The Bushveld Complex was emplaced and cooled in less than one million years – results of zirconology, and geotectonic implications

Zeh, Armin; Ovtcharova, Maria; Wilson, A. H.; Schaltegger, Urs

doi:10.1016/j.epsl.2015.02.035

Cited by 243 publications

(100 citation statements)

References 52 publications

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“…However, until recently, this model lacked clear geochronological support. De Waal et al (2001) determined an age of 2044 ± 8 Ma, not overlapping with recent ages for the Bushveld (2055-2056 ± ∼1 Ma, Zeh et al 2015;2056.88 ± 0.41-2057.04 ± 0.55 Ma, Scoates and Wall 2015). The multi-grain baddeleyite age for Uitkomst of Wabo et al (2015, 2054 shows better overlap with the Bushveld event, but still lacks sufficient precision.…”

Section: Introductionmentioning

confidence: 99%

The Uitkomst intrusion and Nkomati Ni-Cu-Cr-PGE deposit, South Africa: trace element geochemistry, Nd isotopes and high-precision geochronology

et al. 2017

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The Uitkomst intrusion is a tubular maficultramafic layered body that hosts one of South Africa's largest Ni-Cu-Cr-PGE deposits, Nkomati. The sulphide ore occurs in the form of massive lenses in the immediate quartzitic footwall and as disseminations within peridotite. The chromite ore forms an up to ∼10-m-thick layer in the lower portion of the intrusion. Uitkomst has generally been interpreted as a magma conduit, possibly related to the Bushveld event. Here, we present a new high-precision U-Pb zircon date of 2057.64 ± 0.69 Ma that overlaps with the age of the Merensky Reef of the Bushveld Complex and thus demonstrates a coeval relationship between the intrusions. Based on incompatible trace elements as well as O-and Nd isotope data (εNd −4.5 to −6.2), we show that the Uitkomst parent magmas were contaminated with up to 20% Archean upper crust prior to emplacement, and with up to 15% dolomitic country rock during emplacement. Ore formation at Nkomati was critically aided by substantial devolatisation and removal of dolomitic floor rocks leading to hydrodynamic concentration of sulphide and chromite during slumping of crystal mushes into the trough-like centre of the subsiding intrusion and its footwall.

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Section: Introductionmentioning

confidence: 99%

The Uitkomst intrusion and Nkomati Ni-Cu-Cr-PGE deposit, South Africa: trace element geochemistry, Nd isotopes and high-precision geochronology

et al. 2017

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“…For a skewed distribution as shown in Figure 8, our statistical values would be calculated from an arbitrary mixture of analytical variance with protracted duration of zircon crystallization and generate a pure artifact. This implies that our method can be applied using CA-ID-TIMS 206 Pb/ 238 U dates with a state-of-the-art uncertainty of 0.05% up to a maximal age of about 500 Ma; beyond this age we may be able to use highest-precision 207 Pb/ 206 Pb dates of concordant zircon grains in rare cases (e.g., Zeh et al, 2015).…”

Section: In-situ Vs Whole Zircon Techniquesmentioning

confidence: 99%

Estimates of Volume and Magma Input in Crustal Magmatic Systems from Zircon Geochronology: The Effect of Modeling Assumptions and System Variables

2016

Self Cite

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Magma fluxes in the Earth's crust play an important role in regulating the relationship between the frequency and magnitude of volcanic eruptions, the chemical evolution of magmatic systems, and the distribution of geothermal energy and mineral resources on our planet. Therefore, quantifying magma productivity and the rate of magma transfer within the crust can provide valuable insights to characterize the long-term behavior of volcanic systems and to unveil the link between the physical and chemical evolution of magmatic systems and their potential to generate resources. We performed thermal modeling to compute the temperature evolution of crustal magmatic intrusions with different final volumes assembled over a variety of timescales (i.e., at different magma fluxes). Using these results, we calculated synthetic populations of zircon ages assuming the number of zircons crystallizing in a given time period is directly proportional to the volume of magma at temperature within the zircon crystallization range. The statistical analysis of the calculated populations of zircon ages shows that the mode, median, and standard deviation of the populations varies coherently as function of the rate of magma injection and final volume of the crustal intrusions. Therefore, the statistical properties of the population of zircon ages can add useful constraints to quantify the rate of magma injection and the final volume of magmatic intrusions. Here, we explore the effect of different ranges of zircon saturation temperature, intrusion geometry, and wall rock temperature on the calculated distributions of zircon ages. Additionally, we determine the effect of undersampling on the variability of mode, median, and standards deviation of calculated populations of zircon ages to estimate the minimum number of zircon analyses necessary to obtain meaningful estimates of magma flux and final intrusion volume.

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“…This problem has been discussed by Zeh et al (2015) and can be particularly well demonstrated in the Upper CZ of the Bushveld Complex, for which recent high-precision geochronology suggested that the Merensky Reef was at least 2 million years younger than the overlying Bastard Reef (Wall 2016). However, field evidence from the western Bushveld clearly shows the opposite: the Bastard Reef cross-cuts the Merensky Reef (De Klerk 1982;Carr et al 1994).…”

Section: Implications For Layered Intrusionsmentioning

confidence: 97%

“…The immediate underlying floor rocks consist of Late Archean to Early Proterozoic chemical and siliciclastic sedimentary rocks of the Transvaal Supergroup, which were intruded by the RLS over a relatively short period of time (<1 million years) between ca. 2057 and 2056 Ma (Scoates and Wall 2015;Zeh et al 2015).…”

Section: Geological Settingmentioning

confidence: 99%

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

Karykowski

2017

Contrib Mineral Petrol

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by plagioclase, clinopyroxene and other late stage phases, shows variable ratios of these minerals ranging from ca. 21:15:64 to 75:17:8. In comparison to modelled expected ratios, most of the analysed rocks have higher amounts of early crystallising interstitial phases (e.g. plagioclase, clinopyroxene), relative to late crystallising phases (e.g. quartz, alkali feldspar). Therefore, interstitial melt must have migrated at different stages of fractionation during cumulate solidification, as a consequence of either compaction or displacement by convecting interstitial liquids. Two samples, however, show the opposite: late phases are relatively more abundant than early ones, which is consistent with a convection-driven replacement of primitive interstitial liquid by more evolved liquid. These results have important implications for the interpretation of the Lower Zone and, by extension, for layered intrusions in general: (1) interstitial sulphide mineralisation may be introduced into a cumulate through infiltrating melts, i.e. the liquid components of a sulphur-saturated crystal mush are not withheld from further migration, upon interaction with a cumulate pile; (2) most importantly, late stage minerals, such as zircon, rarely crystallise from trapped liquid that was initially in equilibrium with the cumulate. Therefore, dating of interstitial zircon from cumulates is unlikely to record the actual timing of emplacement, but merely the crystallisation of a later episode of residual melt that migrated through the cumulate.

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The Bushveld Complex was emplaced and cooled in less than one million years – results of zirconology, and geotectonic implications

Cited by 243 publications

References 52 publications

The Uitkomst intrusion and Nkomati Ni-Cu-Cr-PGE deposit, South Africa: trace element geochemistry, Nd isotopes and high-precision geochronology

The Uitkomst intrusion and Nkomati Ni-Cu-Cr-PGE deposit, South Africa: trace element geochemistry, Nd isotopes and high-precision geochronology

Estimates of Volume and Magma Input in Crustal Magmatic Systems from Zircon Geochronology: The Effect of Modeling Assumptions and System Variables

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

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