Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa

Lundgaard, K. L.; Tegner, Christian; Cawthorn, R. Grant; Kruger, F. J.; Wilson, Joanne S.

doi:10.1007/s00410-006-0063-x

Cited by 30 publications

(11 citation statements)

References 49 publications

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“…The two An values (one above the other) in each profile are designed to illustrate the fractionationdominated evolution immediately prior to magma addition. The magma chamber was relatively homogeneous and producing plagioclase that was laterally fairly uniform in composition, as shown by Lundgaard et al (2006).…”

Section: Discussionmentioning

confidence: 89%

“…cryptic variation also exists. The differences in the proportions of trapped liquid (based on incompatible trace element abundances of P, Ti, Y, Zr and Ba in whole-rock analyses) in the three different profiles has been documented (Lundgaard et al, 2006). All the samples in the northern profile contain very little trapped liquid (0-10%), whereas progressively more is found in the central (10-30%) and the southern profiles (30-40%).…”

Section: Trapped Liquid Shift Effectmentioning

confidence: 96%

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Lateral variations in plagioclase compositions, Main Zone, Bushveld Complex, South Africa: Evidence for slow mixing of magmas in basinal structures

et al. 2016

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Many layered intrusions are considered to have been repeatedly inflated by magma additions, but rates of magma mixing relative to rates of layer accumulation are difficult to model. The nature of magma recharge through the interval including the Pyroxenite Marker (PM), Main Zone, Bushveld Complex, South Africa, is examined with regard to such processes. The plagioclase compositions (An value) in five previously published and three new profiles (presented here and focusing on the core compositions) that are at least 600 m in vertical extent and spread along a strike length of 110 km are evaluated. The compilation of the eight profiles shows the following trends. Upward reversals in compositions show considerable lateral as well as vertical variations. Lateral variations show a range in: (1) the minimum An value reached in each profile prior to the onset of magma recharge (An51 to An59); (2) the depth below the PM at which the minimum value is observed (50 to 575 m); (3) the An value close to the PM (An54 to An75); (4) the maximum value recorded above the PM (An63 to An76); (5) the height above the PM at which this maximum value is reached (0 to 300 m) – in all cases, the highest values of An occur at the northern end of the studied sections; and (6) the vertical extents over which the reversals occur range from 150 to over 600 m indicating very protracted magma additions and/or slow mixing. The PM terminates toward the south, and close to this termination the immediate footwall rocks to the PM change from north to south from gabbronorite to magnetite gabbronorite. A cross-section through these profiles defines two basins, with an intervening structural upwarp. The magma pulses that were added to produce very gradual and protracted reversals in mineral compositions through the PM interval ponded initially at the base of the northern basin, and did not homogenize the entire magma column. These added magmas did not overflow and have an effect on mineral compositions in the southern basin until after considerable replenishment and crystallization (including the PM) had taken place in the northern basin. We emphasize the prolonged period(s) of magma input and slow rate of vertical homogenization of the magma column during the formation of this sequence of as much as 400 m of the Main Zone.

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Section: Discussionmentioning

confidence: 89%

Section: Trapped Liquid Shift Effectmentioning

confidence: 96%

Lateral variations in plagioclase compositions, Main Zone, Bushveld Complex, South Africa: Evidence for slow mixing of magmas in basinal structures

et al. 2016

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“…Filled diamonds are Zr data from this study (Table 1). Small grey symbols represent Bushveld bulk-rock Zr data from Cawthorn & McCarthy (1985), Cawthorn et al (1991), Bowes et al (1997), Nex et al (2002 and Lundgaard et al (2006). Potential escaped melt compositions in filled squares are Rooiberg and Rashoop data from Schweitzer et al (1997).…”

