Título:	Genesis of the mafic granophyre at Vredefort, South Africa : field, petrographic, and geochemical studies of the impact melt rock dike on farm rensburgdrif
Autor(es):	Maciel, Ana Rita Pereira
E-mail do autor:	maciel,anarita95@gmail.com
Orientador(es):	Reimold, Wolf Uwe
Coorientador(es):	Hauser, Natalia
Assunto:	Vredefort (África do Sul) Fusão por impacto Rochas - análise Granófiro máfico Geoquímica isotópica
Data de publicação:	5-Abr-2023
Data de defesa:	29-Out-2021
Referência:	MACIEL, Ana Rita Pereira. Genesis of the mafic granophyre at Vredefort, South Africa: field, petrographic, and geochemical studies of the impact melt rock dike on farm rensburgdrif. 2021. 117 f., il. Dissertação (Mestrado em Geologia) — Universidade de Brasília, Brasília, 2021.
Abstract:	The Vredefort impact structure in South Africa is currently the largest impact structure known on Earth, with an original diameter estimated 250-300 km. Vredefort is also one of the oldest impact structures known on Earth (~2 Ga). The Vredefort Dome represents the deeply eroded central uplift of this large, complex impact structure. The formation of the impact melt rock of this structure – the Vredefort Granophyre – has been the subject of debate for more than 50 years. This impact melt rock was long considered a regionally remarkably uniform lithology in terms of chemical composition and along clast populations - essentially independent of the compositions of local wall rocks. Mafic inclusions were observed only locally where a dike cuts across an occurrence of metamorphosed dioritic – so-called epidiorite – or basaltic – Dominion Group Metalava - country rock. The genesis of this “normal” Felsic Granophyre has been traditionally explained by wholesale melting of target rocks, at various proportions of mainly granite plus quartzite and shale, or with an additional Ventersdorp Supergroup volcanic component. In 2011, the presence of a Mafic Granophyre phase was reported, allegedly along the margins of a dike of felsic Granophyre on the Kopjeskraal property, in the NW of the Vredefort Dome. It was proposed that two successive intrusions of melt from a differentiating impact melt sheet initially present in the Vredefort crater structure had generated this composite dike. Subsequently, it was recognized that the dike was indeed of composite nature but that the mafic phase occurred in the central part along the extension of the dike. It was hypothesized that mixing Felsic Granophyre with a local occurrence of likely epidiorite could explain the formation of the mafic phase. The second possible mafic component, Dominion Group Lava (DGL), was also considered. The current project was aimed at clarifying the formation of the Vredefort Granophyre by multidisciplinary analysis of the extension of this same dike on Farm Rendsburgdrif. Samples collected along the dike and along a profile perpendicular across the dike were analysed. The analyses indicated the presence of two distinct phases, whereby the mafic (clast-poor) phase was observed in the central part of the dike and the (felsic) clast-rich phase at the margins. Geochemical data indicate that the Mafic Granophyre composition is a mixture between the felsic phase and mafic host rock. The isotope data for Dominion Group metabasalt fall far off the Granophyre and epidiorite data array, which supports that DGL may not have been a precursor for the Mafic Granophyre phase and, apparently, did not play a significant role in the formation of Vredefort Granophyre. In this way, our results strongly mitigate against the hypothesis that Mafic Granophyre represents a differentiation from the original impact melt body and favor the epidiorite assimilation/admixture hypothesis for the formation of Mafic Granophyre.
Unidade Acadêmica:	Instituto de Geociências (IG)
Informações adicionais:	Dissertação (mestrado) — Universidade de Brasília, Instituto de Geociências, Programa de Pós-Graduação em Geologia, 2021.
Programa de pós-graduação:	Programa de Pós-Graduação em Geologia
Licença:	A concessão da licença deste item refere-se ao termo de autorização impresso assinado pelo autor com as seguintes condições: Na qualidade de titular dos direitos de autor da publicação, autorizo a Universidade de Brasília e o IBICT a disponibilizar por meio dos sites www.bce.unb.br, www.ibict.br, http://hercules.vtls.com/cgi-bin/ndltd/chameleon?lng=pt&skin=ndltd sem ressarcimento dos direitos autorais, de acordo com a Lei nº 9610/98, o texto integral da obra disponibilizada, conforme permissões assinaladas, para fins de leitura, impressão e/ou download, a título de divulgação da produção científica brasileira, a partir desta data.
Agência financiadora:	Coordenação de Aperfeiçoamento de Pessoal do Nível Superior (CAPES).
Aparece nas coleções:	Teses, dissertações e produtos pós-doutorado

Os itens no repositório estão protegidos por copyright, com todos os direitos reservados, salvo quando é indicado o contrário.