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TitleShallow 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
AuthorLenhardt, N; Bleeker, WORCID logo; Ngwa, C N; Aucamp, A
SourcePrecambrian Research vol. 338, 105580, 2019 p. 1-24,
Alt SeriesNatural Resources Canada, Contribution Series 20190598
Mediapaper; on-line; digital
File formatpdf; html
AreaJohannesburg; Transvaal; Doornkop Nature Reserve; South Africa
Lat/Long WENS 22.0000 32.0000 -22.0000 -30.0000
Subjectsstratigraphy; tectonics; geochemistry; mineralogy; geophysics; geochronology; igneous and metamorphic petrology; Science and Technology; Nature and Environment; tectonic setting; tectonic evolution; plate margins; volcanism; rifting; subduction; fractional crystallization; magmatism; intrusions; sills; dykes; partial melting; continental margins; marine environments; craton; mantle; mineral enrichment; paleoenvironment; paleocurrents; turbidity currents; lithostratigraphy; lithofacies; facies analyses; bedrock geology; lithology; igneous rocks; volcanic rocks; basalts; lavas; volcaniclastics; breccias, volcanic; tuffs; lava flows; tholeiites; intrusive rocks; dolerites; mafic intrusive rocks; sedimentary rocks; shales; dolomites; siltstones; sandstones; turbidites; geochemical analyses; major element analyses; trace element analyses; modelling; geophysical interpretations; magnetic interpretations; magnetic susceptibility; radiometric dating; uranium lead dating; clasts; Machadodorp Member; Boven Member; Lydenburg Member; Silverton Formation; Pretoria Group; Transvaal Supergroup; Transvaal Basin; Kaapvaal Craton; Paleoproterozoic; Daspoort Formation; Postmasburg Group; Hekpoort Formation; Bushy Bend Member; Precambrian; Proterozoic
Illustrationsgeoscientific sketch maps; cross-sections, stratigraphic; lithologic sections; photographs; photomicrographs; tables; geochemical plots; ternary diagrams; block diagrams
Released2019 12 14
AbstractThe 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.

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