Section: The Roof Of the Bushveld Complexmentioning

confidence: 99%

A Felsic End to Bushveld Differentiation

VanTongeren¹,

Mathez²,

Kelemen³

2010

Journal of Petrology

116

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The similarities in age (c. 2•06 Ga) and isotopic composition of the Bushveld Igneous Complex and the overlying Rooiberg Group felsic volcanic rocks suggest the possibility that the two are cogenetic.To investigate this possibility we derive a robust new estimate of the major and trace element bulk composition of the Bushveld Upper Zone and Upper Main Zone (UUMZ). Using MELTS thermodynamic modeling, we show that this composition fails to reproduce fundamental features of the cumulate sequence, such as the presence of primary orthopyroxene at the base of the magma column.We investigate the possibility that some amount of evolved magma escaped from the magma chamber and derive a new estimate of the UUMZ parent magma composition that reproduces generally the observed cumulate sequence. Our results constrain the amount and composition of the escaped magma and provide evidence of a genetic link between the UUMZ of the Bushveld layered series and the felsic volcanic rocks immediately overlying it. We conclude that between 15 and 25% of the original magma volume was expelled and is manifest either as part of the upper Rooiberg Group lava sequence or the Rashoop Granophyre (distinction is hindered by their almost identical major and trace element compositions). A parent magma in which 15^25% Rooiberg or Rashoop material is added back to the extant cumulate sequence is initially saturated in orthopyroxene and is in major and trace element equilibrium with the phases at the base of the UUMZ.

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“…2). This eld evidence implies that the ~1.0 km and ~3.0 km thick MZ in the Roossenekal and Belfast sub-chambers, respectively, formed synchronously from the same interconnected resident melt, as further substantiated by similar An-content of plagioclase at the base of the MZ at both sub-chambers (67.5% 41,56 and 71% 42,43 , respectively). A greater thickness of MZ cumulates in the Roossenekal sub-chamber likely relates to a redeposition of crystals inside this depression and/or prevailing crystallization in the deeper parts of the magma chamber, due to a pressure-induced increase in the liquidus temperature of the melt 9,52,57 .…”

Section: The Resident Melt Column Of the Bushveld Complexmentioning

confidence: 63%

A 5-km-thick reservoir with >380,000 km3 of magma within the ancient Earth's crust

Latypov

Chistyakova

Hornsey³

et al. 2021

Preprint

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Several recent studies have argued that large, long-lived and molten magma chambers1–10 may not occur in the shallow Earth’s crust11–23. Here we present, however, field-based observations from the Bushveld Complex24 that provide evidence to the contrary. In the eastern part of the complex, the magmatic layering was found to continuously drape across a ~4-km-high sloping step in the chamber floor. Such deposition of magmatic layering implies that the resident melt column was thicker than the stepped relief of the chamber floor. Prolonged internal differentiation within such a thick magma column is further supported by evolutionary trends in crystallization sequence and mineral compositions through the sequence. The resident melt column in the Bushveld chamber during this period is estimated to be >5-km-high in thickness and >380,000 km3 in volume. This amount of magma is three orders of magnitude larger than any known super-eruptions in the Earth’s history25 and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces26. This suggests that super-large, entirely molten and long-lived magma chambers, at least occasionally, occur in the geological history of our planet. Therefore, the classical view of magma chambers as ‘big magma tanks’1–10 remains a viable research concept for some of Earth’s magmatic provinces.

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Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa

Cited by 30 publications

References 49 publications

Lateral variations in plagioclase compositions, Main Zone, Bushveld Complex, South Africa: Evidence for slow mixing of magmas in basinal structures

Lateral variations in plagioclase compositions, Main Zone, Bushveld Complex, South Africa: Evidence for slow mixing of magmas in basinal structures

A Felsic End to Bushveld Differentiation

A 5-km-thick reservoir with >380,000 km3 of magma within the ancient Earth's crust

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Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa

Cited by 30 publications

References 49 publications

Lateral variations in plagioclase compositions, Main Zone, Bushveld Complex, South Africa: Evidence for slow mixing of magmas in basinal structures

Lateral variations in plagioclase compositions, Main Zone, Bushveld Complex, South Africa: Evidence for slow mixing of magmas in basinal structures

A Felsic End to Bushveld Differentiation

A 5-km-thick reservoir with &gt;380,000 km3 of magma within the ancient Earth's crust

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Product

Resources

About

A 5-km-thick reservoir with >380,000 km3 of magma within the ancient Earth's crust