科研之友
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摘要
Incorporation of the Kaapvaal craton within a speculative Neoarchaean-Palaeoproterozoic supercontinent has long been debated, and this idea provides a potential Solution to solving the apparently enigmatic provenance of the huge quantities of gold within the famous Witwatersrand auriferous deposits of Kaapvaal. Within a framework of a postulated Neoarchaean "Kenorland" ("northern"; present-day reference) supercontinent, we examine possible "southern" cratons that may have been contiguous with Kaapvaal: Pilbara, Zimbabwe, Dharwar, Sao Francisco, Amazon, Congo. Brief reviews of their basic geology and inferred evolution in syn-Witwatersrand basin times (c. 3.1-2.8 Ga) show no obvious Support for any such supercontinental amalgamations. An alternative idea to explain a measure Of gross similarity amongst several Neoarchaean cratons is through global events, such as a c. 3125-3000 Ma cratonic-scale erosive event interpreted for both Pilbara and Kaapvaal, and a much more widespread magmatic event at c. 2760-2680 Ma. We postulate that a global superplume event at c. 3.0 Ga included a plume beneath the Kaapvaal cratonic nucleus, thus halting any subduction around that terrane due to the thermal anomaly. Such a speculative global magmatic event is assumed to have enhanced production of juvenile oceanic crust at mid-ocean ridges, including those "offshore" of the thermally elevated Kaapvaal nucleus. Intra-oceanic obduction complexes may have built Lip fairly rapidly under such conditions, globally, and once the plume event had abated, "normal" plate tectonics would have resulted in composite (greenstone-tonalite, possibly also including granite) terranes accreting with nuclei such as Kaapvaal. This enhanced plume-related cratonic growth can be seen as a rapid accretion event. Formation of the envisaged ophiolite complexes possibly encompassed deformation-related first-order concentration of gold, and once accretion Occurred around Kaapvaal's nucleus, from north and west (present-day frame of reference), a second-order (deformation-related) gold concentration may have resulted. The third Order of gold concentration would logically have occurred once placer systems reworked detritus derived from the orogens along the N and W margins of Kaapvaal. Such conditions and placer gold deposits are known from many Neoarchaean cratons. The initial Source of gold was presumably from the much hotter Mesoarchaean mantle and may have been related to major changes in Earth's tectonic regime at c. 3.0 Ga. The unique nature of Kaapvaal is probably its early stabilization, enabling formation of a complex flexural foreland basin system, in which vast quantities of placer sediments and heavy minerals could be deposited, and preserved from younger denudation through a unique post-Witwatersrand history.
- 出版日期2009-6
