Metasomatism of the Kaapvaal Craton during Cretaceous intraplate magmatism revealed by combined zircon U-Pb isotope and trace element analysis

Hoare, Brendan C.; O’Sullivan, Gary; Tomlinson, Emma L.

doi:10.1016/j.chemgeo.2021.120302

Cited by 7 publications

(4 citation statements)

References 86 publications

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“…U–Pb dating of kimberlite zircon is complicated by the fact that zircon crystallization may predate kimberlite magmatism and thus preserve ages that predate emplacement by millions to several billions of years (e.g. Kinny et al 1989; Zartman & Richardson, 2005; Hoare et al 2021). (U/Th)–He dating applied to zircon and perovskite circumvents some sources of error in kimberlite dating by analysing a daughter product retained only at low temperatures; however, the system is susceptible to resetting by post-emplacement tectonomagmatic processes (Stanley & Flowers, 2016), and additionally may not always provide precise age constraints amongst other issues (see Reich et al 2007).…”

Section: Introductionmentioning

confidence: 99%

Uranium–lead geochronology applied to pyrope garnet with very low concentrations of uranium

O’Sullivan

Hoare²,

Mark

et al. 2023

Geol. Mag.

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We present U–Pb dates from peridotitic pyrope-rich garnet from four mantle xenoliths entrained in a kimberlite from Bultfontein, South Africa. Garnet dates magmatic emplacement due to the high mantle residence temperatures of the source material prior to eruption, which were most likely above the closure temperature for the pyrope U–Pb system. We determine a U–Pb date of 84.0 ± 8.1 Ma for the emplacement of the Bultfontein kimberlite from garnet in our four xenolith samples. The date reproduces previous dates obtained from other mineral-isotope systems (chiefly Rb–Sr in phlogopite). Garnet can be dated despite extremely low concentrations of U (median ∼0.05 μg/g), because concentrations of common Pb are often low or non-detectable. This means that sub-concordant garnets can be dated with moderate precision using very large laser-ablation spots (130 μm) measured by quadrupole inductively coupled plasma – mass spectrometry (LA-Q-ICP-MS). Our strategy demonstrates successful U–Pb dating of a U-poor mineral due to high initial ratios of U to common Pb in some grains, and the wide spread of isotopic compositions of grains on a concordia diagram. In addition, the analytical protocol is not complex and uses widely available analytical methods and strategies. This new methodology has some advantages and disadvantages for dating kimberlite emplacement versus established methods (U-based decay systems in perovskite and zircon, or Rb- or K-based systems in phlogopite). However, this method has unique promise for its potential application to detrital diamond prospecting and, more speculatively, to the dating of pyrope inclusions in diamond.

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Section: Introductionmentioning

confidence: 99%

Uranium–lead geochronology applied to pyrope garnet with very low concentrations of uranium

O’Sullivan

Hoare²,

Mark

et al. 2023

Geol. Mag.

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“…An inherited subduction signature manifested in cratonic mantle metasomes is supported by 40 Ar/ 39 Ar age results for phlogopite from strongly metasomatized peridotites along the southern Kaapvaal craton margin, yielding ages of 1.25-1.0 Ga (Hopp et al 2008). In contrast, U-Pb geochronology of zircon in MARID-type xenoliths from the western Kaapvaal craton revealed exclusively Late Mesozoic ages (Konzett et al 1998;Giuliani et al 2015;Hoare et al 2021), but it is uncertain whether these dates represent the timing of K-rich mantle metasomatism or simply the time of isotopic closure as a consequence of lithospheric mantle cooling after a shortlived, yet impactful regional heating event during the Early Mesozoic (Bell et al 2003). Regardless of the timing of lithospheric mantle K-enrichment, it is remarkable how uniform this enriched domain apparently was in terms of its mineralogical and geochemical compositions, as evident from the similar incompatible element systematics and Sr-Nd-Hf isotopic compositions (Figs 12 & 13) of type orangeites.…”

Section: Mesozoic Orangeite Magmatism On the Central Kaapvaal Cratonmentioning

confidence: 87%

Evolution of ultrapotassic volcanism on the Kaapvaal craton: deepening the orangeite versus lamproite debate

