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Granular zircon from Vredefort granophyre (South Africa) confirms the deep injection model for impact melt in large impact structures

Elizaveta Kovaleva, Dmitry A. Zamyatin and Gerlinde Habler
Granular zircon from Vredefort granophyre (South Africa) confirms the deep injection model for impact melt in large impact structures
Geology (Boulder) (August 2019) 47 (8): 691-694

Abstract

The Vredefort impact structure, South Africa, is a 2.02 Ga deeply eroded meteorite scar that provides an opportunity to study large impact craters at their lower stratigraphic levels. A series of anomalous granophyre dikes in the core of the structure are believed to be composed of an impact melt, which intruded downwards from the crater floor, exploiting fractures in basement rocks. However, the melt emplacement mechanisms and timing are not constrained. The granophyre dikes contain supracrustal xenoliths captured at higher levels, presently eroded. By studying these clasts and shocked minerals within, we can better understand the nature of dikes, magnitude of impact melt movement, conditions that affected target rocks near the impacted surface, and erosional rates. We report "former reidite in granular neoblastic" (FRIGN) zircon within a granite clast enclosed in the granophyre. High-pressure zircon transformation to reidite (ZrSiO (sub 4) ) and reversion to zircon resulted in zircon grains composed of fine neoblasts ( approximately 0.5-3 mu m) with two or three orthogonal orientations. Our finding provides new independent constraints on the emplacement history of Vredefort granophyre dikes. Based on the environment, where other FRIGN zircons are found (impact glasses and melts), the clast was possibly captured near the top of the impact melt sheet and transported to the lowermost levels of the structure, traveling some 8-10 km. Our finding not only provides the highest-pressure shock estimates thus far discovered in the Vredefort structure (> or =30 GPa), but also shows that microscopic evidence of high shock pressures can be found within large eroded craters at their lowest stratigraphic levels.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 47
Serial Issue: 8
Title: Granular zircon from Vredefort granophyre (South Africa) confirms the deep injection model for impact melt in large impact structures
Affiliation: University of the Free State, Department of Geology, Blomfontein, South Africa
Pages: 691-694
Published: 201908
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 32
Accession Number: 2019-068813
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2019251
Illustration Description: illus. incl. geol. sketch map
S27°00'00" - S27°00'00", E27°30'00" - E27°30'00"
Secondary Affiliation: Russian Academy of Sciences, Ural Branch, Zavaritsky Institute of Geology and Geochemistry, RUS, Russian FederationUniversity of Vienna, AUT, Austria
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201936
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