Skip to Main Content

Microchemical investigation of small-scale pseudotachylitic breccias from the Archean gneiss of the Vredefort Dome, South Africa

Tanja Mohr-Westheide and Wolf Uwe Reimold
Microchemical investigation of small-scale pseudotachylitic breccias from the Archean gneiss of the Vredefort Dome, South Africa (in Large meteorite impacts and planetary evolution IV, Roger L. Gibson (editor) and Wolf Uwe Reimold (editor))
Special Paper - Geological Society of America (September 2010) 465: 619-643


Pseudotachylitic breccias are the most prominent impact-induced deformation phenomenon in the Vredefort Dome, the eroded central uplift of the 2.02 Ga, originally 250-km-wide Vredefort impact structure in South Africa. Controversy remains about the origin of these melt breccias, and the most popular hypotheses are genesis by (1) shearing (friction melting), (2) shock compression melting, (3) decompression melting immediately after shock propagation through the target or slightly later during the modification phase of cratering, (4) combinations of these processes, or (5) intrusion of allochthonous impact melt. A resolution to this problem requires detailed multidisciplinary analysis in order to characterize the nature of different occurrences of such breccias with the aim of identifying the melt-forming process. Past work has focused mainly on orientation and geometry of Vredefort pseudotachylitic breccia veins, besides a few whole-rock geochemical investigations of mostly decimeter- to tens of meter-sized occurrences, whereas detailed geometric and micro-chemical analysis has not yet been adequately related to microdeformation studies of such melt breccias. Here, we report the results of detailed microchemical analyses of small- to meso-scale pseudotachylitic breccias in a polished 3 X 1.5 m granite slab from a dimension stone quarry in the western core of the Vredefort Dome, supplemented by data for samples from the Rand Granite Quarry in the northern sector of the core. The veinlets selected for analysis do not provide textural evidence for shearing/faulting. Electron microprobe analysis of pseudotachylitic breccia groundmass and X-ray fluorescence bulk chemical analysis of both pseudotachylitic breccias and their host rocks reveal that pseudotachylitic breccia commonly displays a close chemical relationship to its direct wall rock. If groundmass compositions are corrected for the inherent microclast content, correspondence of breccia groundmass and immediate host rock composition is further enhanced. For small veinlets (<1 mm width), melting appears to have occurred locally, with compositions of melt and immediately adjacent host rock minerals commonly being identical. It is, thus, suggested that larger breccia zones could be sites of pooling of melt generated in places throughout the wider environs of dilational sites. For millimeter-scale veinlets, local melt formation and also a lack of lateral mixing are indicated. In contrast, pseudotachylitic breccia veinlets <1 mm, and quite possibly also some larger veins, could conceivably have formed by shock or decompression melting. As previous shock experimental work has demonstrated, this local melting could have been accomplished with or without a friction component.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 465
Title: Microchemical investigation of small-scale pseudotachylitic breccias from the Archean gneiss of the Vredefort Dome, South Africa
Title: Large meteorite impacts and planetary evolution IV
Author(s): Mohr-Westheide, TanjaReimold, Wolf Uwe
Author(s): Gibson, Roger L.editor
Author(s): Reimold, Wolf Uweeditor
Affiliation: Humboldt University Berlin, Leibniz Institute, Museum fuer Naturkunde, Berlin, Germany
Affiliation: University of the Witwatersrand, School of Geosciences, Johannesburg, South Africa
Pages: 619-643
Published: 201009
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 65
Accession Number: 2011-013843
Categories: Igneous and metamorphic petrologyGeochemistry of rocks, soils, and sediments
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch maps
S27°10'00" - S26°55'00", E27°15'00" - E27°34'60"
Secondary Affiliation: Leibniz Institute at Humbaldt University Berlin, DEU, Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201109
Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal