科研之友
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摘要
The Palaeoproterozoic Magondi Supergroup lies unconformably on the Archaean granitoid-greenstone terrain of the Zimbabwe Craton and experienced deformation and metamorphism at 2 06-1 96 Ga to form the Magondi Mobile Belt The Magondi Supergroup comprises three lithostratigraphic units Volcano-sedimentary rift deposits (Deweras Group) are unconformably overlain by passive margin back-arc and foreland basin sedimentary successions including shallow-marine sedimentary rocks (Lomagundi Group) in the east and deeper-water shelf to continental slope deposits in the west (Piriwiri Group) Based on the upward-coarsening trend and presence of volcanic rocks at the top of the Piriwiri and Lomagundi groups the Pit-twirl Group is considered to be a distal deeper-water time-equivalent of the Lomagundi Group The Magondi Supergroup experienced low-grade metamorphism in the southeastern zone but the grade increases to upper greenschist and amphibolite facies grade to the north along strike and more dramatically across strike to the west reaching upper amphibolite to granulite facies in the Piriwiri Group
Carbonates form prominent horizons in the lower Lomagundi Group occur in the Deweras Group as thick packages in the northern part of the basin but form only thin discontinuous beds elsewhere and are rare in the Piriwiri Group Beds of anhydrite and sulphate pseudomorphs are relatively common in the Deweras Group and also occur in the Lomagundi Group Schidlowski et al (1975 1976) found extreme enrichment in C-13 in carbonates of the Lomagundi Group with an average delta C-13 value of +8 2 parts per thousand VPDB Subsequent work in the Magondi Basin has shown that high delta C-13 carbonates are also present in the continental rocks of the underlying Deweras Group
The initiation of the Deweras rift is not well constrained geochronologically but It may have started as early as 2 26 Ga and was followed by deposition of the lower part of the Lomagundi Group on a passive continental margin Assuming that the deepening trend in the upper Lomagundi Group and upward-coarsening trend in the Piriwiri Group reflect subsidence in a back-arc and subsequent foreland basin setting with sediment derivation from an approaching volcanic arc the age of the onset of Magondi deformation at ca 2 0 Ga provides an upper age limit for sedimentation
Combining all available data for the Magondi Basin the Limpopo Belt between the Zimbabwe and Kaapvaal cratons and the Kheis Belt on the southwestern margin of the Kaapvaal craton a new tectonic model is presented for the assembly of the Kaapvaal and Zimbabwe cratons at ca 2 1-2 0 Ga These cratons along with the Archaean cratons in West Africa and South America record an assembly of a large continent at the time when Archaean cratons in North America and Fennoscandia experienced extension and breakup Carbonates of the Magondi Basin the Kheis Belt and the northern margin of the Kaapvaal craton therefore reflect the carbon Isotope composition of the open ocean at 2 2-2 1 Ga and provide further evidence that the delta C-13 values reaching to +10 parts per thousand VPDB and higher record a seawater signal rather than local diagenetic or closed basin conditions The emerging picture is that Earth experienced dramatic tectonic reorganization between 2 1-2 0 Ga which likely influenced oc
- 出版日期2010-10-15
