Geochronology of Layered Intrusions

Scoates, James S.; Wall, Corey J.

doi:10.1007/978-94-017-9652-1_1

Cited by 49 publications

(32 citation statements)

References 247 publications

(298 reference statements)

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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: 85%

“…All handpicked zircon grains (∼250 μm in length) were elongate, euhedral, clear, and colourless to light pink with minor amounts of mineral and fluid inclusions. Chemical abrasion-isotope dilutionthermal ionisation mass spectrometry procedures are modified from Mattinson (2005) and adapted at PCIGR as outlined in Scoates and Wall (2015). Isotopic ratios were measured on a modified single collector VG-54R or 354S (with Sector 54 electronics) thermal ionisation mass spectrometer equipped with analogue Daly photomultipliers.…”

mentioning

confidence: 99%

“…Uranium fractionation was determined directly on individual runs using the EARTHTIME ET535 mixed 205 Pb-233 U-235 U isotopic tracer and Pb isotopic ratios were corrected for fractionation of 0.25%/amu ± 0.03% (2σ) based on replicate analyses of NBS-982 reference material (Scoates and Wall 2015). All dates were Th-corrected for initial disequilibrium in 230 Th/ 238 U using a Th/U magma = 3.5 ± 1.5 (Bushveld marginal rocks: Barnes et al 2010) and 238 U/ 235 U = 137.88.…”

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confidence: 99%

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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: 85%

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confidence: 99%

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confidence: 99%

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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 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%

“…Modern high-precision dating of layered intrusions relies on the availability of zircon in cumulates. However, experimental studies show that zircon does not crystallise until the very late stages of fractionation (see Scoates and Wall 2015 for a detailed review). Therefore, melt migration has the potential to obscure crystallisation ages of cumulates, especially due to the fact that slow cooling of cumulate frameworks in layered intrusions facilitates enhanced porosity and permeability (e.g.…”

Section: Electronic Supplementary Materialsmentioning

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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Comprehensive Pb‐Sr‐Nd‐Hf isotopic, trace element, and mineralogical characterization of mafic to ultramafic rock reference materials

Fourny

Weis

Scoates

2016

Geochem Geophys Geosyst

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Controlling the accuracy and precision of geochemical analyses requires the use of characterized reference materials with matrices similar to those of the unknown samples being analyzed. We report a comprehensive Pb‐Sr‐Nd‐Hf isotopic and trace element concentration data set, combined with quantitative phase analysis by XRD Rietveld refinement, for a wide range of mafic to ultramafic rock reference materials analyzed at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. The samples include a pyroxenite (NIM‐P), five basalts (BHVO‐2, BIR‐1a, JB‐3, BE‐N, GSR‐3), a diabase (W‐2), a dolerite (DNC‐1), a norite (NIM‐N), and an anorthosite (AN‐G); results from a leucogabbro (Stillwater) are also reported. Individual isotopic ratios determined by MC‐ICP‐MS and TIMS, and multielement analyses by HR‐ICP‐MS are reported with 4–12 complete analytical duplicates for each sample. The basaltic reference materials have coherent Sr and Nd isotopic ratios with external precision below 50 ppm (2SD) and below 100 ppm for Hf isotopes (except BIR‐1a). For Pb isotopic reproducibility, several of the basalts (JB‐3, BHVO‐2) require acid leaching prior to dissolution. The plutonic reference materials also have coherent Sr and Nd isotopic ratios (<50 ppm), however, obtaining good reproducibility for Pb and Hf isotopic ratios is more challenging for NIM‐P, NIM‐N, and AN‐G due to a variety of factors, including postcrystallization Pb mobility and the presence of accessory zircon. Collectively, these results form a comprehensive new database that can be used by the geochemical community for evaluating the radiogenic isotope and trace element compositions of volcanic and plutonic mafic‐ultramafic rocks.

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Geochronology of Layered Intrusions

Cited by 49 publications

References 247 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

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

Comprehensive Pb‐Sr‐Nd‐Hf isotopic, trace element, and mineralogical characterization of mafic to ultramafic rock reference materials

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