but recent re-interpretation of the regional gravity signature, taking into account crustal flexure, allows the possibility that at least the Eastern and Western Limbs might be continuous at depth Webb, 2001: Webb et al, 2004). The total areal extent of Bushveld rocks has been estimated at about 65,000 km 2 , and with an average thickness of 6 km for the layered mafic cumulate rocks, the Bushveld Complex must have formed from a volume of basaltic magma of nearly 400,000 km 3 , and perhaps as much as 1,000,000 km 3 if large volumes of volcanic rocks (now eroded) escaped to the surface as theorized by Cawthorn and Walraven (1998).…”

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

Ashwal

Webb

Knoper

2005

South African Journal of Geology

139

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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 17.5º), 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 bet...

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confidence: 97%

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

Ashwal

Webb

Knoper

2005

South African Journal of Geology

139

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“…Meyer and de Beer, 1987) several methodological approaches and developments such as one-dimensional (1D) inversion (Petrick et al, 1977) were tested.…”

Section: Pioneering Em Work In Africamentioning

confidence: 99%

Making and Breaking of a Continent: Following the Scent of Geodynamic Imprints on the African Continent Using Electromagnetics

Weckmann

2011

Surv Geophys

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The African continent inherits a long history of continental accretion and breakup. The stage of "making" a continent goes back to the Archean, when the first continental masses formed cratons which mostly remained stable ever since. Subsequent collision of weaker continental masses was followed by several extension and compression episodes that resulted in the formation of supercontinents. After the assemblage of Gondwana, a period of predominantly "breaking" , i.e. the breakup of supercontinents, took over. The modern day African continent exhibits different types of margins; continental rifting occurs side by side with recent collision. Since the late 1960's Magnetotelluric (MT) experiments have played an important role in studies of the electrical conductivity structure of Africa. The early results significantly shaped the MT community's understanding of continental scale conductivity belts and basic characteristics of cratons and mobile belts on both crustal and lithospheric mantle scale for some decades. Modern MT studies in Africa have generally supported earlier results with high resistivities observed on cratons and low resistivities observed across mobile belts. Advances in instrumentation, data processing and interpretation resulted in higher resolution images of the lithosphere which in consequence induce an improved understanding of tectonic processes and geological prerequisites for the occurrence of natural resources. The high electrical conductivity of mobile belts and their relation to reactivated fault and detachment zones were often interpreted to characterize mobile belts as tectonic weak zones, which can accommodate stress and constitute zones along which continents can break. Recent breaking of the African continent can be studied on land across the East African rift; however, the lack of amphibian MT experiments across today's margins does not allow for good resolution of remnants of continental break-up processes. Naturally, the regions and the focus of the MT studies in Africa are diverse, but they all contribute to the story of making and breaking a continent.

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“…Many authors, particularly those working to the northeast of Marble Hall in Sekhukhuniland, have argued for progressive fractionation of the granites from the base upwards (Lenthall, 1975;Marlow, 1976;DeGraaf, 1982;MacCaskie, 1983;Kleeman, 1985;Walraven, 1986;Meyer and De Beer, 1987;Kleeman and Twist, 1989). This is clearly illustrated in the southwestern lobe north of Rustenburg where plagioclase and pyroxene-bearing granite passes into biotite, mafic mineral-poor granite with height in the sheet (Walraven, 1977).…”

Section: Discussionmentioning

confidence: 98%

Polymetallic mineralization in the granites of the Bushveld Complex - examples from the central southeastern lobe

Bailie

Robb

2004

South African Journal of Geology

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Structure of the Bushveld Complex from resistivity measurements

Cited by 40 publications

References 2 publications

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

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

Making and Breaking of a Continent: Following the Scent of Geodynamic Imprints on the African Continent Using Electromagnetics

Polymetallic mineralization in the granites of the Bushveld Complex - examples from the central southeastern lobe

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