Genesis of the mafic impact melt rock in the northwest sector of the Vredefort Dome, South Africa

Autor(en)

Wolf Uwe Reimold, Natalia Hauser, Alisson Lopes Oliveira, Ana Rita Pereira Maciel, Steven Goderis, Lidia Pittarello, Wencke Wegner, Mario Fischer-Goedde, Christian Koeberl, Vinciane Debaille, Carolinna Silva Maia de Souza

Abstrakt

Genesis and emplacement of Vredefort Granophyre, the impact melt rock exposed on the Vredefort Dome, the erosional remnant of the central uplift of the Vredefort impact structure, South Africa, have long been debated. This debate was recently reinvigorated by the discovery that besides the previously known felsic variety of >66 wt% SiO₂, a second, somewhat more mafic phase of <66 wt% SiO₂ occurs along a Granophyre dike on farms Kopjeskraal and Eldorado in the northwest sector of the dome. Two hypotheses have been put forward to explain the genesis and emplacement of this second phase: (1) successive injections of impact melt into extensional fractures opened in the course of central uplift formation/crater modification, with melts of distinct compositions derived from a differentiating impact melt body in the crater, and (2) generation of the more mafic phase as a product of admixture/assimilation of a mafic country rock component, either the so-called epidiorite of possible Ventersdorp Supergroup affiliation or the Dominion Group meta-lava (DGL), to Felsic Granophyre. In the latter model, contamination with mafic country rock would have occurred during downward intrusion and stoping into and below the crater floor. The so-called Mafic Granophyre has previously only ever been sampled on a single site (Farm Kopjeskraal). In this study, samples of Granophyre occurring along the southerly extension of this dike on farm Rensburgdrif, and from a second dike on the Rietkuil property further southwest were investigated by field work, and petrographic, geochemical, and isotopic analysis. The mafic phase indeed occurs in the interior of the dike at Rensburgdrif, and also on Rietkuil. New geochemical and Sr-Nd isotope data support the hypothesis that the Mafic Granophyre composition represents a mixture between Felsic Granophyre and a mafic country rock. A 20% admixture of epidiorite or DGL to Felsic Granophyre provides an excellent match for the chemical composition of the Mafic Granophyre. The Sr-Nd isotope data indicate that this admixture likely involved the epidiorite component rather than DGL. Together with earlier Sr-Nd-Os-Se isotopic data, and other geochemical data, these results further support formation of the Mafic Granophyre by local assimilation/admixture of epidiorite to Felsic Granophyre.

Organisation(en)

Department für Lithosphärenforschung

Externe Organisation(en)

Universidade de Brasília, Vrije Universiteit Brussel, Naturhistorisches Museum Wien (NHM), Universität zu Köln, Université Libre de Bruxelles

Journal

Meteoritics and Planetary Science

Band

58

Seiten

907-944

Anzahl der Seiten

38

ISSN

1086-9379

DOI

https://doi.org/10.1111/maps.14027

Publikationsdatum

07-2023

Peer-reviewed

Ja

ÖFOS 2012

105105 Geochemie

ASJC Scopus Sachgebiete

Geophysics, Space and Planetary Science

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/ad5dd5c8-b10f-4978-8a69-82d6613a3b3b