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

摘要: Abstract The early Paleoproterozoic Machadodorp Member of the Silverton Formation (Pretoria Group, Transvaal Supergroup), South Africa, is an example ancient submarine explosive volcanism in intra-cratonic setting under incipient rifting conditions. consists three different members, which include, ascending order, Boven Member, and Lydenburg Member. formation primarily characterized by muddy sediments formed a relatively shallow environment, predominantly as sub-storm wave-base pelagic deposits, within epeiric embayment on Kaapvaal Craton. subaqueous volcanic activity mostly distributed eastern part basin. So far, has not been studied detail. Therefore, detailed facies analysis its was initiated, allowing constraints evolution. basal unit basaltic sheet lava, followed fining-upward succession volcaniclastic rocks, ranging from breccias, tuff-breccias tuffs, rich fluidal-shaped clasts, to laminated tuffs that are related mass flows, final lava end section studied. rocks tholeiitic basalts composition, with all samples plotting low-TiO2 suite. Fractional crystallization clinopyroxene, olivine plagioclase predominant evolutionary process illustrated mineral chemistry major element geochemical modeling. AFC (Assimilation Crystallization) processes could account for trace variations, nor arc-like signatures observed primitive mantle normalized plots. Variable enrichment beneath this area more plausible cause elements variations rocks. Trace modeling indicates parental composition these variably enriched spinel lherzolite source. Such must have occurred before 3 Ga during episodes arc collision subduction.