| Titre | Shallow marine basaltic volcanism of the Machadodorp Member (Silverton Formation, Pretoria Group), Transvaal Basin, South Africa - an example of Paleoproterozoic explosive intraplate volcanic activity in
an epeiric embayment |
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| Auteur | Lenhardt, N; Bleeker, W ; Ngwa, C N; Aucamp, A |
| Source | Precambrian Research vol. 338, 105580, 2019 p. 1-24, https://doi.org/10.1016/j.precamres.2019.105580 |
| Image |  |
| Année | 2019 |
| Séries alt. | Ressources naturelles Canada, Contribution externe 20190598 |
| Éditeur | Elsevier |
| Document | publication en série |
| Lang. | anglais |
| DOI | https://doi.org/10.1016/j.precamres.2019.105580 |
| Media | papier; en ligne; numérique |
| Formats | pdf; html |
| Région | Johannesburg; Transvaal; Doornkop Nature Reserve |
| Lat/Long OENS | 22.0000 32.0000 -22.0000 -30.0000 |
| Sujets | cadre tectonique; évolution tectonique; marges plaques; volcanisme; décrochement horizontal; subduction; cristallisation fractionnée; magmatisme; intrusions; filons-couches; filons rocheux; fusion
partielle; marges continentales; milieux marins; craton; manteau terrestre; enrichissement des minéraux; paléoenvironnement; paléocourants; courants de turbidite; lithostratigraphie; lithofaciès; analyses de faciès; géologie du substratum rocheux;
lithologie; roches ignées; roches volcaniques; basaltes; laves; volcanoclastique; brèches volcaniques; tufs volcaniques; coulées de lave; tholéiites; roches intrusives; dolérites; roches intrusives mafiques; roches sédimentaires; schistes; dolomites;
siltstones; grès; turbidites; analyses géochimiques; analyses des éléments majeurs; analyses des éléments en trace; établissement de modèles; interprétations géophysiques; interprétations magnétiques; susceptibilité magnétique; datation
radiométrique; datation au uranium-plomb; clastes; Supergroupe de Transvaal ; Craton de Kaapvaal ; Paléoprotérozoïque; stratigraphie; tectonique; géochimie; minéralogie; géophysique; géochronologie; pétrologie ignée et métamorphique; Sciences et
technologie; Nature et environnement; Précambrien; Protérozoïque |
| Illustrations | cartes géolscientiques généralisées; coupes transversales, stratigraphique; coupes lithologiques; photographies; photomicrographies; tableaux; graphiques géochimiques; diagrammes ternaires;
blocs-diagrammes |
| Diffusé | 2019 12 14 |
| Résumé | (disponible en anglais seulement)
The early Paleoproterozoic Machadodorp Member of the Silverton Formation (Pretoria Group, Transvaal Supergroup), South Africa, is an example of ancient
submarine explosive volcanism in an intra-cratonic setting under incipient rifting conditions. The Silverton Formation consists of three different members, which include, in ascending order, the Boven Member, the Machadodorp Member and the Lydenburg
Member. The formation is primarily characterized by the muddy sediments of the Boven and Lydenburg members, which formed in a relatively shallow submarine environment, predominantly as sub-storm wave-base pelagic deposits, within an epeiric embayment
on the Kaapvaal Craton. The subaqueous Machadodorp Member volcanic activity is mostly distributed in the eastern part of the basin. So far, the volcanism of the Machadodorp Member has not been studied in detail. Therefore, a detailed facies analysis
of its volcanism was initiated, allowing constraints on its volcanic evolution.
The Machadodorp Member consists of a basal unit of basaltic sheet lava, followed by a fining-upward succession of volcaniclastic rocks, ranging from breccias,
tuff-breccias and tuffs, rich in fluidal-shaped clasts, to laminated tuffs that are related to subaqueous mass flows, and a final sheet lava that characterized the end of volcanic activity in the section studied.
The Machadodorp Member rocks are
tholeiitic basalts in composition, with all samples plotting in the low-TiO2 suite. Fractional crystallization of clinopyroxene, olivine and plagioclase was the predominant evolutionary process as illustrated by mineral chemistry and major element
geochemical modeling. AFC (Assimilation and Fractional Crystallization) processes could not account for all the trace element variations, nor for the arc-like signatures observed on the primitive mantle normalized plots. Variable ancient mantle
enrichment beneath this area is a more plausible cause for the trace elements variations observed in the Machadodorp rocks. Trace element modeling indicates that the parental composition of these rocks was a variably enriched spinel lherzolite
source. Such enrichment must have occurred before 3 Ga during episodes of arc collision and subduction. |
| GEOSCAN ID | 321926 |
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