Cold fingers in a hot magma: Numerical modeling of country-rock diapirs in the Bushveld Complex, South Africa

Gerya, Taras; Uken, Ron; Reinhardt, Jürgen; Watkeys, M. K.; Maresch, Walter V.; Clarke, Brendan M.

doi:10.1130/g19566.1

Cited by 28 publications

(19 citation statements)

References 8 publications

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“…= 950 • C) -(for calculations see supplementary materials). The modelling results are therefore consistent with BC cooling from 1150 • C to 700 • C in less than one million years, a timescale which is in good agreement with previous, more complex modelling results of Gerya et al (2003), and Cawthorn and Webb (2013).…”

Section: Thermal Model For Emplacement and Coolingsupporting

confidence: 89%

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

Zeh

Ovtcharova

Wilson

et al. 2015

Earth and Planetary Science Letters

249

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Section: Thermal Model For Emplacement and Coolingsupporting

confidence: 89%

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

Zeh

Ovtcharova

Wilson

et al. 2015

Earth and Planetary Science Letters

249

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“…Bittner & Schmeling, 1995). Thermomechanical modelling suggests that the country rock diapirs were characterized by a negative thermal anomaly; according to Gerya et al. (2003), diapir rims were 200–300 °C hotter than diapir cores at the time of their emplacement, a consequence of colder, less dense floor rocks ascending into the hot overlying RLS.…”

Section: Geological Settingmentioning

confidence: 99%

“…In conjunction with petrographic, bulk‐rock and mineral chemical constraints, we use pseudosections in the MnNCKFMASHT (MnO‐Na 2 O‐CaO‐K 2 O‐FeO‐MgO‐Al 2 O 3 ‐SiO 2 ‐H 2 O‐TiO 2 ) model system to investigate the pressure, temperature and compositional ( P, T , X ) controls on symplectite formation. We suggest that preservation of the symplectites is consistent with suprasolidus decompression on ascent of the partially molten diapiric core, along a P–T path that is broadly consistent with the results of thermomechanical modelling by Gerya et al. (2003).…”

Section: Introductionmentioning

confidence: 99%

Spinel–cordierite symplectites replacing andalusite: evidence for melt‐assisted diapirism in the Bushveld Complex, South Africa

Johnson

Brown

Gibson

et al. 2004

Journal Metamorphic Geology

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Spinel-cordierite symplectites partially replacing andalusite occur in metapelitic rocks within the cores of several country rock diapirs that have ascended into the upper levels of layered mafic/ultramafic rocks in the Bushveld Complex. We investigate the petrogenesis of these symplectites in one of these diapirs, the Phepane dome. Petrographic evidence indicates that at conditions immediately below the solidus the rocks were characterized by a cordierite-, biotite-and K-feldspar-rich matrix and 5-10 mm long andalusite porphyroblasts surrounded by biotite-rich fringes. Phase relations in the MnNCKFMASHT model system constrain the near-solidus prograde path to around 3 kbar and imply that andalusite persisted metastably into the sillimanite + melt field, where the fringing relationship between biotite and andalusite provided spatially restricted equilibrium domains with silica-deficient effective bulk compositions that focused suprasolidus reaction. MnNCKFMASHT pseudosections that model these compositional domains suggest that volatile phase-absent melting reactions consuming andalusite and biotite initially produced a moat of cordierite surrounding andalusite; reaction progressed until all quartz was consumed. Spinel is predicted to grow with cordierite at around 720°C. Formation of the aluminous solid products was strongly controlled by the receding edge of andalusite grains, with symplectites forming at the andalusite-cordierite moat interface. Decompression due to melt-assisted diapiric rise of the floor rocks into the overlying mafic/ultramafic rocks occurred close to the thermal peak. Re-crossing of the solidus at P ¼ 1.5-2 kbar, T > 700°C resulted in preservation of the symplectites. Two features of the silica-deficient domains inhibited resorption of spinel. First, the cordierite moat armoured the symplectites from reaction with crystallizing melt in the outer part of the pseudomorphs. Second, an up-T step in the solidus at low-P, which may be in excess of 100°C higher than the quartz-saturated solidus, resulted in high-T crystallization of melt on decompression. Even in metapelitic rocks where melt is retained, preservation of spinel is favoured by decompression.

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“…Importantly, palaeomagnetic evidence indicates the RLS was emplaced horizontally (Hattingh, 1986;Letts et al, 2009) and later subsided isostatically to form the current, basin-like geometry of the RLS. The Pretoria Group in the Burgersfort area was not subjected to pre-or post-Bushveld penetrative deformation and there is no evidence of the diapirism that affected much of northeastern contact aureole (Uken and Watkeys, 1997;Gerya et al, 2003). Deformation related to diapiric deformation of the Steelpoort pericline is restricted to the RLS sequence within ~5 km of the pericline (Clarke et al, 2005), whereas the Derde Gelid structure is not of diapric origin (Clarke et al, 2009).…”

Section: Regional Geological Setting and Previous Investigationsmentioning

confidence: 90%

“…Kruger, 2005) has enabled the application of sill emplacement mechanics to explain features within the contact aureole. These features include the Spruitfontein Inlier (Clarke et al, 2000), the diapiric domes in the northeastern contact aureole (Uken and Watkeys, 1997;Gerya et al, 2003) and the conduit-like Uitkomst satellite intrusion (Hornsey, 1999). These emplacement-related features have also been used by Clarke et al (2009) to propose an intrusion model for the RLS as a sill emanating from a feeder dyke.…”

Section: Emplacement Mechanics Of Planar Intrusionsmentioning

confidence: 99%

The Geometry and Emplacement Mechanics of a Bushveld Complex Peridotite Body, South Africa

Clarke

Uken

Reinhardt

2009

South African Journal of Geology

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The Apiesdooringdraai peridotite is a syn-Bushveld Complex, dominantly harzburgitic intrusion of Lower Zone parentage. Detailed mapping and three-dimensional modeling shows the intrusion to comprise several discordant segments linked by concordant, silllike segments. Deformation in the contact aureole is strongly partitioned adjacent to the discordant segments of the intrusion and is detectable at distances from the contact on the order of the intrusion width. In contrast, areas above and below the intrusion, and at greater distances from the discordant intrusive segments, are effectively undeformed. The geometry of structural features suggests that the peridotite is a variably structurally-evolved stepped-sill system that was emplaced as kilometre-scale magma fingers from the northwest to the southeast, a trend heavily utilised by the mafic-ultramafic magmas of the Bushveld Complex. Deformation in the country rock was affected by the lateral expansion of the magma fingers at high postulated strain rates, and linkage of offset fingers formed prominent steps in the intrusion. Emplacement-related country rock deformation is postdated by a shallow dipping foliation that records compaction and crustal loading during contact metamorphism. Figure 5. Simplified geological map of the Burgersfort-Steelpoort area, showing the transgressive nature of the ADP with respect to its country-rock stratigraphy.

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Cold fingers in a hot magma: Numerical modeling of country-rock diapirs in the Bushveld Complex, South Africa

Cited by 28 publications

References 8 publications

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

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

Spinel–cordierite symplectites replacing andalusite: evidence for melt‐assisted diapirism in the Bushveld Complex, South Africa

The Geometry and Emplacement Mechanics of a Bushveld Complex Peridotite Body, South Africa

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