Tappe

Shaikh

Wilson

et al. 2021

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Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a ca. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic for the bulk rock compositions and is associated with strongly depleted Th-U-Nb-Ta contents at high Cs-Rb-Ba-K concentrations. The resultant high LILE/HFSE ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones.The Bushveld-intersecting orangeite dykes have strongly enriched Sr-Nd-Hf isotopic compositions (initial 87Sr/86Sr = 0.70701-0.70741; εNd = −10.6 to −5.8; εHf = −14.4 to −2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti-Nb-Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2-1.1 Ga Namaqua-Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma.Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H2O-CO2-F in the petrogeneses of these potentially diamondiferous alkaline rocks.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5440652

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“…Worldwide, over the last few decades, much information about the magma evolution and lithology determination of kimberlites has been gleaned from the zonal structure of olivine elsewhere in the past [2,[29][30][31][32][33][34][35][36]. Currently, global research on minerals in kimberlite primarily focuses on olivine, garnet, perovskite, and spinel [37][38][39][40][41][42][43][44][45][46]. The olivine found in southern Liaoning's kimberlite typically exhibits pronounced serpentinization, posing challenges for comprehensive investigation.…”

Section: Introductionmentioning

confidence: 99%

Revealing the Magmatic Impulse Emplacement and Evolution Path of Kimberlite in Southern Liaoning through Mineralogical Characteristics of the Phlogopite Zone

Ma,

Wang,

2024

Applied Sciences

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Phlogopite is a crucial indicator for effectively constraining the magmatic evolution and emplacement mechanism of kimberlite. In this study, samples were collected from the No. 110 kimberlite pipe within diamond belt I and the No. 50 kimberlite pipe within diamond belt II in the southern Liaoning diamond mining area in the eastern North China Craton (NCC). Zonation is highly developed in the phlogopite; the major and trace element compositions of the phlogopite zonation in the samples were analyzed. In this study, phlogopite from the No. 50 pipe kimberlite (#50 phlogopite) zonation is divided into the following components: (1) The cores, low Ti-Cr xenocryst, average Mg# = 90.6, has a resorption structure, the presence of serpentine and talc minerals in low Ti-Cr cores (xenocrysts) can be used as evidence for hydrothermal metasomatism; (2) cores/inner rims (between core and outer rim), high Ti-Cr, it is thought to be related to the assimilation of mantle materials by deep kimberlite magma, average Mg# = 88.2; (3) outer rims, low-Cr/Cr-poor, average Mg# = 82.4, Fe, Al and Ba contents increased, and there was a trend of evolution to biotite composition believed to be related to the metasomatic metamorphism of melt and wall rock during the late magmatic evolution or ascent; (4) rinds, it is characterized by re-enrichment of Mg, rind I (low-Ti-Cr, average Mg# = 88.4), rind II (high-Ti-Cr, Mg# = 88.6), rind II may be formed earlier than rind I. Rind is very rare and has been reported for the first time in southern Liaoning kimberlite. This study was only accidentally found in the outermost part of #50 phlogopite, the Mg-rich feature represents an environment in which oxygen fugacity has increased. The phlogopite in samples from pipe No. 110 (#110 phlogopite) exhibits relatively homogeneous characteristics across different zones and is more enriched in Al and Ba, which is likely the result of mantle metasomatism. Due to its euhedral characteristics and limited composition variation, it is considered that #110 phlogopite is more likely to be derived from direct crystallization from magma than from xenocrysts. In addition, based on the simultaneous enrichment of Al and Fe in phlogopite from the core to the outer rim, pipe No. 50 was determined to be a micaceous kimberlite, while pipe No. 110 more closely resembles group I kimberlites. This paper proposes that successive pulses of kimberlite magma emplacement gradually metasomatized the conduit, and subsequent kimberlite magma ascended along the metasomatized conduit, thereby minimizing the interaction between the later magma and the surrounding mantle lithosphere.

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Metasomatism of the Kaapvaal Craton during Cretaceous intraplate magmatism revealed by combined zircon U-Pb isotope and trace element analysis

Cited by 7 publications

References 86 publications

Uranium–lead geochronology applied to pyrope garnet with very low concentrations of uranium

Uranium–lead geochronology applied to pyrope garnet with very low concentrations of uranium

Evolution of ultrapotassic volcanism on the Kaapvaal craton: deepening the orangeite versus lamproite debate

Revealing the Magmatic Impulse Emplacement and Evolution Path of Kimberlite in Southern Liaoning through Mineralogical Characteristics of the Phlogopite Zone

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